Download a review of the microbial geochemistry of banded iron

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The CanadtonMincralo gist
Vol. 33, pp. 132l-1333 (1995)
A REVIEWOF
THEMICROBIALGEOCHEMISTRY
OF BANDEDIRON.FORMATIONS
D. ANNBROWN
Departnewof Geolnglcal
ManitobaR3T2N2
Scienres,
Univenityof Manitoba,Vtliw1ipsg,
GORDONA. GROSS
Geological
Suneyof Canada,
601BoothSteet,Ottawo,OwarioKIA 0E8
JERZYA. SAWICKI
AECI" Chalk Riverlaboratories, ChalkRiver, Ontario KOJ1J0
Ansrnesr
The role of micrmrganirms in the depositionof bandediron-formations(BF) has been discussedfor many years,as it
has beendifficult to explain the accumulationof iron and silica in thesedepositsby chemicalprocessesalone.In this paper,
we survey fte significanceof the presenceof microorganismsin microfossils and stromatolitesassociatedwittt BIF. The
requirementsfor the growth of bacteriaare discussed.Microbially mediatedgeochemicalFocessesinvolved in the deposition
of iron and sitca in thesesedimentsare considered.The influencethat microbial mats,hot springs,and deepseahydrothermal
ventsmight haveon currentideasin geochemistryalsoareexamined.A recentlydiscoveredbiofrln *as able to sorb silica and
presipitateircn both ashematiteandsideritein closeproximity; we suggestthat the microtrialreactionsoccurringin this biofikn
could be similar to thosethat may haveplayed a part in the depositionof BIF. Biofilms createmicroenvirotrmentsthat allow
the precipitation of minerals with distinctly different fields of stability, Since biomineralizationcan occur both by physico
chemicalsorptionandby metabolicallyinducedreactions,suchreactionscould haveplayedan importantrole in the deposition
of iron and silica in BIF and in metalliferoussedimentsfrom the early Archeanto the presentday.
Keryords: bandediron-formations,biofilm, biomineralization,microbial reactions,microfossils,stromatolites.
Somaanr
Ir role desmicro-organismesdansla ddpositiondesformationsde fer rubann6esfait lbbjet de discussionsdepuisplusieurs
ann6es,parce qu'il est difrcile d'expliquerI'accumulationde fer et de silice dans ces gisementsuniquementpar processus
chimiques.Dans ce travail, nous dvaluonsI'importancede la prdsencede microorganismes dans les microfossileset les
stromalolitssassosi6saux formationsde fer. Nous passonsen revtreles exigeancesdesbact6riesen croissance,ainsi que leur
rdle dans les pocessus g6chimiques menante h ddpositiondu fer et de la silice dans les structuresgdologiques.Nous
examinonsaussilinfluence qu'auraientpu avoir les coloniesmicrobiennes,les sourcesthermales,et les 6ventshydrothermaux
sous-nrarins.Une biopellicule d€couverter&emment qui avait labilitd dabsorberla silice et de precipiter le fer sousforme
dhdmatiteet de sid6rited"ns le mOmemilieu, nousfait trFnserque desr6actionsmicrobiennessemblablesauraientpu jouer un
qui p€rmettent
r6le important dansla ddpositiondesformationstle fer. De tels biopelliculescr€entdesmigro-environnements
la pr6cipitation de mindraux ayant des champs de stabilitd nettement diffdrents. Comme il est possible d'assurcrune
biomin€ralisationpar sorption physicochimiqueet par r6actionsiniti&s par mdtabolismed'organismes,de telles rdactions
pourraientavoirjoud un r0le importantdansla d6positiondu fer et de la silice danr les formationsde fer et dansles s6diments
m6tallifdrasdepuisI'arch6enpr6cocejusqu'i nosjours.
(Iraduit par la Rddaction)
Mots-cl6s:formationsde fer nrbann€es,biopellicule biomin#lisation, rdactionsmicrobiennes,microfossiles,stromatolites.
t322
InrnoouctroN
Speculationon the various processesthat produced
the enormousconcentrationsof iron, silica and other
metalsin bandediron-formations(BIF) has stimulated
researchfor many years, for it has been difficult to
explainhow suchvast amountsof iron and silica could
have been concentratedin sedimenlsonotably during
the Archean.Although there were various reports of
the detectionof stromatolitesand microfossilsbv the
tum of the century, it was not until Tyler's and
Barghoorn'sdiscovery n 1954 of coccoid and filamentousmicrofossilsin the early hoterozoic Gunflint
chert(-2.0 Ga old) that reportsof microfossilsin ironformations have proliferated in the literature. The
potential role of microorganismsin the genesis of
metalliferous sediments,puticularly in bandedironformations, now provides an incentive for further
research.
Bacterial activity has been demonstated (Mann
L992) n the deposition of metals as well as in the
weatheringand leachingofrocks. Ferris er al. (L987)
andBeveridge(1989)alsohaveshownthat bacteriaare
ableto accumulatemetalsfrom solutionby biosorption
on a cell surface,as well as by active oxidative or
reductive metabolism. A microbial biofilm (Brown
et al. L994, Sawicki et al. 1995)recently discovered
in an Underground ResearchLaboratory ((JRL) in
ManitobaCBrownet al. 1989)hasshownthat iron from
biotite in the granitic host-rock can be mobilized by
microbial action. The iron is used as a source of
energyby a microbial consortiumandthenprecipitated
both in the oxidizedandreducedforms ashematiteand
siderite, respectively. This biofilm provides new
evidence for the role of biogenic processesin the
deposition of metalliferous sediments,and since it
preferentially accumulatessilica as well as iron, we
suggestthat this type of microbial mediation could
have beeninvolved in the depositionof bandedironformations.
