Modern Views on Desilicification: Biosilica and Abiotic Silica Dissolution in Natural and Artificial

34 Pages·2010·4.93 MB·English

4656 Chem.Rev.2010,110,4656–4689
Modern Views on Desilicification: Biosilica and Abiotic Silica Dissolution in
Natural and Artificial Environments
Hermann Ehrlich,*,† Konstantinos D. Demadis,*,‡ Oleg S. Pokrovsky,§ and Petros G. Koutsoukos|
InstituteofBioanalyticalChemistry,DresdenUniversityofTechnology,D-01069Dresden,Germany,CrystalEngineering,GrowthandDesign
Laboratory,DepartmentofChemistry,UniversityofCrete,VoutesCampus,GR-71003Heraklion,Crete,Greece,LaboratoryofMechanismsand
TransferinGeology,ObservatoryMidi-Pyrenees(OMP),UMR5563,CNRS,14AvenueEdouardBelin,31400Toulouse,France,andFORTH-ICEHT
andLaboratoryofInorganicandAnalyticalChemistry,DepartmentofChemicalEngineering,UniversityofPatras,GR-26504Patras,Greece[007f]
Received October 7, 2009
Contents recentlyreviewedbyEhrlichetal.2canbestbedemonstrated
by the existence of approximately 128000 species of
1. Introduction 4656 molluscs, about 800 species of corals, 5000 species of
2. Biosilica: Diversity in Structure and Composition 4657 sponges(including550speciesofglasssponges),700species
2.1. Biosilica at the Microlevel: Protozoa and 4658 of calcareous green, red, and brown algae, more than 300
Diversity of Siliceous Structures speciesofdeep-seabenthicforaminifera,and200000diatom
2.2. Biosilica at the Macrolevel: Marine Glass 4660 species.3
Sponges Thereisnodoubtthatforthecompleteanddetailedstudy
2.3. Pathological Silicification: Silica Calculi and 4661 of the role of organic matrices (frameworks or templates)
Stones involvedinbiomineralization,theymustbefirstisolated(in
3. Biodegradation and Silica Utilization in Nature 4662 a “pristine”, undamaged form, ideally) from biocomposites
3.1. Silica Bioleaching: Bacteria and Fungi 4663 using appropriate demineralization techniques. Therefore,
3.2. Silica Bioleaching: Diatoms 4665 demineralization as a tool is an inevitable step in most
3.3. Silica Bioleaching: Sponges (Porifera) 4665 modernstrategiesrelatedtoinvestigationsofbiomineraliza-
3.4. Silica Bioleaching: Worms (Annelida) 4666 tion mechanisms. The isolation of an organic component
fromanynaturallybiomineralizedmaterial,whethermineral-
4. Possible Mechanisms of Biologically Mediated 4666
Desilicification izedwithcalcium-orsilica-containingcompounds,isindis-
pensable.Themostefficientandeffectivetechniqueshould
4.1. Interaction between Silica and Cells and 4666
Tissues be not based upon fast dissolution of the inorganic compo-
nent;itisaslowandgentle,biomimeticallyinspiredprocess
4.2. Enzymatic Degradation of Silica 4668
that enables the preservation of the organic component in
5. Mechanism and Kinetics of Silica Dissolution 4669
thebiomineral-basednaturallyoccurringcompositeswithout
5.1. Dissolution of Inorganic Silica and Quartz 4669
artifacts.2,4
5.2. Dissolution of Biogenic Silica: Diatoms and 4675
Thebuildingofdiscreteorextendedorganicarchitectures
Their Frustules
inbiomineralizationofteninvolveshierarchicalprocessing.
5.3. Dissolution of Biogenic Silica: Plants and Their 4678
In these reactions, the molecularly based construction of
Phytoliths
organic assemblies is used to provide frameworks for the
6. Desilicification and Isolation of Organic Templates 4681
synthesis of organized inorganic materials, which, in turn,
6.1. Alkali versus Hydrofluoric Acid 4681
are exploited as prefabricated units in the production of
6.2. Isolation of Protein-Based Templates 4682 higherordercomplexmicrostructures.5-10Animalskeletons
6.3. Isolation of Polysaccharide-Based Templates 4683 appear to have been optimized by natural selection to
7. Epilogue, Conclusions, and Outlook 4684 physicallysupportandphysiologicallymaintaindiversetissue
8. Acknowledgments 4684 types encompassing a variety of functions.11,12 Increased
9. References 4684 understanding of biomineralization has initiated develop-
ments in biomimetic synthesis with the production of
1. Introduction synthetic biomimetic materials fabricated according to
biologicalprinciplesandprocessesofself-assemblyandself-
Of the intriguing topics that are receiving renewed organization.13
attention nowadays, the study of the “triangle” biomineral-
Among different biominerals, silicon dioxide (silica) in
ization/demineralization/remineralizationisamongthemost
its different amorphous (and occasionally porous) forms is
fascinating.Biomineralsmaybedepositedwithinthetissues
probablythemostintriguingone,forthefollowingreasons:
of biota, as well as within the corresponding immediate
(a)itisdefinitelythefirstandtheoldestnaturalbioskeleton;
surroundings or environment, as a result of the metabolism
(b)ithasuniquemechanicalproperties;(c)ithasextremely
of the living organism.1 The variety of biomineralizers
highspecificsurfaceareaand,therefore,adsorptionproperties
with respect to dissolved components of external milieu.
*To whom correspondence should be addressed. E-mail addresses:
These properties made silica the most widely distributed
[email protected];[email protected].
†DresdenUniversityofTechnology. biomineral.
‡UniversityofCrete.
Although biomineralization/demineralization/remineral-
§ObservatoryMidi-Pyrenees(OMP).
|UniversityofPatras. ization phenomena are probably among the most widely
10.1021/cr900334y 2010AmericanChemicalSociety
PublishedonWeb05/04/2010
ModernViewsonDesilicification ChemicalReviews,2010,Vol.110,No.8 4657
HermannEhrlichreceivedhisPh.D.degreefromtheUniversityofLemberg Oleg S. Pokrovsky was born in 1972 in Moscow, Russia. He studied
in1984.HermannservedasapostdoctoralresearcheratMax-Bergmann geology and geochemistry at the Geochemistry Department in Moscow
Centre of Biomaterials and Institute of Materials Science in Dresden, State University where he received his Ph.D. in 1994. After two
Germany,andcurrentlyholdsapositionasaStaffScientistattheInstitute postdoctoralstaysatFloridaStateUniversity(Laboratoryoff-elements)
ofBioanalyticalChemistryattheDresdenUniversityofTechnology.His and Laboratoire de Ge´ochimie of Toulouse, he entered CNRS in 1999.
researchisfocusedonthephenomenaofbiomineralization-demineraliza- Since2004,hehasbeenseniorCNRSresearcherattheObservatoryof
tion-remineralization,aswellasonbiologicalmaterialsofmarineorigin. Midi-Pyreneesworkingonmineraldissolutionandprecipitation,weathering,
Usingbiochemical,cellular,molecular,andanalyticalapproaches,heand speciation and migration of trace elements, and metal-microorganism
hisco-workershaveforthefirsttimediscoveredandcharacterizedchitin interactionandbiomineralization.Hehaspublishedover80peerreviewed
andnovelhydroxylatedcollageninskeletalformationsofmarinesponges. articlesininternationalandRussianjournalsandseveralbookchapters.
