In the Penninic nappe stack of the Western Alps, high- to ultrahigh-pressure metamorphic ophiolites of the Zermatt-Saas Zone are associated with slivers of continental crust. In one of these slivers, Monte Emilius, the overprinting of pre-Alpine granulite-facies rocks by subduction-related, Alpine eclogite-facies meta-morphism can be studied. Mafic granulites were initially transformed into blueschists. In a second step, shear zones were developed in which the blueschists recrystallized to fine-grained, foliated glaucophane eclogites, and eclogite veins. The combination of petrographic and field observations as well as whole-rock compositions suggests that the eclogite assemblage formed only in shear zones where Ca-metasomatism induced a change in major element composition. These substantial differences in bulk rock composition demonstrate how spatially limited eclogitization may be controlled by chemical redistribution, the degree of fabric development, and associated metamorphic reactions along fluid pathways. Thermodynamic modelling of selected bulk rock compositions yielded only slightly different conditions of 1.8 +/- 0.1 GPa/550 +/- 50 degrees C for blueschist and 1.9-2.3 GPa/550 +/- 50 degrees C for eclogite, constraining Ca-rich fluid infiltration and transformation to a depth of similar to 60-70 km. Eclogitization occurred in the Early Eocene at 52.96 +/- 0.91 Ma, as indicated by a well-defined Lu-Hf garnet isochron.

At present most knowledge on the impact of iron on 18O / 16O ratios (i.e. δ18O) of dissolved oxygen (DO) under circum-neutral conditions stems from experiments carried out under controlled laboratory conditions. These showed that iron oxidation leads to an increase in δ18ODO values. Here we present the first study on effects of elevated Fe(II) concentrations on the δ18ODO in a natural, iron-rich, circum-neutral watercourse. Our results show that iron oxidation was the major factor for rising dissolved oxygen isotope compositions in the first 85 m of the system in the cold season (February) and for the first 15 m during the warm season (May). Further along the course of the stream, the δ18ODO decreased towards values known for atmospheric equilibration around +24.6 ‰ during both seasons. Possible drivers for these changes may be reduced iron oxidation, increased atmospheric exchange and DO production by oxygenic phototrophic algae mats. In the cold season, the δ18ODO values stabilized around atmospheric equilibrium, whereas in the warm season stronger influences by oxygenic photosynthesis caused values down to +21.8 ‰. In the warm season from 145 m downstream of the spring, the δ18ODO increased again until it reached atmospheric equilibrium. This trend can be explained by respiratory consumption of DO combined with a relative decrease in photosynthetic activity and increasing atmospheric influences. Our study shows that dissolved Fe(II) can exert strong effects on the δ18ODO of a natural circum-neutral spring system even under constant supply of atmospheric O2. However, in the presence of active photosynthesis, with supply of O2 to the system, direct effects of Fe oxidation on the δ18ODO value become masked. Nonetheless, critical Fe(II) concentrations may indirectly control DO budgets by enhancing photosynthesis, particularly if cyanobacteria are involved.

Cyanobacteria induced biomineralization of atmospheric CO2 is a natural process leading to the formation of carbonates by spontaneous precipitation or through the presence of nucleation sites, under supersaturated conditions. As importance of basaltic rocks in the carbon cycle has already been highlighted, basaltic glass was chosen to test its ability to release cations needed for carbonate formation in presence of Synechococcus sp. cyanobacteria. Active cyanobacteria were expected to generate a local alkaline environment through photosynthetic metabolism. This process produces oxygen and hydroxide ions as waste products, raising the pH of the immediate cell surface vicinity and indirectly enhancing the carbonate CO32- concentration and providing the a degree of saturation that can lead to the formation of calcite CaCO3 or magnesite MgCO3. In the presence of active cells, the saturation index (SI) increased from -10.56 to -9.48 for calcite and from -13.6 to -12.5 for magnesite, however they remained negative due to the low Ca2+ and Mg2+ activities. Dead cells were expected to act as nucleation sites by the stepwise binding of carbonate with Ca2+ and Mg2+ on their surface. In the presence of inactive cells, SI values were closer to 0 but still negative due to the low pH and cation concentrations. Our results highlight that our current understanding of the carbon cycle suggests that Earth’s climate is stabilized by a negative feedback involving CO2 consumption and especially during chemical weathering of silicate minerals.