In this review, we examinethe history of researchon
BIF, the requirementsof bacterialgrowth so the reader
may be able to comprehendtle various microbially
mediated mineral deposits occurring today, and the
implications of these microbial reactions and their
relationshipto the conceptsand models proposedfor
the formation of both recentand ancientmetalliferous
sediments.
llsrony or MrcnosrAl Grocrcnrrsrny
Assocr^rm wml BIF
The suggestion that biological processesare
importantin the depositionof iron-rich sedimentswas
initially put forward by Ehrenberg (1836), but it
was not until 1888that Winogradskywas ableto show
that a bacterium,Izptothrix, is able to live and grow
only where ferrous iron is presentin solution. These
organismsglow a sheathsurroundingthe cells that is
stainedbrown by accumulationof iron oxide. Since
this coloration occurs only where living colls are
present,Winogradsky(1888) proposedthat they must
actively produce the ferric iron that becomes
enmeshedin the sheath.He concluded,therefore,that
oxidation of ferrous compoundsto fenic hydroxide
is necessaryfor the growth of the organism. Since
this processprovides energyfor cell growth, organic
matter would not be required for energy production.
However,Lieske(1911)proposedinsteadthat carbon
derived from ferrous bicarbonatewould provide the
necessaryenergy,and despiteWinogradsky'sfindings,
the consensusuntil recently has been that organic
matteris indeednecessaryfor cell growth.
Since weathering of ordinary rocks should have
continued on the same scale throughout the
Precambrian and Phanerozoic.Van Hise & kith
( 19I 1) questionedwhetherthis couldbe the solesource
of the iron and silica in BIF. Harder & Chamberlain
(1915) suggestedthat, although the precipitation of
ferric hydroxide from solution during the deposition
of the Itabira (Brazil) iron-formation could have been
purely chemical, it was more likely to have resulted
from the operation of 'owell-known iron bacteria"
(their term). Harder(1919)reviewedthe early ideason
the relationship between iron-depositing bacteria
and the geological environment.Gruner (1922) consideredit probablettrat both algae and hon bacteria
contributed to the precipitation of organic colloids
during the depositionof the Biwabik (Minnesota)ironformation.
LaBergeet al. (1987)proposedthat the hecambrian
iron minerals originated mainly as fine-grained,
biologically precipitated ferric hydrates that later
maturedto ferrihydrite and hematite.This ferric iron
was then reducedby concomitantmicrobial degradation of organic matter to siderite, which later altered
to magnetite. They suggestedfirther that in the
sedimentary environment, ferric and ferrous iron
minerals would have coexisted in metastable
equilibrium.
Current reviews of energymetabolismby Nealson
& Myers (1990) and Lovley (1991) show that
metabolic oxidation and reduction of inorganic ions,
notably ofiron, by bacleriaaxenot only important but
have a widespreadinfluence also upon the natural
environment.
If the sourceof ferrous iron had been the siagrant
oceansbeneathan anoxic atmosphere,as proposedby
Cloud & Licari (1968), this iron could have been
convertedinto insolubleferric oxidesonly when there
wasa changein the Precambrianahosphere dueto the
evolution of oxidative photosynthesis(Cloud 1973,
1983).Major BIF would thereforebe found only within
a shortinterval during the Early Proterozoic,beforethe
atmospherebecamefully oxygenated.
The highly metamorphosedIsua iron-formation in
MICROBESAND BANDED IRON-FORMATIONS
Greenlandis probablythe oldestknown BIF (3.8 Ga).
It contains rounded carbonaceous,iron-coated
microstructures(mainly oxides) that are more typical
of organisms than of inorganic recrystallization
@obbins 1987).The oldest definite microbial fossils
yet describedare from the WarawoonaGroup (3.3 to
3.5 Ga) in WesternAustalia (Schopf& Packer1987),
where the cells of filamentous aud colonial microfossils me preservedby the formation of stromatolites
embeddedin carbonaceouscherts. Mcrocrystalline
siderite and hematite in the Transvaal BIF (25 to
2.1 Ga) are considered to reflect the chemical
environmentin the depositionalbasins,sincethe other
mineralsappearto be eitherdiageneticor metamorphic
in origin @eukes1983).James(1954)pointedout that
the composition and sffucture of the precursor sedimentaryfaciesmust havebeenconfrolledby environmental factorsin the depositionalbasins,and that this
would be reflected in the mineralogy and textural
featuresof the lithofaciesderivedfrom them.
Although major depositionof BIF was suggestedto
have ceasedwhen eukaryoticphotosynthesisevolved
sufftcientlyto producea stableoxygenatedatmosphere
about 2.0 Ga ago (Cloud 1973),there are today many
environments,smaller in extent,whereiron and silica
arebeing depositedunderboth oxidizing and reducing
conditions (Gross & Mcleod 1987, Rona & Scott
1993).Initially, the iron andsilica in BIF werethought
to be derived exclusively from sedimentarysources,
but now hydrothermalplumes are acknowledgedto
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havebeenthe main source(Gross1991,Isley 1995).