During last five years, he has published over 30 peer reviewed articles
andthreebookchapters,andheholdsthreepatents.
Petros Koutsoukos is a Professor of the Department of Chemical
Engineering of the University of Patras, Director of the Laboratory of
KostasDemadiswasbornin1967inKomotiniofThraceinGreece.Hereceived InorganicandAnalyticalchemistry.HereceivedthePtichionofchemistry
aB.S.DegreefromtheNationalUniversityofAthens,Greece,in1990.Hethen fromtheUniversityofPatras(1972),DiplomaofSpecializationinBusiness
movedtotheUniversityofMichigan,AnnArbor,whereheearnedhisPh.D.Degree Administration (1974) from the Athens School of Economics, and Ph.D
in1995,workingwithProf.DimitriCoucouvanis.HethenjoinedProf.TomMeyer’s from the State University of New York at Buffalo (1980) under the
researchgroupattheUniversityofNorthCarolinaatChapelHill.In1998,he supervision of Prof. G. H. Nancollas. During 1980-1981, he was a
movedtotheChicagolandareaandjoinedtheResearch&DevelopmentDivision postdoctoral fellow in the Physical and Colloid Chemistry Laboratory at
ofthe(then)NalcoChemicalCompanyasaSeniorChemist.In2003,hewas theAgriculturalStateUniversityofWageningen(TheNetherlands),under
electedAssistantProfessorattheChemistryDepartment,UniversityofCrete, theguidanceofProf.J.Lyklema.In1981,hejoinedtheDepartmentof
Greece,whereheiscurrentlyAssociateProfessor.Hiscurrentresearchinterests ChemistryoftheUniversityofPatras(chairofphysicalchemistry)where
extendoverarangeofscientificareasandincludecrystalgrowthandinhibition, hewasawardedthetitleofIfigesia(D.Sc.)andwasgraduallypromoted
bioinspiredmaterialsscience,metalphosphonatechemistry,materialscorrosion totherankoftheAssociateProfessor.In1989,hejoinedtheDepartment
control,andtechnologyofpolymersandcolloids.Kostasisamemberofthe ofChemicalEngineeringasafullprofessor.Researchinterestsinclude
AmericanChemicalSociety,theNationalAssociationofCorrosionEngineers,and the crystallization of inorganic salts from aqueous solutions, physico-
theAssociationofGreekChemists.Hehaspublished∼100peerreviewedpapers chemicalphenomenaatinterfacesbetweensolidsandwater,andmetal
and8bookchapters,andheholds2patents. corrosion.Over160publicationsinrefereedinternationaljournals,more
than 10 book chapters, and six patents have resulted from this work.
studied topics in modern geochemistry, materials science, Betweentheyears1994and2004,heservedasapresidentandCEO
biomedicine, and biomimetics, a review relating to modern of the board of the Patras Science Park S.A. and during 2006-2007
views on biosilica and abiotic silica in ViVo as well as in served as Deputy Director of the Institute of Chemical Engineering and
Vitro with the basic information on principles of desilicifi- High Temperature Chemical Processes (ICEHT) of the Foundation of
ResearchandTechnologyHellas(FORTH).
cation has been lacking up to now.
2. Biosilica: Diversity in Structure and
dimensional siliceous skeletons of marine glass sponges
Composition
(Figure2),areobserved.Basedonthoroughanalysisofthe
Biogenicsilicaexhibitsdiversityinstructure,density,and literature, numerous examples of siliceous structures de-
composition and can exist in several structural forms, scribed in plants, bacteria, yeast, fungi, protists, sponges,
including sheet-like, globular, fibrillar, helical, tubular, and molluscs,ascidians,crustaceans,brachiopods,fishes,mam-
folded sheets. Different levels of structural organization of mals,andhumansaresummarizedinTable1.Alsoexamples
biosilica, from nanogranules in bacteria (Figure 1) or in of pathological biosilicification observed in animal and
mitochondria to up to 2 m high and 70 cm wide three- human organisms are included and discussed below.
4658 ChemicalReviews,2010,Vol.110,No.8 Ehrlichetal.
A common secretory mechanism has been reported in
testateamoebae,heliozoaandheliozoon-likeamoebae,and
radiolariansasfollows.86Silicadepositionvesicles(SDVs),
either situated in the cell cytoplasm (as in testate amoebae
andheliozoansandrelatives)orwithinanexpandedportion
of the peripheral cytoplasm known as a cytokalymma (in
radiolaria), are the site of silicification. Moreover, in some
testate amoebae Golgi-derived vesicles fuse with the mem-
brane surrounding silica deposition sites. These vesicles
possibly contribute to additional incorporation of silica-
secreting membrane into the surface of the SDV while
increasing the membrane surface area. Silica products of
testateamoebaeandheliozoaaredepositedonthecellsurface
by exocytosis. The cytokalymma of radiolaria, while con-
tainingasilica-secretingvacuolarspace,isdecidedlydifferent
Figure1. TEMmicrographofacyanobacterialcellencrustedin
in form and activity from the intracellular secretory spaces
silicacolloids.C)cell;S)polysaccharidesheath;arrow)layer
ofcolloidalamorphoussilica.Scalebar)5µm.Reproducedwith of testate amoebae and heliozoa. The cytokalymma is a
permissionfromref14,copyright2006,Wiley. dynamicstructureexhibitingcytoplasmicflowingactivityand
in a mold-like manner determines the remarkable species-
specific shape of the skeleton. Consequently, the deposited
silicate product of the radiolarian is an endoskeleton and is
not released on the surface by exocytosis. Further research
is needed to determine whether Golgi-derived vesicles,
designated Golgi-fibrillar vesicles (GFV) in some testate
amoebae, are also the source of SDV membranes in other
silicate-secreting sarcodines. To date, none of the various
SDVs have been isolated.
NormalRaphidiophrysambiguacellshavearoundedbody
formthatextendsasmallnumberofstiffradiatingaxopodia
that are used in food capture. Around the cell body lies a
periplastoflooselyadheringboat-shapedscales(Figure3).