Cyanobacteria induced biomineralization of atmospheric CO 2 is a natural process leading to the formation of carbonates by spontaneous precipitation or through the presence of nucleation sites, under supersaturated conditions. . Basaltic glass was chosen in this study to test its ability to release cations needed for carbonate formation in presence of Synechococcus sp. cyanobacteria. Active cyanobacteria were expected to generate an alkaline environment through photosynthesic metabolism. This process produces oxygen and hydroxide ions as waste products, raising the pH of the immediate cell surface vicinity and indirectly enhancing the carbonate CO 32- concentration and providing the a degree of saturation that can lead to the formation of calcite CaCO 3 or magnesite MgCO 3 . In presence of active cells, the saturation index (SI) increased from -10.56 to -9.48 for calcite and from -13.6 to -12.5 for magnesite, however they remained negative due to the low Ca 2+ and Mg 2+ activities. Dead cells were expected to act as nucleation sites by the stepwise binding of carbonate and Ca 2+ and Mg 2+ on their surface. In the presence of inactive cells, SI values were closer to 0 but still negative due to the low pH and cation concentrations.

RationaleInvestigations of the isotope ratios of dissolved oxygen (δ18ODO) provide valuable information about the oxygen cycle in aquatic systems. However, oxidation of Fe(II) may change pristine δ18ODO values during storage and can lead to a misinterpretation. We sampled an Fe(II)‐rich spring system and measured δ18ODO values at various time intervals in order to determine influences of Fe‐oxidation.MethodsWater samples were collected from an Fe‐rich spring and related stream and the δ18ODO values were measured in fresh, 4‐ and 13‐day‐old samples with an isotope ratio mass spectrometer. Three replicates were measured for each sample with a 1σ of ± 0.2‰. On‐site parameters and Fe(II) contents were also measured over the course of the spring system by multi‐parameter probes and spectrophotometry.ResultsThe δ18ODO values over the course of the spring system in fresh, 4‐ and 13‐day‐old samples revealed differences of up to 8‰. We explain this increase by the consumption of DO by Fe(II)‐oxidation. After a flow length of 85 m the differences in δ18ODO values between fresh and older samples decreased because most of the Fe(II) was consumed.ConclusionsFalse interpretations of δ18ODO values are possible if Fe‐rich water samples are measured after too long storage, and we recommend measurement immediately after sampling.

Banded iron formation (BIF) deposition was the likely result of oxidation of ferrous iron in seawater by either oxygenic photosynthesis or iron-dependent anoxygenic photosynthesis—photoferrotrophy. BIF deposition, however, remains enigmatic because the photosynthetic biomass produced during iron oxidation is conspicuously absent from BIFs. We have addressed this enigma through experiments with photosynthetic bacteria and modeling of biogeochemical cycling in the Archean oceans. Our experiments reveal that, in the presence of silica, photoferrotroph cell surfaces repel iron (oxyhydr)oxides. In silica-rich Precambrian seawater, this repulsion would separate biomass from ferric iron and would lead to large-scale deposition of BIFs lean in organic matter. Excess biomass not deposited with BIF would have deposited in coastal sediments, formed organic-rich shales, and fueled microbial methanogenesis. As a result, the deposition of BIFs by photoferrotrophs would have contributed fluxes of methane to the atmosphere and thus helped to stabilize Earth’s climate under a dim early Sun.