The discovery in the late 1960s of hydrothermal
ventsalong spreadingridgeson the seafloor(Jannasch
& Wirsen 1981) and the investigationof surfacehot
springs at Yellowstone @rock & Madigan 1991)
provide partial analoguesfor the study of iron-rich
metalliferoussediments.Modemhydrolithic sediments
rich in iron, manganese,silica and sulfide minerals
sunounding sites of hydrothermaldischargeprovide
corroborationof the nature of the precursorsand the
lithofaciesthat could havedevelopedftom them.
In this review, we suggestthat microbial processes
could have mediatedthe precipitation of iron, either
oxidized or reduced,in narrowbandsinterlayeredwith
beds of silica Microfossils and stromatolitesassociated with banded iron-formations indicate that
prokaryotic organismswere presentby the t'me these
sedimentswere being formed, althoughtheir specific
role in the depositionof BIF hasyet to be determined.
Rrqunmamrrs oF BAcIRTALGnowru
Since microbial activity has such an impact on
geologicalprocesses,
it is importantto understandwhat
these reactions might be. Microorganismshave the
abilify to selectivelygathernutrientsinto the cell and
to modily them so that they may serve either as cell
building blocks or as an energysourcefor growth, the
ofwhich has a considerableeffect on the
consequence
pH and Eh of the naturalenvironment.
Frc. 1. Typical microfossil texture in magnetite- hematite - chert lithofacies in the
Gunflint ircn-formation,ThunderBay, Ontario.
t324
TTIE CANADIAN MINBRAIOGIST
FIc. 2. Stromatolites,20 to 30 cm in diameter,from the Gunflint iron-formation,Thunder
Bay, Ontario.
It is difflcult to demonstate how microorganisms
may have participatedin the deposition of iron-rich
sediments,since the only evidencewe now have is
their fossilized lemains, and it is not possible to
determine from the cell morphology alone what
misrobial reactionsmight have taken place. Evidense
of microfossilsand stromatolitesin eady Precambrian
BIF is relatively rare, and even where they were
originally present,in many casesthey would havebeen
desffoyed by later metamorphism.Figure 1 shows
microfossils from the Gunflint iron-formation in
Ontario,andFigure 2, a stromatolilefrom the Biwabik
iron-formationin Minnesota.
Microbes preferentially utilize the lightest isotope,
so that an assessment
of isotopedepletioncan provide
further information on possiblemicrobial activity. fn
somesediments,chemicalremnantssuchascarbonates
and kerogen contain anomalously Light carbon,
indicating that they were most likely formed tbrough
biological activity (Winter & Knauth 1992).
Microorganisms are usually visible only under
the microscope. They are mainly unicellular and
prokaryotic (without a separalenucleus), commonly
consistingof a singlecell enclosedby a membranethat
separates the metabolically active unit from the
extemal medium. Eukaryotes, on the other hand,
enclose their genetic material within a membranebound nucleus; some are unicellular, but most are
multicellular organisiui. This group include the atgae
and plants,as well as all of the alrimaltr1og6o-.
Bacteri4 howeveq are the most important organisms that participatein chemicalinorganicreactionsin
the natural environment;althoughthe basic metabolic
pathwaysfor cellular growth and internal transfer of
energyare similar in all organig6s,only bacteriashow
such extraordinarydiversity in their ability to obtain
energyfrom externalsources.Bacterialspeciesarealso
versatilein their capacityto quickly produceenzymes
(biological catalysts)to utilize any nutrient tiat may
become available, 16os saaUing many extreme
ecologicalnichesto be rapidly filled (bgaham et al.
1983). The availability of carbon substrates,energy
sourcesandreducingpowsr in the naturalenvironment
has furfluencedthe evolution of the energy reactions
that bacteria are able to accomplish. Mauy geochemicalprocesses
in naturalsyslemstakeplasewithin
a wide rangeof pH, redox, and salinity, and theseare
often detenninedfrom the result of reactionsof tle
indigenous bacteria at water-rock interfaces @erris
et al. 1987).
Nutrients
All life on Earttr is based on carbon. and this
constitutes half of the bacterial cell mass. Other
important elementsare oxygen, nitrogen, hydrogen,
phosphorusand sulfir, together witl trace amounts
of other elements, including iron. In the natural
environmen! these elements are generally leached
from ttre bedrock either by weatheringprocessesor
microbial mediation. The requirementsfor cellular
growtl are, tlerefore, the availability of these
ngcessaryelements,a sourceofenergy, and a suitable
habitat @rown 1994).
In marine and terrestrial groundwatersowhere
nutrientsare limited bacteriausually live in consortia
within a biofilm or maq wherethe member$may also
be symbiotic. The survival stategy of these asso-
MCROBES AND BANDED IRON-FORMATIONS
ciations is to adhereto a surfacein a film of organic
material.Nufrientstendto concentrateat rock - biofitn
- groundwater interfaces, where fluid movement
continuallybrings new suppliesofnutrients to provide
a richer food-source than that of the bulk fluid
environment (Costerton et al. 1980. Sessile
(sedentary)bacteria on these surfacesthus have an
ecological advantageoyer those that are plar:ktonic
(free-swimmingin the bulk fluid).