The scales (1.5 µm × 4 µm) are constructed as a thin, flat
mesh of siliceous material, with the edges curved over to
Figure2. Uniquehierarchicallystructuredskeletonsofupto2m form a rim.
high and 70 cm wide of Aspidoscopulia sp. (Hexactinellida,
Testateamoebae(ortestaceans),whichoccurabundantly
Porifera)deep-seaglassspongeshavenotbeeninvestigated.Image
and ubiquitously from the tropics to polar soils, contribute
courtesyofK.R.Tabachnick,copyright2010.
considerablytotheterrestrialsilicacycleassilicaaccumula-
tors.Siliceoustestsareformedinoneoftwoways:theyare
2.1. Biosilica at the Microlevel: Protozoa and composed either of ingested and subsequently agglutinated
Diversity of Siliceous Structures siliceous material such as sand grains and diatom frustules
orfromingesteddissolvedsilicathatisinternallymetabolized
Siliceous products deposited at the cell surface of amoe- into uniformly shaped plates and held together by organic
boid protists include a wide variety of species-specific cement.87,88
structures:spicules,scales,solidplates,granules,meshwork
Testateamoebaeareclassifiedintotwotypesbytheirtest
frustules,andotherelaborategeometricforms(Table1).The
morphology, idiosomes and xenosomes. The test of idio-
siliceous nature of certain protozoan plates was first noted
somesiscomposedofself-secretedsiliceousscales,andthat
byDujardinin1841.77Siliceousprotistscalescanpersistin
of xenosomes is composed of exogenous materials. Idio-
marine and fresh water sediments once the organism has
somes, in particular, absorb dissolved silica from the
perished, leaving a record of their presence. The siliceous
environmenttoproducetheirsiliceousscales.Thescalesare
scales can be preserved over time as subfossils or as
very small (ca. 10 µm) and thin (ca. 1 µm) and probably
microfossilsinsedimentsoffreshwaterandmarinehabitats,
dissolvereadilyinthesoilsolutions.Siliceousplate-bearing
providing a window into modern and, potentially, ancient generaincludeEuglypha,Assulina,Placocista,Sphenoderia,
protistcommunities.78,79Theroleoftheseunicellularorgan- Tracheleuglypha,Cyphoderia,Campascus,Heleopera,Les-
ismsinthesilicacycleanddistributionintheworldoceans quereusia, Nebela, Quadrullela, Paulinella, Trinema, and
isverysignificant.Forexample,radiolariansarethesecond Corythion.87
(after diatoms) major producers of suspended (dispersed) Recently, Aoki et al.,89 found that testate amoebae Eug-
amorphous silica in waters. They occur from the Arctic to lypharotundaandTrinemaenchelysconsumed55%ofsilica
the Antarctic, being most abundant in the equatorial zone. absorbed by microbes in the incubation experiments. Al-
In waters of the equatorial Pacific, there are about 16000 thoughbiosilicapreservedbylivingtestateamoebaewasas
specimens per cubic meter.80 Because the diversity of smallas0.45-1.57kgofSiO ha-1month-1perhapsdueto
2
structuralformsobservedindiatomsiswelldocumentedin the fast turnover of amoebean silica in the terrestrial silica
the literature,40,41,81-85 we took the initiative to present in cycle,invariousforestsoilsitwasestimatedtorangefrom
this review silica-based representatives of unicellular Pro- 10to227kgSiO ha-1year-1.Theseauthorsconcludedthat
2
tozoa, other than diatoms. testate amoebae have a high potential as silica consumers
ModernViewsonDesilicification ChemicalReviews,2010,Vol.110,No.8 4659
Table1. StructuralDiversityofBiosilica
organism structureandformofbiosilica ref
Bacteria
cyanobacteria siliceous“geysereggs” 15
filamentousmicroorganisms siliceousshrubs 16
hyperthermophilicmicroorganisms siliceoussinters 17
siliceousoncoids 18,19
Nostoc-typecyanobacteria “biospeleotherms”andshrubs 20
biofilmcommunity siliceousmicrostromatolitelaminae 21
ThermusthermophilusTMY silicascalefromgeothermalpowerplant 22
Thermusspp.,Hydrogenobacterspp. 23,24
Phormidium,Fischerella lilypadstromatolites(upto3mlongand1.5mwide) 25
Cyanidium,Alicyclobacillus siliceousspicules(upto3cmhighandupto5mmindiameter) 26
Cyanidiumcaldarium granularsilicaspherules 27
Yeast
silicifiedcellwalls 28,29
Fungi
siliceouscoatedgrains 30
desertvarnish 31
Protozoa
sarcodines 32
Gymnamoebae siliceousboat-shapedscales 33
testateamoebae siliceouscurvedrods 34
siliceousbodyplates
siliceousparticles
heliozoa siliceousneedles,spines
radiolaria siliceousspicules
siliceousporousshells
siliceousspongioseshells
flagellates stomatocysts,scales,spines,bristles 35
chrysophytes,synurophytes 36
choanoflagellates siliceousloricae
dinoflagellates siliceousskeletons,cysts,granules,externalskeleton,scales
ebridians statosphores(siliceousrestingcysts) 37
silicoflagellates silicifiedcellwalls(quartz-cellulose-calcitecomposite) 38
thaumatomastigids silicifiedcellwalls
chlorophytes
xanthophytes
Foraminifera
Silicosigmoilinafutabaensis siliceouscement 39
Diatoms
frustule 40
spines 41
heavilysilicifiedrestingspores 42
nanogranulesinmitochondria 43
Sponges
Hexactinellida skeletalframeworksand 44
Demospongiae spicule(macrosclerae)microsclerae 45,46
Mollusca
Patellaceaspp.(Gastropoda) silicainradulateeth 47
Onchidiellaceltica siliceousspiculesandpenialspines 48-51
Ascidians
StyelaclaVa intracellulargranules 52
Crustaceans
(Copepoda) opalandwillemite-basedteeth 53,54
Aeartiatonsa 55,56
Neocalanusspp.