Although trace element distribution and mobility have been investigated in agricultural soils, precise information about the extent to which anthropogenic activities influence soil quality remains limited. This study quantified trace element distribution (As, Bi, Cd, Cu, Ni, Pb, Pd, Sb, Zn) in agricultural and grazing land soils in metropolitan France based on the results from the GEMAS project [1]. The dataset is composed of214 samples from agricultural soils (Ap) and 218 samples from grazing land soils (Gr) for total and mobile fractions. The geological substrates have been divided into five main classes to correlate with geology: (i) shales, (ii) sandstones and sands, (iii) carbonate rocks, (iv) metamorphic rocks, and (v) intrusive and igneous rocks. These nine metal elements were selected using the enrichment factor higher than 40 in most of the five rock classes, specifically As and Cd. Arsenic had a median of 9.04 mg/kg and 9.5 mg/kg for Ap and Gr, respectively, on all kind of bedrocks. These values are similar, and this is also the case for Cd : 0.215 mg/kg and 0.249 mg/kg for Ap and Gr, soils respectively. Kriging interpolation was used to analyze the spatial distribution of the both elements in French soils. A correlation between geology and mining was observed for the distribution of As in France, for example southern part of Massif Central [2]. However, Cd shows a different pattern. The highest concentrations are situated in the sedimentary basin (Aquitaine and Parisien), as well as in the South Massif Central and South Vosges. This could be related to anthropogenic activities [3]. If the high concentration level of

Particulate material plays a major role in the transport of sparingly soluble nutrients such as P and Fe in natural surface waters. Microbes might gain access to these nutrients either indirectly through particulate dissolution or directly through microbial attack. As such, it seems reasonable to expect a link between the particulate material concentration and bacterial growth in natural surface waters. To explore this link, a series of microcosm growth experiments were performed in the presence of a typical freshwater cyanobacteria, Synechococcus sp., grown in dilute BG-11 culture media in the presence and absence of basaltic and continental riverine particulate material. Results demonstrate that riverine particulates can increase bacterial biomass by 1) triggering bacterial growth in otherwise unfavourable conditions, 2) increasing total maximum biomass concentration, and 3) inducing bacteria growth during the post-exponential phase. These effects are found to be enhanced by increasing particulate concentration. Results also indicate a positive feedback between the nutrient release from the particulates and growing bacteria, where dissolving particulates enhance bacterial growth, which further promotes particulate dissolution by altering fluid pH. Microscopic analysis showed direct physical contact between particulates and cyanobacteria, suggesting that bacteria attach directly on mineral surfaces to gain required nutrients. Furthermore, frequent bacteria clusters were observed associated with particulates, indicating an increasing aggregation of bacteria in the presence of particulate material, which may facilitate a higher burial efficiency of organic carbon.

The goal of this study was to quantify the mobility and partitioning of trace elements originating from mine waste rocks derived from open pit coal extraction activities. The results showed that native rice plants were adapted to growing in metal contaminated soils, posing a severe health risk to local population. Sequential extraction procedures and bulk soil chemical analyses both suggest enrichment of Cd, Pb and Cu in rice paddy soils. Lead was shown to be evenly partitioned among all mineral and organic phases. Copper was associated with carbonates and organic matter. Smaller fractions of Pb and Cu were also bound to Fe and Mn oxides. Only 25% of Cd, 9% of Pb and 48% of Cu were associated with the exchangeable fraction, considered mobile and thus bioavailable for plant uptake. Effects of Cd, Cu and Pb on local Cam Pha Nep cai Hoa vang, and control Asia Italian rice, showed marked differences in growth. The local Vietnamese variety grew close to control values, even upon exposure to higher trace metal concentrations. Whereas the development of the control rice species was significantly affected by increasing trace metal concentrations. This result suggests toxic trace elements accumulation in the edible parts of crops.