In the natural environment today, biofilns are
initiated by planktonic organismstlat colonize rock
surfaces.Bacterial successionthen leads to a mature
community,similar to that found in other ecosysterns.
Exopolymeric substances,generally polysaccharides,
are excretedby the bacteria, which accumulateand
form a chargedlayer of slime surroundingthe consortia
(Characklis & Marshall 1990). Utimately, owing to
transfer limitations, the inner layer of a biofilm may
become anoxic, stimulating the developmentof an
anaerobicpopulation (Costertonet al.1994). Finally,
ciliale prptozoawithin the biofilm may graze on the
bacterialpopulationandthushelp to maintaina climax
community(Locket al. 1984).
Energy
The major sourceof energyfor organismsliving on
earthtoday is the sun.Solarenergyis utilized by plant
photosynthesisto produceoxygenwhilst fixing carbon
into organiccompounds(thathavereducingpotential).
This carbon inco'rporatedas plant biomass is then
utilized by animalsfor tleir growth.
However, in the Archean,before oridative photosynthesishad evolved, and even today, in environments where neither sunlight nor biomass
from solar energy can penetrate,primitive organisms
(bacteria)must find other sourcesof energy.Sincethe
maintenance of complex biological structures of
the living cell is thermodynamicallyunfavorable,to
sustainthemselvesand grow, organismsmust couple
energy-producing oxidation-reduction reacfions to
their endergonic biosyntletic ones. These redox
reactionsare able to tansfer elecfronsfrom a donor
(reducing potential), which becomesoxidized, to an
acceptor,which is thenreduced.
Oxygen is the most common elecfron donor to be
utilized by living orggnis6s, but in environments
where this is not available, such as the eady anoxic
Earth, other donors are required. The sequenceof
elecfron donorsinvolved in nature are, in descending
order of thermodynamicallyavailableenergy:oxygen,
nitrate and nifrite, N[ne, Fe], sulfate and carbon
dioxide. The reduction of thesecompounds(electon
acceptors) is generally coupled to the oxidation
of organic matter (electon donor). This sequenceof
bacteriallymediatedredox reactionsis the sameasthat
found in the waler columns of lakes and sediments
(Sfumm& Morgan1981).
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Environment
Bacteria play a major role in the control of the
natural aqueousmilieu by their influence on the biological reactions of photosynthesis,respiration, and
oxido-reductivechangesin the iron, sulfur and carbon
systemsolargely determiningthe pH and Eh limits of
the natural environment@aasBecking et al. 196O).
Terrestial life requireswater in the liquid phase,thus
the temperaturelimits for growth may range from
below -5"C in Arctic seas to above 100oC (and
possiblymuch higher) in deep-seathermalvents.
When living organismsfirst arose approximately
4.0 Ga ago on this planet, tle atmosphere was
reducing, so the early organisms evolved in the
absence of oxygen. Early anaerobes would also
have had to develop mechanismsto prevent being
poisoned by oxygen radicals, the successful ones
lscaming aerobes,and thosethat could not copewith
oxygenpoisoningremaininganaerobic;as well, some
bacteria (facultative) are able to live in either
environment.
Although filamentous bacteria -3.5 Ga old have
been found that have fossil morphology similar to
cyanobacleria@lue-greenprokaryotic algae) (Schopf
& Packer 1987),it is almost certainthat stromatolites
older than -25 Ga were composedof anoxygenic
photohophs (purple and green bacteria) rather than
oxygen-evolving cyanobacteria@rock & Madigan
1991). Recently, it has been reported @brenreich&
Widdel 1994) that purple bacteriaare able to oxidize
Fe2*to Fe* when this is coupledto the reduction of
organic matter. Biological oxidation of iron would
therefore have been possible before the evolution
of oxidative photosynthesis,so that the formation of
hecambrian stomatoliles would have been possible
without the participation of oxygen-producing
cyanobacteria(Castenholz1994).
Eventually, as life evolved and became more
complex,efficient oxidative photosynthesisdeveloped
in eukaryoticalgaeandgreenplants,but beforeoxygen
could have begun to accumulatein the atmosphere,
the surface waters would first have had to become
safurated. However, once the atmosphere did
become oxic, eukaryotic and multicellular animals
would have been able to proliferate (Woese 1994).
Theseeukaryotesmay have grazedon the prokaryote
communities,possiblyendingthe ageof stomatolites,
since microbial mats cannot persist where tlere are
efficient grazers(Castenholz1994).
MtcnosIALMI.IRAL DEposrnoNToDAy
There are various geological features being
fashionedtoday by microbial mediation that can be
relatedto ancientfossilizedforms. Descriptionsofthe
biological involvement in several of these stuctures
are describedbelow.
7326
THE CANADIAN
MINERALOGIST
both iron and silica are being deposited.Moreover,
modern stromatolites are usually found only in
Microbial matscontainlayersof different microbial carbonateenvironmenls"but the siliceousstomatolites
communities;today, they generally consist of cyano- beingforrnedherearecomparableto thosefound in the
bacteriain the uppermostlayer, various phototrophic Gunflint cherts. This similafiry supportsthe concept
bacteriain deeperlayers until light becomeslimiting, that hot springs were a primary source of silica in
and then degradingbacteria such as sulfate-reducers Precambrianiron-formations,and alsothat many early
occupying the lowest and darkest layer @rock & fossil sfromatolitesmust havebeenbacterial,not algal
Madigan 1991).Theseare dynamic systemso
where in origin, sincealgaeonly evolvedlater ($Ialtet et al.
photosynthesisthat occursin the surfacelayer of the L9s4).
mat is balancedby decompositionat its base.Microbial
mats are found in a variety of environments,notably Shallowseas
thoseofvolcanic hot springsandshallowmarinebasins.