Calanuspacificus
Brachiopods
Disciniscatenuis siliceoustablets 57
Fishes
Psammobatisextenta(Rajidae) chalcedonyinelectrocytesandcholinergicnerves 58-60
Mammals
dog silicacalculiandstones 61
cattle silicacalculiandstones 62
monkey(Macacafuscata) dentalcalculi 63
ratlivermitochondria silicananogranules 64
65
Human
cerebralcortex chalcedony 59,66
urinarybladderandurethra silicacalculiandstones(urolithiasis) 67
kidney renalsilicacalculi 68
humanglialmalignanttumors chalcedony 69
Plants
Chaetocerosgracilis(Bacillariophyceae) siliceoussetae 70
canarygrass(Phalariscanariensis) silicafiber 71
foxtailmillet(Setariaitalica) inflorescencebristles 72
Pleioblastuschino(Poaceae,Bambusoidea) silicacells 73
heathgrass(Sieglingiadecumbens) silicabodies(phytoliths) 74
Bambusoideae tabasheer 75
higherplants siliceousnodulardeposits 76
4660 ChemicalReviews,2010,Vol.110,No.8 Ehrlichetal.
Figure 5. TEM image (left) of Acanthoeca spectabilis siliceous
loricashowingtheleft-handedorientationofcostaecomprisingthe
loricachamberandthecontinuityofindividualhelicalcostaewith
thecostalstripssubtendingthespines(bar)2µm).SEMimage
(right)ofaloricashowingthearrangementofhelicalcostaeforming
thechamberincontinuitywithbasesofthespines(bar)2µm).
Theinnerlayerofcostalstripswithinthelanternofspinesisalso
apparent(arrow).Figurereproducedwithpermissionfromref92,
copyright2008,Elsevier.
Figure3. SiliceousscaleoftheR.ambigua(Protista,Centrolhe-
lates,theAcanthoecidaenorris,areexclusivetomarineand
liozoa).Patterncenter(1),isthmus(2),sternum(3),ribs(4),mesh
(5), rim (6). Figure reproduced with permission from ref 33, brackishwater environments and are distinguished by the
copyright1988,TheCompanyofBiologistsLtd. possessionofabasket-likeloricacomprisinganarrangement
of siliceous costae. The lorica typically contains an outer
arrangementoflongitudinalcostaewithaninnerarrangement
ofhelicalortransversecostae;thecombinedpositioningof
these structures gives the lorica its overall mechanical
stability.Individualcostaearemadeupofrod-shapedcostal
strips attached to each other end-to-end.
Each costal strip is deposited within the cell in a
membrane-boundvesicleand,whencomplete,isexocytosed
totheexterior.93-95Costalstripsareaccumulatedoutsidethe
cell until a sufficient number has been produced to form a
lorica. Assembly of the lorica occurs as a single, discrete
movement that lasts 2-5 min after which no further
adjustments can be made. While all loricate species adhere
tothesebasicprinciples,thereare,nevertheless,twodistinc-
tivetypes.96Inonevariant,thenudiformcondition,alorica-
bearingcelldividestoproducea“juvenile”flagellatedcell,
which swims away from the parent lorica, settles onto a
substratum, accumulates costal strips on its surface, and,
Figure4. PeriplastofPolyplacocystisambigua:(1)fusiformscales; when a complete set have been produced, assembles a
(2)naviculoidscales;(3)ovalscales.Scalebar10µm. lorica.97 In the second variant, the tectiform condition, a
lorica-bearing cell produces a complete set of costal strips
and suppliers and play a role comparable to that of higher priortocelldivisionandstoresthematthetopofthecollar.
plants in the terrestrial silica cycle. Whenacompletesetofstripshavebeenproduced,thecell
Heliozoa are ubiquitous unicellular phagotrophs with divides and the resulting “juvenile” is inverted and pushed
slender radiating axopodia for trapping prey.90 In most out backward from the parent lorica taking with it the
centrohelids, the axopodia (bearing numerous kinetocysts accumulated strips. Within minutes of the “juvenile” being
used for prey capture) project through a thick coat, often liberated from the parent a new lorica is assembled.94,97
double, of silica scales, which are well characterized by
electron microscopy. Recently, Gaponova and Dovga91 2.2. Biosilica at the Macrolevel: Marine Glass
reportedaboutdiversityofthesiliceousscalemorphologies
Sponges
withinonespecies,Polyplacocystisambigua(Protista,Cen-
trohelida). A scanning electron microscopy image (Figure Sponges(Porifera)arethesimplestandoldestmulticellular
4) showed strong evidence of the presense of the fusiform, animals on earth and live attached to the seabed or another
naviculoid,andovalformsoftheP.ambiguaperiplastscales. substratum.Spongesdivergedfromotheranimalsearlierin
Thebasicstructureofthechoanoflagellateloricacomprises evolutionary history than any other known animal group,
a pattern of siliceous costae arranged in longitudinal and extantorextinct,withthefirstsponge-relatedrecordinearth
transverse (or helical) arrays (Figure 5). history found in 1.8 billion year old sediments.98 The huge
Theabundanceofloricatechoanoflagellatesintheoceans diversitywithrespecttotheirnaturalhabitatisprobablythe
today can be directly related to the enormous scope for reason for the estimated number of approximately 15000
variation as is illustrated from the basic structure of the different sponge species.99 The phylum Porifera is divided
siliceous lorica.92 For example, one group of choanoflagel- into three classes, Hexactinellida and Demospongiae that
ModernViewsonDesilicification ChemicalReviews,2010,Vol.110,No.8 4661
comprise a siliceous skeleton and the Calcarea with a 2.3. Pathological Silicification: Silica Calculi and
calcareous skeletal network.100 In contrast to the high Stones
diversity of spicules, there are relatively few basic types of
Silica has been recognized as an essential trace element
skeletal frameworks in demosponges. Six elemental types
in the metabolism of mammals.114 Silicon deficiency gives
of skeletons with intermediate forms can be differentiated:
risetodisordersofbone,cartilageformation,andconnective
hymedesmoid, plumose, axial, radiate, reticulate, and ar-
ranged in strength confusion.44 Hexactinellida Schmidt tissuemetabolism.Silicon,assilicate,ispresentinvegetables
and the husks of whole grains. The blood-plasma level is
(Porifera)aremainlydeep-watermarinespongesdefinedby
approximately 1 mg per 100 mL and usually does not rise
theirproductionofsiliceousspiculesofhexactinic,triaxonic
since any excess is rapidly excreted in the urine.115 In
(cubic)symmetryorshapesclearlyderivedfromsuchforms
humans, silica is also a normal metabolite, absorbed from
byreductionofprimaryraysorterminalbranchesaddedto
the gastrointestinal tract into the blood, and present in the
the ends of primary rays. Hexactinellids include about 600
urine in concentrations of about 10 mg/day116 in normal
describedspecies,7%ofallPorifera,distributedin5orders,
subjects. It is present in urine in amounts proportional to
about 17 families, and about 118 genera (as reviewed by
Ehrlich4).Thespiculesofmosthexactinellidsarelarger(see that in a silica-rich diet such as vegetables, whole grains,
and seafood.