The goals of this study were, (1) to quantify the effects of rare earth elements (REE) on rice plant growth and (2) to determine whether the presence of iron (III) oxides on the plant root surface (i.e. the iron oxide plaques) played a role in impeding any toxic effects caused by the presence of the REE. Hydroponic experiments were designed to grow rice plants in a greenhouse under controlled conditions, exposed to all rare earth elements simultaneously, and to iron (II) sulfate or iron (II) chloride. The results showed a significant decrease in root and plant height and biomass at rare earth element concentrations of 0.5 mg/L and 1 mg/L. Negative growth effects were observed for plant roots and shoots upon addition of 100 mu mol/L Fe(II) chloride or Fe(II) sulfate. Even when the root biomass was enhanced upon addition of Fe (II) chloride at a 1 mg/L rare earth concentration, however, statistically significant decreases in root length and plant height were recorded. In the presence of Fe (II) sulfate, a negative growth effect was present for all REE concentrations, being more pronounced at the highest REE levels. For the Fe(II) chloride experiments, speciation modeling showed that the rare earth elements would remain "free" as hydrated ions (Ln 3(+)) or would be complexed by Fe(III) oxyhydroxides. With Fe(II) chloride, the light rare earths (La, Ce, Pr, Nd; LREE) remained mostly soluble, whereas the middle (Sm, Eu, Gd; MREE) and heavy (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; HREE) elements were for the most part bound by Fe(III) solids. As negative growth effects were observed with Fe(II) chloride, the most soluble LREE could be concluded to play a role in rice plant growth inhibition. Furthermore, upon addition of Fe(II) sulfate, the MREE and HREE were significantly associated with SO42- and a regained toxic effect for rice plants was observed at the highest REE concentration, suggesting also an effect of the dissolved MREE-SO4+ and HREE-SO4+ species on rice growth. This observation, coupled to the knowledge that SO42- is an essential nutrient for plants, suggests absorption of the REE-SO4+ species by the rice plant. The recorded negative growth effects for both the Fe(II) chloride or Fe(II) sulfate conditions, strongly suggest that all REE are detrimental to the development of rice. For both the Fe(II) chloride and Fe(II) sulfate conditions the negative growth effects may have been attenuated as a consequence of REE sorption to Fe(III) oxyhydroxides (i.e. iron oxide plaques) identified on the root surface, as suggested by surface complexation modeling of the REE to iron (III) oxides, at the pH and ionic strength conditions in this study.

In this study, models were used for the first time to investigate the fate and transport of rare earth elements (REE) in the presence of hydrous manganese and ferric oxides in groundwaters from the coastal Bohai Bay (China). Results showed that REE sorption is strongly dependent on pH, as well as hydrous manganese and ferric oxide content. Higher proportions of REE were sorbed by hydrous manganese oxide as compared to hydrous ferric oxides, for example in the presence of neodymium. In this case, a mean 28% of this element was sorbed by hydrous manganese oxide, whereas an average 7% sorption was observed with hydrous ferric oxides. A contrasting REE sorption behavior was observed with hydrous manganese and ferric oxide for all investigated groundwaters. Specifically, REE bound to hydrous manganese oxides showed decreasing sorption patterns with increasing atomic number. The opposite trend was observed in the presence of hydrous ferric oxides. In addition, these results suggested that light REE (from La to Sm) rather than heavy REE (from Eu to Lu) are preferentially scavenged by hydrous manganese oxide. However, the heavy REE showed a greater affinity for hydrous ferric oxides compared to light REE. Therefore, both hydrous manganese and ferric oxide are important scavengers of REE. This study shows the implication of hydrous manganese and ferric oxide sorption for the sink of REE in groundwater.