Algal mats up to 30 cm thick have beendescribed
Contemporaryhydrothermal sedimentsare being
from Baja California (Horodyski 1977, Horodyski formedat severalplaceswithin the Santorinicaldera,in
et aL L977).A variety ofmicroorganismscontributeto the eastern Mediterranean (Bostr0m & Widenfalk
their formation, but their morphologyis conffolled by 1984). The sediments are largely authigenic iron
filament abundance,flooding history, water depthand hydroxide,carbonate,and sulfide mineralsmixed with
mostimportantly,by the degreeof desiccation.Several rock fragmentsand some clay. The topmost surface
types of filamentous organisms contribute to the of the sediments,howeyer,consistsalrnostentirely of
primary organic deposit, which is subsequently biogenic iron oxide-hydroxide formed on the stalls
degradedand modified by further bacterial activity. of the bacteriumGalionellafemrginea (Holm 1987).
If heavy rains flood these mat communities, the Owing to the proximity of the surface, there is
depositionof larninatedsedimentceases,and instead, extensivedownwardadvectionof oxygen,which reacts
up to 10 cm of siliceous sedimentmay be deposited with the vent hydrogensulfide to form sulfuric acid.
over the surfaceof the mat (Stolz & Margulis 1984). This acid producesa local environmentquite different
This new layer of sedimentis then recolonizedby a to that of the deepse4 andhencethe sedimentsalsoare
different suite of organisms.Meanwhile,the siliceous dissimilar.
sedimentsare reworked inlo an anaerobicmud that
containsmetals,sulfur, and well-preservedsheathsof Deep-seafloorvents
filamentou$and coccoidbacteria.An active microbial
communitycanthusproducea largeamountof organic
Metalliferous sedimentsaroundhydrothermalvents
material which, althoughpreservedas a thin layer in in deepbasinsin the Red Seacontainfaciesof soft red
the underlying mud, provides little other evidenceof to grey-brownbandedsedimentsthat are composedof
the organisms that produced it Stolz & Margulis iron and base-metaloxides, hydroxides, sulfides,
(1984)suggestedthat theserecentlaminatedsediments silicates and clay minerals @l Shazly 1990). These
of Baja California bear a striking similarity to partially consolidatedsedimentsare generallymuddy,
the laminalsd sedimentsand associatedcherts of the with micrometer-sizeparticles.The organismsfound
ArcheanSwazilandSystem.
associatedwith thesesedimentsare mainly planktonic
foraminifer4 with only a few sulfate-reducingbacteria
Hot sprtngs
at the seawater interface, altlough isotopic data
indicatethat pyrite wasbiologically formed.
Sftomatolitesand algal mats are laminatedorgaooSedimentsdepositedaround deep seafloorthermal
sedimentary structures that form through repeated ventsappearto provide a goodanalogyfor the studyof
cycles of sediment accretionoin general reflecting BIF (Jannasch& Wirsen 1981, Jannasch& Taylor
periodic intemrptions in their deposition. They are 1984).The submarinefood-chain.hereis basedon
generally built at present by eukaryotic algae, but hydrogen sulfide, which chemosynthetic bacteria
before eukaryotes evolved, prokaryotic organisms utilize to provide the primary sourceof food for the
had formed similar organiccomplexes.Today,stroma- invertebratepopulation.The major sites of microbial
tolites are being formed at Yellowstone both by activity are suspensionsof bacterial cells (Haymon
eukaryotic algae and by prokaryotic bacteria. et al. 1993), biofilms gpwing on various surfaces
hokaryotes preferentiallygrow where eukaryotesare (Tunnicliffe & Fontaine 1987) and filamentous
unableto survive,usually when the conditionsare too microorganisms(Juniper& Fouquet 1988),as well as
hot. Phylogenetic characterizationof the hot-spring symbiotic associations between chemoautotrophic
sedimentsby moleculargeneticshasrecentlyrevealed prokaryotesand vent invertebrates.Most surfacesare
a remarkablediversity of prokaryotic species@arnes coveredwith bacterial biofikns that contain iron-rich
et ql. L994). Although Yellowstone waters generally encrustations.The biological productivity mound the
haveonly a low concentrationof iron, in somegeysers vents is
high compared to that of the
Microbial mats
MICROBESAND BANDED IRON-FORMATIONS
L327
was irrigated by surface water used for mining
activities (Brown et al. 1994).The water containeda
range of microorganisms, and it had also been
contaminatedwith blastingmaterialscontainingcarbon
and nitogen, both essentialfor microbial growth. The
biofilm grew over a period of several months to a
thickness in places of 10 mm. Initially, it appeared
grayrshwith light-brown sfreals, but orangemarkings
developedlater. Runnels,formed within the biofilm by
flowing water, were stainedblack internally. Lateq a
white precipitateformed over theserunnels.
