A series of laboratory-controlled microbial experiments using gram-negative sulphate-reducing bacteria (Desulfovibrio brasiliensis) inoculated with natural uranium were performed to investigate 238U/235U fractionation during bacterially-mediated U reduction. Control experiments, without bacteria to drive U reduction, were conducted in parallel. Paired measurements of 238U/235U and U concentration for both the residual growth medium solution and the accumulated biologically-mediated precipitate were obtained using multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS). The control experiments show that only minor (<0.1‰), if any 238U/235U fractionation occurs during co-precipitation with calcite. This implies that carbonate sediments are capable of faithfully recording the signature of the global ocean during Earth’s major climatic events, including oxygenation and de-oxygenation transitions in the marine environment. The results for the microbial experiments demonstrate that the 238U/235U composition of the unreacted growth medium containing U(VI) is isotopically lighter than the composition of the U(IV)-bearing precipitate as U(VI) is consumed, in agreement with field-based observations of microbially-mediated U reduction. Uranium isotopic shifts of up to 0.8‰ were observed between the liquid and solid phases. These observations can be modelled using a Rayleigh distillation approach describing kinetic uptake in a closed system, which yields a fractionation factor α of 0.99923±0.00004 (ε=−0.77±04‰) for U(VI)–U(IV) reduction mediated by the D. brasiliensis microbe. This fractionation behaviour is consistent with that observed in field-based redox environments, which give rise to similar α values. Competing processes such as U co-precipitation (e.g. adsorption) may act to lower the apparent value for α and possibly play a secondary role both in the microbial experiments of this study and in natural U reduction settings where variable α values are found. These results may suggest that microbes adept at inducing U(VI) reduction play a crucial role in facilitating significant 238U/235U isotope fractionation in nature.

Integrated Ocean Drilling Program (IODP) Expedition 310 (Tahiti Sea Level) offered an opportunity to study the geomicrobiology of a reef framework. Offshore drilling was conducted on the coastal reefs of Tahiti (French Polynesia) at 22 sites in water depths of up to 117 m. Up to 80% of the retrieved core material comprises authigenic grey microbial carbonates with laminated or thrombolitic morphologies, which are associated with corals. Microbialites infilled the cavities during reef development and stabilized the coral reef framework. Rock-surface analyses were performed to track ongoing microbial activity in biofilms that could represent a modern counterpart of the processes at the origin of the formation of fossil microbialites. Significant concentrations of adenosine 5'-triphosphate, indicative of the presence of living microorganisms, were detected at relatively shallow depths, 0-6 m below the seafloor. Exoenzyme activities confirmed the presence of an active metabolizing microbiota forming biofilms in reef cavities. Onshore investigations of the recovered microbes and biofilms completed our picture that the rapid postglacial formation of carbonate microbialites was mediated by the activity of anaerobic microbes, such as sulphate-reducing bacteria and iron-respiring organisms, stimulated by the highly productive reef environment.

Some species of sulphate-reducing bacteria (SRB) are known to mediate the formation of dolomite and Mg-calcite. However, their exact role in the mineralization process remains elusive. Here, we present the result of a laboratory experiment that was designed to test whether formation of carbonate minerals by SRB can occur in the absence of living cells, through passive mineralization of their exopolymeric substances (EPS). SRB capable of mediating dolomite were cultivated in the laboratory, allowing them to secrete EPS. Microbial activity within the cultures was subsequently inhibited with antibiotics. Only after this step, Ca2+ and Mg2+ were added to the solution and carbonate minerals could form. Mg-calcite and disordered Ca-dolomite precipitated in association with EPS. The mol.% of Mg2+ in the crystals increased with longer incubation times. This result demonstrates that organic compounds produced by SRB can mediate the formation of Ca-Mg carbonates in the absence of an active metabolism.

Terra Nova, 24, 248–254, 2012AbstractThe study of microbe‐mineral interactions and the identification of biominerals in sedimentary rocks provide crucial information on the coevolution of life and Earth surface environments. Desulfovibrio brasiliensis, a sulfate‐reducing bacterium isolated from Lagoa Vermelha (Brazil), is known for its ability to precipitate dolomite, a mineral that is common in the geological record, but difficult to obtain in laboratory experiments simulating Earth's surface conditions. Here, we report on a new bacterial strain capable of precipitating Mg‐calcite and Ca‐dolomite, isolated from a microbial mat in the sabkha of Abu Dhabi (UAE). The 16S rRNA gene sequence of the isolate revealed a 99.6% identity (i.e. same species) with Desulfovibrio brasiliensis. The presence of the same microbial species at two geographically distant dolomite‐forming locations is difficult to explain in terms of pure coincidence. Rather, it suggests that the ability of precipitating dolomite may be a unique characteristic associated with specific strains.

Since the description of stromatolites a century ago by Kalkowsky (1908) as products of ancient microbial activity, the exact understanding of its formation remained unclear and is still matter of debate.