Banded iron formations are economically important sedimentary deposits in Earth's Precambrian rock record, consisting of alternating iron-rich (hematite, magnetite, and siderite) and silicate/carbonate (quartz, claylike minerals, dolomite, and ankerite) layers. Based on chemical analyses from banded iron formation units of the 2.48 Ga Dales Gorge Member of the Hamersley Group in Western Australia, it has been previously suggested that most, if not all, of the iron in banded iron formations could have been oxidized by anoxygenic phototrophic bacteria (photoferrotrophs) at cell densities considerably less than those found in modern iron-rich aqueous environments. However, oxygen-producing phytoplankton may have also been capable of supplying the necessary oxidizing power. Here, we revisit the question of the anoxygenic and oxygenic phytoplankton populations necessary to account for banded iron formation deposition and quantify the amount of selected trace elements (P, Mn, Co, Ni, Cu, Zn, Mo, Cd) that could have been associated with their biomass. Using an expanded geochemical data set for the Dales Gorge Member as an example, we find that with turnover times comparable to those seen in modern ecosystems, the same phytoplankton populations required to form banded iron formations could have supplied the entirety of trace elements found in this iron-rich deposit. Further, spurred by the similarities between banded iron formation and anoxygenic phytoplankton trace-element stoichiometries, we suggest that much of the trace-element inventory preserved in the banded iron formation was at some point biologically assimilated in the water column, released from degrading photoferrotrophic biomass at the seafloor and in the sediment pile, and ultimately fixed in the iron-rich sediment in approximately stoichiometric proportions by near-quantitative adsorption to ferrihydrite. Our observations suggest that, as today, phytoplankton and the recycling of their biomass exerted control over the trace-element composition of ancient seawater and sediment.

ABSTRACT Microaerophilic Fe(II) oxidizers are commonly found in habitats containing elevated Fe(II) and low O 2 concentrations and often produce characteristic Fe mineral structures, so-called twisted stalks or tubular sheaths. Isolates originating from freshwater habitats are all members of the Betaproteobacteria , while isolates from marine habitats belong almost exclusively to the Zetaproteobacteria . So far, only a few isolates of marine microaerophilic Fe(II) oxidizers have been described, all of which are obligate microaerophilic Fe(II) oxidizers and have been thought to be restricted to Fe-rich systems. Here, we present two new isolates of marine microaerophilic Fe(II)-oxidizing Zetaproteobacteria that originate from typical coastal marine sediments containing only low Fe concentrations (2 to 11 mg of total Fe/g of sediment [dry weight]; 70 to 100 μM dissolved Fe 2+ in the porewater). The two novel Zetaproteobacteria share characteristic physiological properties of the Zetaproteobacteria group, even though they come from low-Fe environments: the isolates are obligate microaerophilic Fe(II) oxidizers and, like most isolated Zetaproteobacteria , they produce twisted stalks. We found a low organic carbon content in the stalks (∼0.3 wt%), with mostly polysaccharides and saturated aliphatic chains (most likely lipids). The Fe minerals in the stalks were identified as lepidocrocite and possibly ferrihydrite. Immobilization experiments with Ni 2+ showed that the stalks can function as a sink for trace metals. Our findings show that obligate microaerophilic Fe(II) oxidizers belonging to the Zetaproteobacteria group are not restricted to Fe-rich environments but can also be found in low-Fe marine environments, which increases their overall importance for the global biogeochemical Fe cycle. IMPORTANCE So far, only a few isolates of benthic marine microaerophilic Fe(II) oxidizers belonging to the Zetaproteobacteria exist, and most isolates were obtained from habitats containing elevated Fe concentrations. Consequently, it was thought that these microorganisms are important mainly in habitats with high Fe concentrations. The two novel isolates of Zetaproteobacteria that are presented in the present study were isolated from typical coastal marine sediments that do not contain elevated Fe concentrations. This increases the knowledge about possible habitats in which Zetaproteobacteria can exist. Furthermore, we show that the physiology and the typical organo-mineral structures (twisted stalks) that are produced by the isolates do not notably differ from the physiology and the cell-mineral structures of isolates from environments with high Fe concentrations. We also showed that the organo-mineral structures can function as a sink for trace metals.