Deposits of iron and silica (delectedby electron
microscopy, X-ray diffraction and energy-dispersion
spectroscopy)wererapidly forrnedon the bacterialcell
walls and exfrapolymericslime @g. 4), althoughthe
concentrationsof iron andsilica in the water werelow.
Mdssbauerspectroscopy(Sawicki et al. 1995)showed
that smallparticlesof protoferrihydritewereformedon
the outer aerobic face of the biofilm" which later
showedalterationto hematite.By contrast,at the inner
anaerobicface of the biofilm next to the rock wall,
ferrous iron was formed, which was then either
Brornnarnou alr
Uxpm.cnouxr RTSnARCH
Leronaronv (JRL)
sorbed direclly onto the biofilm i*elf, or reacted
with carbonate from microbial fermentation to be
The stimulusfor this paperwas our discoveryof a precipitated as siderite. The white precipitate was
biofilm that was able to precipitateboth hematiteand found to be gypsum. Both siderite and gypsum are
sideriteaswell asto adsorbsilica. This biofilm formed rarely found in this pluton.
in the URL excavatedin the Lac du Bonnet granitic
pluton of the CanadianShield (Brown et al. 1994,
Mcnonru GSocHHvILSTRv
Sawicki et al. L995).The excavationis naturally dry,
The microbial reactionsthat cancausefhe formation
with only two water-bearingfractures,but the biofilm
(Frg. 3) was able to form on the surfaceof a lmge of differentmineralsin theseenvironmentswill now be
boreholein the floor of a drift (4'2frm deep),whereit considered.Mobility of mineral ions in the geological
surounding water, and dense swarms of shrimp
feeding near the points of emissionindicate theseare
optimal sites for the chemosyntheticgrowth of their
filamentous misrobial epiflora (Wirsen et al. 1993).
Tunnicliffe & Fontaine(1987)consideredthat although
initial precipitationof the iron is bacteriallymediated,
further deposition of metals is determined by the
geochemicalconditionsof the ambientenvironment.
Extensivedepositsof polymetallic sulfide andoxide
sedimen8aroundvent sitesare formed by the transfer
of geothermalenergyto microbial biomassby bacterial
lithoautotrophy (growth on inorganic compounds).
This fansfer ofenergy contributessignificantly to the
large-scalegeochemicalprocessesof the oceanfloor,
so that present-dayhydrothermalenvironmentscould
be comparableto the processesthat occurredin the
hecambrian seas.Baross & Hofinan (1985) further
suggestedthat complexmicrobial ocean-floorcommunities in the Archean could have evolved relativelv
rapidly aroundsuchactivehydrothermalvents.
Ftc. 3. Biofilm formed in a boreholein the UndergroundResearchlaboratory, resulting
from the precipitationof ferrihydrite, siderite,and gypsumin the slime layer.
L328
TTIE CANADIAN
MINERAI'GIST
Ftc. 4. Thin section of the biofilm showing in sira biomineralization(arrows) around
bacterialcells.
environmentis influenced by the baclerial processes ferrous ions to form an initial precipitation of
of sorption, accumulation, and precipitation. These amorphousferrous sulfide, altering later to pyrite.
reactionsinclude dissimulatorymetabolismof various When these iron sulfide minerals are exposedat the
compoundsas energy sources,active assimilation of surface,either by erosionor by mining, they can form
metal ions into the cell, notably as metalloenz;rmeso the substratefor chemolithic sulfur-oxidizing microandpassivephysicochemicalaccumulation.
organisms, notably Thiobacillus ferrooxidans
The greatesteffect of microbial activity upon the (Jorgensen1983). These bacteria are able to derive
environmentis tbrough dissimilatory metabolism(not suffrcient energy for gfowth from the oxidation of
incorporatedinto ttre cell biomass),notably of iron pyrite. The optimal pH for this reactionis 2.0, whereas
and manganeseas a sourceofbacterial energy.Since the internal pH of the cell is close to neutrality. This
these metals do not have to enter the cell and can difference in pH provides a sufficient proton-motive
be relatively ineffrcient, va$t amountsmay have to be force acrossthe cell membranethat iron and sulfur can
altered to provide sufflcient energy for cell growth. be oxidized by the removal of electons. To maintain
hon is the fourtl mostabundantelementon Earth.such electroneutality,thereis a concomitantinward flow of
that it is readily available to provide an almost protonsthat are transforrnedinlo cell energywith the
unlimiled inorganic sonrce of energy, albeit not a consumptionof oxygen (Cox & Brand 1984). In a
particulady efficient one. Bacterial reactionsutilizing similar manner, manganesemay be oxidized by
iron canhavea considerableeffect on the environmenl bacterial mediation and form manganesenodulesand
by allering the pH and Eh, producing micronichesof concretions@hrlich 1963).
different fields of stability that allow the deposition
of various iron compoundsdependentupon the redox Reductionof ferric iron
state.
The dissimulatoryreductionof ferric to fenous iron,
Oxidation of ferroas iron
coupled to the oxidation of organic matter, is widespread in the natural environment (Lovley 1991).
Pyrite is formed in sedimentary rocks by the Although an early study concluded that microbial
bacterial reduction of sulfate (generally from marine reductionof ferric iron was non-enzymatic(Starkey&
waters)to hydrogensulfide; this reactschemicallywith Halvorson Dn), it has since proved impossible to
MICROBESAND BANDM IRON-FORMATIONS
demonstratethat ferric iron canindeedbe reducednonenzrymatically.In fact, it has beenconsistentlyshown
that ferric iron in microbial culturesis only reducedas
a result of direct enzymaticaction (Lovley 1991).