Figure 2) and more luxuriously architectured than those in
demosponges.101Forexample,Antarcticgianthexactinellids, According to Keeler,117 urinary pH, urinary silicate
concentration, and the concentration of matrix constituents
suchasRossellanudaandScolymastrajoubini,whichmay
have an influence on the formation mechanism of silica
be up to 2 m tall, 1.4 m in diameter, and up to 600 kg wet
weight, contain up to 50 kg biogenic silica each.102 They urinary stones. Many patients have received silicate in the
form of antacids without resulting stone formation in the
are extremely slow-growing and seemingly reproduce only
atlongtimeintervals103andbecomeold,probablymorethan urinarytract.Nosignificantdifferenceintheurinarysilicate
1500years.104,105Whiletheirlivingtissuesrepresentonlya concentration and the daily urinary excretion of silicate is
foundbetweensilicatestoneformersandcontrols.Although
modest biomass, the silicious spicules of hexactinellids
urinary silicate levels were within the normal range in
become an important ecological factor. After the death of
described cases, a siliceous fragment and other matrices
thesponges,themegascleresdonotdissolvebutaccumulate
existedsimultaneously.Thesefactssuggestthattheurinary
on the bottom and over large areas to form spicule mats
concentrationofmatrixconstituentshasthegreatestinfluence.
commonly about 50 cm thick but occasionally exceeding
2 m.104,106 The silica stone was first described by Herman and
Goldberg118 in the USA and was attributed to the ingestion
Sponges seem to be important players in numerous
ofmagnesiumtrisilicatefortreatmentofesophagitis.Silica
geobiological processes including the silica demineraliza-
tion-biomineralization-remineralization cycle.107 For ex- occursincalculiintheopalinestate,whichisnoncrystalline,
butonheating,itcanbetransformedintovariouscrystalline
ample, siliceous sponge reefs had a wide distribution in
formsdependingonthetemperatureofignition.68However,
prehistoric times and once constructed the largest reefs
silicacalculiinhumansareknowntobeextremelyrare.119-121
knownonearth,reachinganacmeintheUpperJurassicwhen
Inaseriesofelectronmicroscopicstudiesofhumanurinary
a deep-water reef belt on the northern Thetis shelf existed
that was 7000 km long.108 In present times, hexactinellid stones,discretesiliceousdepositswereencounteredin3out
sponges of the order Hexactinosida have constructed reefs of 180 stones. The deposits differred in their morphology
atseverallocalitiesoffthecoastofBritishColumbia,Canada. from any other components of urinary stones.122 Three
urinarybladderstonescomposedofpuresilicondioxidewere
Thereefsoccuronrelictglaciatedseafloorareaswithalow
foundinapatientwhohadbeentakingmagnesiumtrisilicate
sedimentationrateandahighdissolvedsilicaconcentration
in the Queen Charlotte Basin and in Georgia Basin,109,110 compound three times a day for 40 years.67
and the reefs represent stable communities that have been Long-term oral intake of silica-containing antacids (e.g.,
growingforupto9000years.Themainbuildersofthereef magnesium trisilicate) is thought to be a causative factor
frame include the hexactinosidan species Aphrocallistes inducing silicate urolithiasis.122,123 Briefly, when a silicate
Vastus, Farrea occa, and Heterochone calyx in Queen reacts with acid in the alimentary tube, especially in the
Charlotte Basin and A. Vastus and H. calyx in Georgia stomach, part of the silica is precipitated as a gel and part
Basin109Theframeworkofthesereefsisconstructedthrough remainsasasolutionintheformofacolloid.Thebreakdown
several processes of framebuilding, and the reef matrix is products can be readily absorbed across the intestinal wall
derived from trapping of suspended sediments.108 Similar and excreted rapidly in the urine. Alkalization by the gut
framebuilding processes are thought to have contributed to fluid changes a silica colloid into a more soluble and
the formation of the ancient reefs.111 Upwelling and down- absorbable compound. Part of the silica can be reabsorbed
wellingoceanographicprocessesinabiologicallyproductive andeliminatedintheurine.Urinaryexcretionrisesdramati-
coastalseahavecontributedtothedevelopmentoflargereef cally in proportion with the increase in silicate intake.
complexes. The reefs form as bioherms (mounds) and Syntheticmagnesiumtrisilicate(2MgO·3SiO ·nHO),which
2 2
biostromes(bedsorsheets)thatmayriseupto21mabove isawhiteinsolublepowder,wasshownnottobeabsorbed
the seafloor, and they cover around 1000 km2 on the from the intestine and to have absorbent and antacid
continentalshelf.Individualreefcomplexesmaycoverareas properties of particular value in the treatment of gastric
of more than 300 km2 and occur in the 90-240 m depth acidity and chronic peptic ulceration.124,125 The value of
range.110,112,113 Trapping fine-grained siliciclastic sediments magnesium trisilicate and the silica resulting from the
in suspension in near-bottom currents is an important part reaction with gastric hydrochloric acid apparently depends
of the mound-forming process, because it prevents the reef onthefactthatbothareactivehydratedcolloids.Although
frameworkfromcollapsingunderitsownincreasingweight. the chemical reaction can be represented as
4662 ChemicalReviews,2010,Vol.110,No.8 Ehrlichetal.
2MgO·3SiO ·nH O + 4HCl f 2MgCl + 3SiO + contain several kinds of minerals.130 Rocks and minerals
2 2 2 2
(n + 2)H O (1) represent a vast reservoir of elements, many of which are
2
essential to life and which must be released from their
inanimate prison into forms that may be assimilated by the
the conditions of interaction are complex and are not
biota.Silicateiswidelydistributedinnature:inadditionto
completely understood. Page et al.126 suggested that more
the 28% of the Earth’s land surface formed by quartz, an
complex end-products than silicon dioxide (or silica) were
estimated49%ofsurfaceisoccupiedbyothermineralswith
produced,namely,variouscombinationswithwatertoform
asilicatestructure.131Thefollowingformsofsolidsilicaare
differentsilicicacidsandsilicates.Theyconsideredthatthese
encountered in nature.132
productscouldincludesolubleorthosilicicacid(H SiO ,that
4 4 (a) R-quartz, the most abundant single mineral in sedi-
is,SiO +2H O),partiallysolublemetasilicicacid(H SiO ,
thatis,2SiO +2H O),partiallysolubletrisilicicacid(H2Si O3, ments
that is, 3Si2O +22H O), and nearly insoluble disilici4c a3ci8d (b) opal,oropalinesilica,adisorderedhydratedformof
(H Si O , th2at is, 22SiO + H O). The variation in the silica
2 2 5 2 2 (c) amorphous silica, a term used to describe materials
hydration of the SiO molecule thus induces changes in
2 whosecompositionisrelativelypureSiO thatarein
solubility. This factor, as well as the physicochemical state 2
the mesomorphous state.