Chemical weathering of silicates represents an essential process of both the rock and carbon cycles. The application of this natural process on a global scale could be used to mitigate excess CO2 in the atmosphere. This concept is known as enhanced weathering where fine mineral powder is spread over the land surface for carbonation of silicates and expected CO2 sequestration. Within this context, organic matter, may promote or inhibit the processes of enhanced weathering, however this has not yet been fully quantified. Motivated by this knowledge gap, the present work studied the dissolution behaviour of olivine under inorganic conditions and in the presence of inert Synechococcus sp. biomass in batch reactor setups at various ionic strengths with a constant input of atmospheric CO2. Olivine, which represents one of the most obvious candidates for use in enhanced weathering, showed no significant statistical differences in the release of dissolved cations for all studied organic and inorganic experimental setups. For batch reactors containing inert biomass, a moderate increase in alkalinity and pH is found with respect to the inorganic conditions, which points to the buffering of H2CO3 by deprotonated functional groups on the inert biomass surface. Overall, the experimental results of this study indicate a negative combined effect of inert biomass and high ionic strengths on the olivine dissolution rates in natural aquatic systems. This suggests reduced carbonation rates on the Earth's surface and a lower potential of artificially dispersed olivine powder for CO2 sequestration.

It is widely accepted that anoxygenic photosynthetic bacteria (photoferrotrophs) oxidized dissolved Fe(II) in anoxic Archean seawater, leading to the sedimentation of cellular biomass and the Fe(III) minerals that comprised banded iron formations (BIF). However, it remains unknown to which extent the sedimented bacteria may have transported trace metals to BIF through their ability to sorb ions from seawater. We therefore chose the marine anoxygenic photoferrotroph Rhodovulum iodosum to quantify Cd2+, Co2+, Cu2+, Mn2+, Ni2+ and Zn2+ adsorption to bacteria/Fe(III)-oxyhydroxide composites at neutral pH and an ionic strength of 0.1M. Acid base titration data were modeled using a linear programming optimization method to yield pKa values of 4.83±0.13, 6.21±0.18, 7.74±0.24 and 9.28±0.27 and corresponding site densities of 5.7, 12.0, 3.3 and 6.5×10−4mol/g, respectively. FTIR spectroscopy confirmed the presence of carboxyl groups as the most acidic sites corresponding to the lowest pKa values. The analysis of Cd2+ sorption data showed two metal complexing sites with pKS values of 3.44±0.14 and 4.80±0.21 and corresponding binding site concentrations of 1.1±0.3×10−4 and 0.9±0.2×10−4mol/g on the ferrihydrite/R. iodosum composite. The pKS values were used to calculate metal-ligand binding constants, Km. This confirmed Cd2+ binding to the most acidic carboxyl groups on the ferrihydrite/R. iodosum composite surface, as the calculated Km was consistent with reported Cd2+ binding constants for simple organic acids (e.g., lactic, acetic, pyruvic and citric acid). This was used to further calculate the concentration of Co2+, Cu2+, Mn2+, Ni2+ and Zn2+ sorbed to ferrihydrite/R. iodosum composites. The resulting concentrations were ultimately compared to the concentrations of trace elements in BIF to demonstrate that is plausible that the trace metal content in BIF was derived from plankton growing in the ancient ocean's photic zone.

In the present study, a mixed-flow steady-state bio-reactor was designed to biomineralize CO2 as a consequence of photosynthesis from active Synechococcus sp. Dissolved CO2, generated by constant air bubbling of inorganic and cyanobacteria stock solutions, was the only source of inorganic carbon. The release of hydroxide ion by cyanobacteria from photosynthesis maintained highly alkaline pH conditions. In the presence of Ca2+ and carbonate species, this led to calcite supersaturation under steady state conditions. Ca2+ remained constant throughout the experiments showing the presence of steady state conditions. Similarly, the Synechococcus sp. biomass concentration remained stable within uncertainty. A gradual pH decrease was observed for the highest Ca2+ condition coinciding with the formation of CaCO3. The high degree of supersaturation, under steady-state conditions, contributed to the stabilization of calcite and maintained a constant driving force for the mineral nucleation and growth. For the highest Ca2+ condition a fast crystal growth rate was consistent with rapid calcite precipitation as suggested further by affinity calculations. Although saturation state based kinetic precipitation models cannot accurately reflect the controls on crystal growth kinetics or reliably predict growth mechanisms, the relatively reaction orders obtained from modeling of calcite precipitation rates as function of decreasing carbonate concentration suggest that the precipitation occurred via surface-controlled rate determining reactions. These high reaction orders support in addition the hypothesis that crystal growth proceeded through complex surface controlled mechanisms. In conclusion, the steady state supersaturated conditions generated by a constant cyanobacteria biomass and metabolic activity strongly suggest that these microorganisms could be used for the development of efficient CO2 sequestration methods in a controlled large-scale environment.