The organisms responsible for the enzymatic
reduction of ferric iron have only recently been
isolated,althoughthey havebeenknown for morethan
a century(Winogradsky1888).Geochemicalevidence
indicates il1a1fissimilafqry respiratory reduction of
ferric iron may have evolvedprior to other respiratory
processes,such as the reduction of sulfate or nitrate
(Walker 1984). Since ferric iron reducersare able to
outcompeteboth sulfate-reducersand methanogens
(reducers of carbon dioxide) as electron acceptors
(Stumm& Morgan 1981),this reactionis the one that
will preferentiallyoccur in nature.Thesebacteriahave
beenidentified in sediments,suggestingthat they could
have also been responsiblefor late post-depositional
reductionof ferric iron (Lovley et al. 1990).The iron
reactionszuerunongthe most important geochemical
processesin anaerobic sedimentary environmentso
since they can releasethe highly insoluble ferric iron
from the rock massinto the groundwaterby reducingit
to the more solubleferrousstate.
Until recently, all black iron precipitatesin natural
environments were consideredto be iron sulfrdes.
mainly pyrite as described above. However, it has
now beenfound that sincebacterialreductionof ferric
iron is energetically preferred to the reduction of
sulfate, ferous iron is producedrather than sulfide.
The ferrousiron thenreactswith dissolvedcarbonateto
form siderite (Colemanet al. 1993,Roden & Lovley
1993). The formation of siderite has also been
demonstratedin the biofilm from the URL @rown
et al. L994,Sawicki et al. 1995).
Magnetiteformation
Many bacterialen4nnesrequire a metal to conduct
catalysis;theseare usually fransition metalsthat have
several oxidation stales in the natural environment
(Wackettet aI. 1989).The metalsare assimilatedinto
the cell by specifiic transport-systems,allowing the
bacteria to collect the metals from an environment
where they may be presentin only minute amounts.
Some bacteria contain chains of magnetosomes
(magnetitecrystals),which make them magnetotactic
and causethe cells to become orienled in the geomagneticfield @rock & Madigan 1991).However,the
amountof magnetiteincorporatedinto thesebacteriais
minuscule in comparison to that formed by the
dissimilatory energy-producingreactions. Studies of
anaerobicsedimentssuggestthat bacteria are able to
producelarge quantitiesof ultafine-grained magnetite
whenreductionof ferric iron is coupledto oxidationof
organic matler (Lovley et al. L987). This processis
likely to have caused the accumulation of large
amountsof magnetitein iron-formations,presumably
t329
under conditions that are more oxidlzing than where
sideriteis formed.
Examinationof modern depositsand experimental
synthesessuggestthat ironstonesare forrned by the
gels @nhig et al.
crystallization of Si-Fe-O{H
1992),which occur as a result of hydrothermalfluids
mixing with seawater.Since it is unlikely that tlere
would have been a sufficient period of time for
significant quantitiesof iron to be precipitated,other
factors,suchasiron-oxidizing bacteriaableto facilitate
low-temperatureprecipitationof ferric iron, may well
havebeeninvolved.
Sorption: the cell as a polyionic trap
Passivephysiochemicalbiosorption is the type of
microbial mediationthat occursbetweeninorganicions
and microbial cellular components,whether dead or
alive. Many bacteriaproduce an outennostenvelope
of slime that servesas a buffer betweenthe cell and
the external environment. which has unsatisfied
chargesthat promote a polyionic trap for ionic and
electrostaticbinding ofmetals and silicates(Geesey&
Jang 1990). Bacterial cells are efficient in forming
metallic silicatesandfine-grainedminerals,sincethese
reactionsreducefhe level of toxic metalsin solution.
Silicate binding to Bacillus subtilis has been
demonsftated(Umrtia & Beveridge1993),in a casein
which the depositionof silicate is dependenton the
elecffopositive charge of the cell. The formation of
fine-grainedmineralsalso occursin the S-layer(slime)
a cyanobacteriumfound in freshof Synechococcus,
water lakes (Schultze-Lam& Beveridge1994).In this
caseomin916l formation begins as the deposition of
minute crystals of glpsum, but as the pH of the
microenvhonmentis increasedby metabolic activity,
the precipitatechangesto calcite.Active cells can shed
the mineralizedS-layer,but as they age they become
entombed.andform calcitebioherms.
Epilithic microbial communitiesfound ubiquitously
on submergedrosksarshighly mineralized(Konhauser
et al. 1994), and the biofilns, regardlessof their
substratum lithology, consistently form iron and
aluminum silicates, indicating that biomineralization
is a surface process associated with the anionic
nature of the cell wall. These biofilms seem to
dominate the rock-water interface and to determine
which minerals will ultimately become part of the
riverbed sediment.
Sitca is one of the major componentsof BIF, but
any microbially mediatedsystemfor silica deposition
posesa problem,sincesilica-metabolizingprokaryotes
are unknown, and eukaryotes only evolved later.
Thereforeomediation by bacteria of the precipitation
of silica in Archean iron-rich banded sediments
would have had to be by passive physiochemical
sorptionsuchas describedabove,ratherthan by active
metabolism.