of the silica (which may remain in solution as a colloidal
sol or may partially separate as a gel) probably determine Under the classification of amorphous silica, the various
its absorption from the intestine. kinds of hydrated and dehydrated silica gels, silica glass,
Literaturedealingwithsilicaurolithiasiswithouttheintake siliceous sinters, powders from the condensation of vapor
ofsilica-containingantacidisscant.Onlytwocasesofsilica phases, and skeletal materials of numerous silica-secreting
calculi without oral intake of magnesium trisilicate were organismsareincluded.Allformsofsolidsilicalistedabove
reported.125 andrepresentedinFigure6areinvolvedintothesilicacycle
Siliceousurinarycalculiarecommonamongherbivorous on the Earth.
animals such as cattle and sheep.117,127 Silica calculi are Through the weathering process, silicate minerals con-
reported also in dogs.61,128 As many as 4% of range steers tribute to the dissolved silica in surface waters and may be
wintered in the northwestern plains of North America concomitantly altered to colloidal aluminosilicates (clays).
develop silica calculi with urethral obstruction. The high In the ocean, via the biochemical precipitation by siliceous
incidence of silica in these animals has been attributed to organisms, silica is precipitated as amorphous silica or
theconsumptionofforagewithahighcontentofSiO2.The opalinesilica.Duringdiagenesis,amorphoussilicadeposited
grassincertainareasmaycontainupto8%ofsilicaondry intheoceansrecrystallizestoquartzandthesilicainsolution
weight basis and a large number of steers in these areas in formation waters gradually comes to equilibrium with
develop silica calculi.62 Precipitation of silica in the urine quartzbyprecipitationofsecondaryovergrowths,authigenic
of these animals tends to occur when their urine level of crystals,etc.Hotspringswatersprecipitatesilicaastheycool
siliconexceeds70-80ppm.However,followinganexperi- at the surface and become supersaturated, forming opaline
mentalincreaseofsiliconinsteerurine,silicacalculifailed or siliceous sinters through intermediate stages of gel
to develop, and therefore, it has been postulated that
formation.
proteinaceous matrix plays a role in silica calculi forma-
Fromabiologicalperspective,siliconcanbeaccumulated
tion.117
by diatoms and other living organisms such as silicoflagel-
Localized silicon distribution was also found in both
lates, some xanthophytes, radiolarians, actinopods, plants
fracture and oral surfaces of the dental calculi in Macaca
suchasgrasses,ferns,horseradish,sometrees,andflowers,
fuscatamonkeys.63Theseauthorshavesuggestedthatsilicon-
somesponges,ascidians,molluscs,andbacteriaandfungi.134
rich areas containing opal as well as clay minerals may
regulate the formation also of human dental calculi.
Tothebestofourknowledge,therearenoworksrelated
to demineralization of silica stones and calculi with res-
pect to isolation of organic matrix. However, the presence
of organic macromolecules (including fibrin) as nucleators
ofthesebiomineralformationswassuggested.Forexample,
Lagergren122 considered that the structure of the calculi in
his cases and the finely crystalline form of the silica
supportedtheviewthatthesilicawasprecipitatedinthegel
form apparently mixed with fibrin. Also Bailey129 reported
results that suggested that urine protein contributes to the
formation of siliceous urinary calculi, by affecting the
solubility of polymerized silicic acid. He suggested that
the reaction involves the silanol groups of the silicic acid
polymers and the secondary amide groups of the protein.
3. Biodegradation and Silica Utilization in Nature
Mineralsarenaturallyoccurringinorganicsolidsofdefinite
chemical composition and possessing an ordered internal
Figure6. Classificationofsiliconcompoundsinthesoil.Figure
structure; rocks can be considered to be any solid mass of reproduced with permission from ref 133, copyright 2006,
mineral or mineral-like material and may therefore often Springer.
ModernViewsonDesilicification ChemicalReviews,2010,Vol.110,No.8 4663
Given these roles in living organisms, it is reasonable to activities that involve fungi, bacteria, and actinomycetes.
analyze here the peculiarities of silica biodegradation in Thus, plants and microbes, through their intricate interplay
Nature. withsoilminerals,contributeappreciablytotheglobalsilicon
cycle.142
3.1. Silica Bioleaching: Bacteria and Fungi Bacteria(andpossiblyarchaea)acceleratesilicadissolution
in the sea by colonizing and enzymatically degrading the
Microorganisms are intimately associated with global phe-
organic matrix of diatom frustules.143 The resolubilization
nomenasuchasbioweathering,soilformation,andthebiode-
and internal recycling of the silica in diatom frustules can
terioration of rocks and minerals. The biologically mediated
providethesolublesilicathatisessentialforfurthergrowth
destruction of silicate minerals to liberate limiting inorganic
ofdiatoms.144Empiricalevidencefromanumberofoceanic
nutrientsisafundamentalobservationofsubsurfacemicrobial
systemshasrevealedthatSiregenerationintheuppermixed
ecology.135,136Interactionsbetweenmicrobiotaandsilicatesmay
layer,viadissolutionofbiogenicsilica,isacriticalSisupply
beclassifiedintothefollowingreactions:137
mechanism to diatoms. These regions, where diatom silica
(1) Dissolution and destruction of the silicate crystal
is recycled more rapidly, possess a regenerating source of
lattice
therequiredSiinadditiontotheexchangewithnew,nutrient-
(2) Uptake and utilization of silicon
richsubsurfacewater.Ithasbeenestimatedthatanatomof
(3) Releaseofsiliconandsedimentationintheenvironment.
Si is cycled 39 times before burial to the seabed,145 which
Thecycleofbiologicaldegradation(Figure7)ofpolymeric
illustrates that silicon regeneration is robust and efficient.
silicates is completed by an abiotic polymerization and
Moreover, the extent of Si regeneration within the upper
separation.Therearenumerouswaysofinteractionbetween
water column is quite substantial and sustains significant
thebiosphereandsiliconinitsvariousformsandmaintaining
diatom production.143
thiscycleisofvitalimportanceforaplethoraoflowerand
higherorganisms.138Becauseofitsgreaterchemicalmobility Why do bacteria utilize silicon and silica? It was shown
than crystalline silica phases, biogenic opal plays a major that silicic acid increased numbers of both aerobic and
role in the cycling of silica in soils as well as in aquatic facultative anaerobic bacteria in ultrapure water incubated
environments.139 understrictoligotrophicconditions.146Soilextractsactedas
Plantscontributesignificantlytothebiogeochemicalcycle thebacterialinoculum.Theseauthorsspeculatedthatsilicic
of silicon.140 Alexandre et al.,141 reported that 92% of the acid, produced by the hydrolysis of silicates on the early
biogenic silica in the soil is recycled by plants and is the Earth,couldhavestimulatedthegrowthofthefirstbacterium,
mainsourceofthisnutrient.Theytakeupsiliconfromsoil thereby allowing it to become established in the then
water in the form of water-soluble silicic acid, which is prevailing conditions (presumed to be oligotrophic). It was
polymerizedandprecipitatedasamorphoussilica,frequently reported that addition of Si in culture media showed a
in close proximity to the transpiration conduit. After plant remarkable growth accelerating effect on Staphylococcus
death, silica returns to the soil, and plant decay generates aureus147 and in Nocardioform microorganisms.148 Silicon
humic acid, which increases weathering activity in soils. is also known to enhance the growth and pathogenicity of
Biocycling of silica in soil also occurs through microbial Mycobacterium tuberculosis.149
Figure7. Biodegradationofpolymersilica,andtherelease,uptake,andrepolymerizationofmonomersilicabyvariousorganisms,leading
toacyclingbetweenthepolymerandmonomerformsofsilica.