Microbial mediation is the only demonstrated mechanism to precipitate dolomite under Earth surface conditions. A link between microbial activity and dolomite formation in the sabkha of Abu Dhabi has, until now, not been evaluated, even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences. Such an evaluation is the purpose of this study, which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of Abu Dhabi, along a transect from the intertidal to the supratidal zone. This investigation revealed a close association between microbial mats and dolomite, suggesting that microbes are involved in the mineralization process. Observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats. In current models, microbial dolomite precipitation is linked to an active microbial activity that sustains high pH and alkalinity and decreased sulphate concentrations in pore waters. Such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone. By contrast, these models cannot be applied to the supratidal zone, where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity. As no abiotic mechanism is known to form dolomite at Earth surface conditions, two different hypotheses can reconcile this result. In a first scenario, all of the dolomite present in the supratidal zone formed in the past, when the mats were active at the surface. In a second scenario, dolomite formation continues within the buried and inactive mats. In order to explain dolomite formation in the absence of active microbial metabolisms, a revised microbial model is proposed in which the mineral-template properties of exopolymeric substances play a crucial role.

Microbially mediated high-Mg calcite and dolomite precipitation occurs under oxic conditions in Brejo do Espinho lagoon, Brazil, within the upper 5 cm below the sediment water interface. With burial to < 25 cm in the sediment sequence, early diagenesis associated with sulphate-reducing bacterial activity transforms the mixed carbonate mineralogy to 100% dolomite, as the pore-water becomes undersaturated with respect to calcite, while remaining supersaturated with respect to dolomite. Laboratory culture experiments using moderately halophilic aerobic bacteria (Virgibacillus marismortui and Marinobacter sp.) isolated from the uppermost part of the microbial mat in Brejo do Espinho demonstrate that microbially mediated dolomite precipitation can occur under ambient Earth's surface conditions in the presence of oxygen. These results add an additional metabolic process, aerobic respiration, to bacterial sulphate reduction and methanogenesis, which have previously been identified with dolomite formation. Furthermore, the formation of carbonate minerals with spherulitic structures in both the natural environment and laboratory culture experiments points to microbial involvement, as recognized in numerous other modern environments and ancient systems. This study suggests that previously recognized modern dolomite-forming environments, such as the supratidal areas of Andros Island, Bahamas, with recent dolomite crusts should be revisited to evaluate the importance of aerobic respiration in dolomite precipitation.

The magnetic characteristics of intact magnetosome chains in Magnetospirillum gryphiswaldense bacteria were investigated by means of static and dynamic magnetic analyses and ferromagnetic resonance spectroscopy. The nano-sized magnetosomes are generally in a stable single-domain state, but magnetosomes smaller than 30 nm characteristic of superparamagnetic magnetite particles were also found. Alternating current (AC) susceptibility indicates that all magnetosomes are blocked below 150 K. At room temperature the anisotropy of M. gryphiswaldense is dominated by the shape of the magnetosome chains. Low-temperature ferromagnetic resonance (FMR) spectroscopy indicates that this dominant shape anisotropy can affect the detection of the Verwey transition at 100 K. The static and dynamic magnetic analyses show that the Verwey transition is smeared and that our magnetotactic bacteria fail the Moskowitz test. This failure is explained by the biomineralization of non-stoichiometric magnetosomes. This interpretation is based on the increase in high-field susceptibility and the distinct peak in the out-of-phase component of the AC susceptibility below 50 K. These results are attributed to freezing of spins associated with defect structures in the core and at the surface of nano-sized magnetosomes. The results obtained from M. gryphiswaldense demonstrate that intrinsic properties of nano-sized magnetosomes are significantly influenced by non-stoichiometry and by the anisotropy excited from their arrangement in the bacteria. (c) 2008 Elsevier B.V. All rights reserved.

Microsedimentary structures referred to as nanobacteria-like particles were described from modern carbonate environments, where they form in close spatial association with sulfate-reducing bacteria (SRB). However, the exact mechanism of their formation, as well as their paleontological significance, remains controversial. Here we report on an investigation of microbe-mineral interactions in experimentally produced carbonate globules. The experiments were carried out under anoxic conditions at 30 degrees C with Desulfovibrio brasiliensis, a SRB known to mediate dolomite formation. We observed that extracellular polymeric substances (EPS) secreted by the microbial community play a key role in the mineralization process. Nanobacteria-like particles represent the early stage of carbonate nucleation within the EPS, which progressively evolve to larger globules displaying a grainy texture. We excluded the possibilities that these structures are fossils of nanobacteria, dissolution surfaces, or artifacts created during sample preparation. D. brasiliensis cells are predominantly located outside of the EPS aggregates where mineral growth takes place. As a result, they remain mobile and are rarely entombed within the mineral. This self-preservation behavior may not be limited to D. brasiliensis. Other microbes may produce, or may have produced during the geological past, biogenic minerals through a similar process. Mineralization within EPS explains why microbial relics are not necessarily present in biogenic carbonates.

Bacterial populations, microbial and mineral-forming processes, and their products were analyzed in Lagoa Vermelha, Brazil. The microbial mat and underlying sediment were studied as a unique system to define the boundary conditions responsible for high Mg-calcite and dolomite formation. In the uppermost layers of the microbial mat, oxygenic photosynthesis and aerobic respiration resulted in calcite precipitation, whereas, in the underlying anoxic layers of the mat, sulfide oxidation and sulfate reduction induced formation of a range of carbonate minerals with increasing Mg concentrations.The chemical, mineralogical, and biological conditions presently found in Lagoa Vermelha may have been more common in the Precambrian. The microorganisms performing the metabolic processes related to carbonate mineral formation within Lagoa Vermelha's hypersaline microbial mat may have already been present in the Precambrian. Thus, microbial carbonate as a biomineral could be a record of metabolism throughout geological time. (c) 2005 Elsevier B.V. All rights reserved.