1330
THE CANADIAN MINERALOGIST
Btournrr.elze.noN MopsIFoRrrrE Gmwsn or BIF
Microorganismshavebeenpresenton Earthfor 907o
of its history, and so it is to be expectedthat they
shouldhaveplayeda fundamentalrole in the evolution
ofour naturalenvironment.Significantalterationofthe
biosphereby microorganismsis causedby their needto
obtain nutrients and energy for growth. One of the
major effects of this requirement is the change in
valency in the caseofinorganic ions that are utilized
by bacteriaas a sourceof energy;this is reflected in
the modification of pH and Eh, thus altering the
solubilities of the dissolvedions.
Organic life may leave its signature in the sediments, and evidence of organic remains have been
found in BIF and the associatedblack shales.In many
cases, the carbon is isotopically light, indicating
microbial activity; this also can be correlatedwith an
increasein the concentation of iron (Walker 1984).
Sfromatolites are fossilized microbial mats that
contain entrappedsediment between layen of filamentous organisms.Although silica is not directly
metabolizedby prokaryotes,it can be precipitatedby
physicochemicalreactions with the microbial slime
layer or cell wall. Modern $tromatolitesare usually
carbonaceous,not siliceous, and contain oxygenproducingorganisms,but in appropriateenvironments,
suchasYellowstoneaudBaja Califomi4 stromatolites
are today being formed that contain prokaryotic
organismsin a siliceous matrix. These sfiomatolites
have a similar morphology to those of the Archean,
suggestingthat they could havebeenformedby similar
means.
Microorganisms require a source of energy for
growth, but this doesnot necessarilyhave.to be light
from tle sun. Metalliferous sediments presently
forming at the deep-seavents utilize chemolithotrophic, not photosyntletic, energy. Furthermore,
bacterial photosynthesis did not initially produce
oxygen; it was only later when cyanobacteriaevolved
that oxygen was directly generated,but it would still
have been limited in amount and rapidly utilized.
Oxygen could only have accumulated in the
amosphere when the water became saturated,after
eukaryotic algae and greenplants evolved in the late
Proterozoic.
Ho'ffever,oxidationof ferrousiron probablywasthe
earliest photosyntheticabiJity evolved by primitive
organisms (Ehrenreich & Widdel 1994), which
suggeststhat free oxygen would have been unnecessaryfor the deporl6oasf 6xidizediron beds.Microbial
organismsin shallow seasduring the Archean would
have received unrestrictedlight from the sun, which
they couldhaveutilized to lay down thin layersofironrich matsover large lateral areas.Episodic or seasonal
flooding of thesemats may have coveredthem with
siliceous sediments.which would ttren have been
recolonizedby photosyntheticbacteriaoncethe water
retreated.The questionis not, therefore,whether the
atmosphere contained sufficient oxygen for the
deposition of oxidized iron, but rather, whether
precipitation of large quantities of ferric iron could
have occurredby microbial mediationin the aqueous
depositionalenvironmentof that time. To judge from
the BIF that we know today,this whole processwould
havehad to be repeatedmany times over the millennia
to allow suchnumerousbedsto accumulate.
Many BIF contain thin bands of silica alternating
yiX[ millimeter-thick bands of minerals containing
ferrous or ferric iron (or both). It has beenditftcult to
explain how chemical processesalone could have
producedtheseresults. Our recent study of the URL
microbial biofilm does offer an explanation,since it
showsthat microbial consortiaare capableof forming
different microenvironments(stability fields) within a
biofikn, thus allowing precipitation of both oxidized
iron as ferrihydrite and reducediron as siderite.This
microbial mediation permits two very dissimilar iron
minerals to be precipitatedin close proximity. Both
oxidized and reducedstatesof iron are found in BIF,
but the statein which they were originally precipitated
would have been detenninedby the oxygen potential
(possibility of metabolic microbial oxidation) in the
environmentat the time of deposition.Since iron can
be precipitated in both oxidized and reduced forrns
by bacterial mediation, it is no longer necessary
to postulatelater metamorphicalteration in order to
explain the stateof the iron in BIF.
Stomatolites at Yellowstoneand the biofilm in the
URL show that iron, evenin very low concentations,
may be precipitatedby microbial actionfrom solution.
Although the present ocean environments do not
provide large areasofiron-rich shallow watersunder
an anaerobicafuosphere,suchaswererequiredfor the
massive Archean and Proterozoic BIF, there are
metalliferous sedimentsbeing depositedtoday under
comparableenvilonments,but in smallerareasaround
hot springsand hydrothermalvents (Gross& Mcleod
1987).
Coxcr-usroNs
The microbial precipitationof iron and silica in BIF
would have dependedupon the natural environment
at the time of deposition. Silica could have been
accumulated by physicochemical sorption on the
microbial biomass, whereasiron would more likely
have been metabolically utilized by the bacteriaas a
sourceof energythat could havebeendepositedeither
in an oxidized or reducedstate, dependentupon the
prevailing pH and Eh. In the absenceof oxygen,iron
would have beenprecipitatedin the reducedform as
sideriteor iron silicate,or possiblyaspyrite in a marine
environmenl Where oxidizing potentialwas available,
the iron would more likely have beenprecipitatedas
MICROBES AND BANDED IRON-FORMATIONS
r331
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ReceivedDecem.ber5, 1994, revised.manuscript accepted
August8, 1995.