4664 ChemicalReviews,2010,Vol.110,No.8 Ehrlichetal.
Theactionofbacteriaonsilicate-basedmineralshasbeen treatment. The bacterial action was connected with the
characterized as nonenzymatic.150 Microorganisms capable formationofmucilaginouscapsulesconsistingofexopolysac-
of growing on the silicates in the absence of organic charides. Chemoorganoheterotrophic bacteria Bacillus cir-
substances may have developed autotrophically and have culansJordanandB.circulansCHBC-1wereusedinorder
been proposed to be categorized as “silicate microorgan- to eliminate silicates from oil shale organic matter.165 This
isms”.151 The existence of “silicate bacteria”, defined as processwasexpectedtosubstitutethedrasticHF-treatment
chemolitho-autotrophicbacteriathatgaintheirenergybreak- in the kerogen concentrate preparation procedure. The
ingSi-Obondsinsilicates,hasbeensuggestedbyVernad- desilicification efficiency of the zymogenous strain B.
sky.152 Brussoff153 has isolated a thermophilic Bacillus circulans CHBC-1 was about 70%. These experiments
siliceous, which abundantly deposited silica in its cells. B. indicated that siliceous bacteria may have potential as an
siliceousiscapableofbreakingdowntheinsolublepotassium alternative, biochemical agent for the isolation of native
aluminosilicates in the soil and may even be cultured in a kerogen from the silicified oil shales. Tests showed that
granitebowlorinamediumofpowderedglass.154Further- bacteriawerenotusingtheshaleorganicmatterasasource
more,aplatemethodforstudyingthebreakdownofsynthetic of carbon.166
andnaturalsilicatesbysoilbacteriawasestablished.155Also, Inmostofthepreviousstudiesconcernedwiththeeffect
silicatesthatcontainsulfidemineralscanbepresentduring of bacteria on silicate dissolution, the action of media
heapbioleachingformetalextraction.Weatheringofsilicates acidification and organic acid excretion due to bacterial
is catalyzed by heterotrophic bacteria and fungi via the metabolism was the main factor responsible for element
productionofacidicmetabolites,aswellasbytheactionof release from the solid via metal-O-Si bond breakage. In
acidophilic microorganisms.156,157 Acidophilic microorgan- addition, it was very difficult to separate the effect of live
isms preferentially attach to sulfide over silicate minerals, bacteriaandtheirexometabolitesfromthatofthechemical
andtheirattachmentisattributedtotheirabilitytogrowon components of nutrient solution. Thus, experiments on live
theFe2+releasedfromoxidationofthemineralsulfidebond. anddeadculturesofrhizosphericsoilbacteriaPseudomonas
However,thesemicroorganismsdonotcleavetheSi-O-Si aureofaciensinteractionwithcalciumsilicate(wollastonite,
bond. CaSiO ) revealed that the release rates of both Ca and Si
3
Heinen158 was able to show that silicon is actively were only weakly affected by the presence of live or dead
metabolized by certain bacteria. The same author reported bacterial cells in inert electrolyte solution and in nutrient
that incubation of Proteus mirabilis in media containing media: there is only ∼20% increase of dissolution rate in
silicate instead of phosphate, lead to the formation of experimentswithliveculturescomparedwithdeadcultures.
carbohydrate-silicate esters.159 It was shown that these Therefore,theeffectofextracellularorganicproductsonthe
bacteria possess an energy-dependent silicon uptake from weathering rate of Ca-bearing minerals is expected to be
quartzinitsnaturalenvironment.138OberliesandPohlmann160 weak,andtheaccelerationof“basic”silicaterockdissolution
alsoprovedthatbacteriathathadattachedtofeldsparorglass in natural settings in the presence of soil bacteria is most
producedetchingsinthesurfacestructuresthatcorresponded likely due to the pH decrease alone.167
insize,shape,anddepthtothedimensionsofthesebacterial In contrast to alumosilicates and other multiple oxides
cells. The mechanisms by which they act may involve requiringfordissolutionthebreakingofAl-O-SiorMg-,
productionofmetabolicproductssuchasorganicacidsthat Ca-,orFe-O-Sibonds,promotedbybothpHchangeand
can act as acidulants or ligands, base in the form of NH3, ligandattackonpositivelychargedmetalsites,thetSi-O-Sit
andspecificcapsularslimefrombacteria.Amongtheacids, bondsinquartzoramorphoussilicacanbepolarizedmostly
2-ketogluconicacidformedbysomebacteriahasbeenshown byprotons(tSiOH°ftSiOH+)orhydroxylions(tSiOH°
2
veryeffectiveinthedissolutionofsilicates.155,161Theyfurnish ftSiO-).Indeed,Heinenetal.168reportedonleachingof
protons that help in breaking Si-O bonds through proton- silicainathermalhotspring.Theformationofsiliceousgels
ationandcatalysis.Atthesametime,someoftheacidsmay facilitatedsettlingofmicroorganismsandwasaccompanied
also act as ligands that pull cations from the framework of by leaching of silica from the underlying rock, resulting in
thecrystallattice,facilitatingsubsequentbreakageofframe- destruction of rock material to a fine-grained dust beneath
workbondsormechanicalframeworkcollapse.Bacterialacid microbial mats. The mean silicon concentration was 34.6
polysaccharide-based slimes have been reported to form µg of Si/mL in the thermal spring water as opposed to a
complexeswithsilicatethatleadtosilicatesolubilization.162 mean value of 2.9 µg of Si/mL in the cold waters. Silica-
Recently,Brehmetal.163reportedontheroleofmicroorgan- leachingbacteriaarehighlytolerantwithrespecttodissolved
ismsandbiofilmsinthebreakdownanddissolutionofquartz. silica. For example, Bacillus licheniformis, an indigenous
Theydescribedmicroenvironmentalphotosynthesis-induced strain from high silica-containing magnesite ore was found
alkaline leaching of quartz by cyanobacteria-containing to tolerate 250 µg of Si/mL in the medium. During growth
biofilms. Due to their photosynthesis, microorganisms can the bacterium was capable of accumulating about 21 µg of
create a local shift in the pH from 3.4 to evidently higher Si/mLwithin8h.169TheapplicationofThiobacillusthiooxy-
than9(necessaryforquartzdissolution).Thequartzcovered dans,whichformedasaresultofmicrobialsulfuroxidation
withabiofilmispartiallyperforatedtoadepthofmorethan sulfuricacidandstronglyacidifiedleachingmedium,brought
4 mm. 131 mg of Si/L into solution after 45 days. In the absence
Differentstrainsof“silicate”bacteriawereusedtoleach ofthemicroorganismandwithadditionofsulfuricacid,65.7
silicon from low-grade bauxite ores in which the silica mg of Si/L were leached.137
connectedwithaluminaintheformofaluminosilicateswas Bacteria are also involved in the transformation between
themainimpurity.164Thestrainswereisolatedfromsoiland thedifferentformsofsilicon.Singhetal.170recentlyreported
rocksamplesandweretaxonomicallyrelatedtothespecies on the formation of silicon/silica nanocomposites using
Bacillus circulans and B. mucilaginosus. The leaching of Actinobacter sp. bacterium that was exposed to K SiF
2 6
siliconbythesebacteriagreatlydependedonthemethodof precursor under ambient conditions. The authors hypoth-
ModernViewsonDesilicification ChemicalReviews,2010,Vol.110,No.8 4665
Figure8. SEMimagesofunidentifiedfungi(arrows)thatdissolvezirconiumsilicatecollectedattheseabottom.