The past two decades have seen a significant advancement in our understanding of bacterial surface chemistry and the ability of microbes to bind metals from aqueous solutions. Much of this work has been aimed at benthic, mat-forming species in an effort to model the mechanisms by which microbes may exert control over metal contaminant transport in soils and groundwater. However, there is a distinct paucity of information pertaining to the surface chemistry of marine planktonic species, and their ability to bind trace metals from the ocean’s photic zone. To this end, the surface properties of the cyanobacterium Synechococcus sp. PCC 7002 were studied as this genus is one of the dominant marine phytoplankton, and as such, contributes significantly to metal cycling in the ocean’s photic zone. Zeta potential measurement indicates that the cell surfaces display a net negative charge. This was supported by potentiometric titration and Fourier transform infrared spectroscopy analyses demonstrating that the cells are dominated by surface proton releasing ligands, including carboxyl, phosphoryl and amino functional groups, with a total ligand density of 34.18±1.62mmol/g (dry biomass). Cd adsorption experiments further reveal that carboxyl groups play a primary role in metal adsorption, with 1.0g of dry biomass binding an equivalent of 7.05×10−5M of Cd from solution at pH=8. To put this value into context, in 1L of seawater, and with an open-ocean population of Synechococcus of 105cells/mL in the photic zone, approximately 10nmol of Cd could potentially be adsorbed by the cyanobacteria; an amount equivalent to seawater Cd concentrations. Although we have only focused on one microbial species and one metal cation, and we have not considered trace element assimilation, our results highlight the potential role of surface sorption by phytoplankton in the cycling of metals in the ocean.

Fe(III) (oxyhydr)oxides are ubiquitous in modern soils and sediments, and their large surface area leads to scavenging of trace elements. Experimental trace element partitioning between Fe(III) (oxyhydr)oxides and aqueous solutions have been used to elucidate the geochemical composition of the Precambrian oceans based on the trace element concentrations in Precambrian banded iron formations (BIFs). However, previous partitioning experiments did not consider the potential influence of microbially-derived organic material, even though it is widely believed that bacterial phytoplankton was involved in Fe(II) oxidation and the deposition of BIF primary minerals. Therefore, the present study focuses on sorption of Ni to, and co-precipitation of Ni with, both biogenic ferrihydrite precipitated by the freshwater photoferrotroph Rhodobacter ferrooxidans SW2 and the marine photoferrotroph Rhodovulum iodosum, as well as chemically synthesized ferrihydrite. We considered the influence of cellular organic material, medium composition and the availability of dissolved silica. Our results show a preferential association of Ni with ferrihydrite, and not with the microbial cells or extracellular organic substances. We found that the addition of silica (2mM) did not influence Ni partitioning but led to the encrustation of some cells with ferrihydrite and amorphous silica. The two- to threefold lower Ni/Fe ratio in biogenic as compared to abiogenic ferrihydrite is probably due to a competition between Ni and organic matter for sorption sites on the mineral surface. Additionally, the competition of ions present at high concentrations in marine medium for sorption sites led to decreased Ni sorption or co-precipitation. Based on our data we conclude that, if the Fe(III) minerals deposited in BIFs were – at least to some extent – biological, then the Ni concentrations in the early ocean would have been higher than previously suggested. This study shows the importance of considering the presence of microbial biomass and seawater ions in paleomarine reconstructions.