esizedthatthisbacteriumsecretesreductasesandoxidizing observed.Itseemsthatbioerosionofthisveryancientmineral
enzymes,whichleadtotheSi/SiO nanocompositesynthesis. is also of correspondingly ancient origin.
2
Thesynthesisofsilicananoparticlesbybacteriademonstrates
Therefore,microorganismshavebeenshowntoaccelerate
the versatility of the organism.
the dissolution of silicate-based minerals by the production
Endolithic and epilithic microbial communities pruduce of excess proton and organic ligands and in some cases by
polyols as osmotic protectants in response to desiccation. theproductionofhydroxylanion,extracellularpolysaccha-
Low molecular weight polyols and polysaccharides bind to rides, enzymes, and proteins135,142,174 or, in the special case
the siloxane layers within layered siliceous minerals, such of nitrogen-rich systems, from the creation of alkaline
as micas and soapstone, by hydrogen bonding.171 These environments.175
interlaminar complexes cause expansion of the crystalline
layer, weakening the structure, and may allow entry of 3.2. Silica Bioleaching: Diatoms
chelatingagentsthatmobilizetheionsstabilizingthecrystal
structure.Duringperiodsofdesiccation,thepolyolsbecome The marine biogeochemical cycle of silicon largely
concentrated, forming nonaqueous systems. Basic catalysis depends upon the balance between biological precipitation
in such water-deficient ecosystems favors the formation of ofamorphoussilicabysiliceousorganisms(mainlydiatoms)
water-solubleorganosiliconcompounds,principallyorganic andpostmortemdissolutionoftheirskeletons.176Priortothe
siloxanes.Polyolsandcomplexorganicacidscanalsoattack diatom appearance, biogenic silica burial was probably
siliceous minerals under alkaline conditions. Extracellular equitablydividedbetweenradiolariansandsiliceoussponges
carbohydrate polymers released by fungi and bacteria can asmoreancientorganisms.177Dissolutionbeginsinthewater
react with organic siloxanes to form water-soluble organic columnwhenthesilicaskeletonisexposedtoundersaturated
siloxanes.171 seawater and can continue after deposition at the seafloor.
Recently, Penna et al.178 reported in Vitro investigations
Biocorrosion of historical and optical glasses is often
of the influence of particulate silica sources on growth and
linkedtothegrowthoffungi.172Fungiprovokedremarkable
siliconuptakeofdifferentdiatomspecies.Eachdiatomstrain
etching rates due to their ability to synthesize ligands
wasincubatedincontrolledconditionswithmineral(quartz
(siderophores)specificforferricandothermetalions.Initial
sand and two pure quartz dusts with variable degree of
symptomsofglassbiodeteriorationcompriseofetching,pit
hydrophilicity/hydrophobicity)andbiogenic(diatomaceous
corrosion, and leaching.173
earth and sponge spicules) silica substrates. It was shown
Recently, Bansal et al.174 have shown a Fusarium ox-
that investigated species grew much better with crystalline
ysporumfungus-basedbioleachingapproachforthesynthesis
than amorphous substrates, free from their solubility in
of hollow silica nanospheres from sand. The formation of
artificial seawater. Furthermore, diatom biomass increased
thesilicananoparticlesisbelievedtoproceedbyatwo-step
when the pure hydrophilic quartz was converted into the
process. The first step involved leaching of silica from the
hydrophobic one; this may be due to an enhancement of
sandgrainsintheformofsilicicacidbyproteinspresentin
diatomadhesionontothesilicaparticles,probablymediated
fungalbiomass,whileinthesecondstepthesilicatecomplex
by protein-surface adsorption. The authors suggested a
ishydrolyzedbyotherspecificproteinsinthefungustosilica.
hypothetical surface driven uptake mechanism as follows.
The silica is in the form of nanoparticles capped by
Marinediatomsmightbeabletomakebioavailabledissolved
stabilizing proteins from 2 to 5 nm in size and is released
silicon from particulate silica by excretion of some com-
intosolutionbythefungus.Anapproachwasalsoreported142
pounds,whichfacilitatesiliconreleaseintheformofsilicic
thatinvolvestheuseofthesameplantpathogenicfungusin
acid, either by cell-particle contact for benthic diatoms or
thebiotransformationofnaturallyoccurringamorphousplant
by diffusion into the medium for pelagic species.
biosilicaintoquasi-sphericalcrystallinesilicananoparticles
anditsextracellularleachingintheaqueousenvironmentat
3.3. Silica Bioleaching: Sponges (Porifera)
roomtemperature.Thesilicabiotransformationisfairlyrapid
and occurs within 1 day of reaction of the fungal biomass According to Maldonado and co-workers,102 our current
with the rice husk. understanding of the silicon cycle in the ocean is based on
Bioerosion of very hard silica-based minerals also takes the presumption that diatoms dominate not only the uptake
place in natural environments. Recently, bioleaching trails of silicic acid but also the production and recycling of
made by fungi that are responsible for dissolution of biogenic silica and also that other organisms with siliceous
zirconiumsilicateinmarineenvironment(Figure8A,B)were skeletons, including sponges, radiolarians, and silicoflagel-