Recent dolomitic sediment samples from Lagoa Vermelha, Brazil, were examined microscopically to study the process of bacterial fossilization in carbonate sediments. Bacteria-like bodies were intimately associated with carbonate mineral surfaces, and coatings on the former demonstrate the calcification of single bacterial cells. The bacterial fossilization process in Lagoa Vermelha sediments was simulated in the laboratory by cultivation of mixed and pure cultures of sulphate-reducing bacteria, which were isolated from the Lagoa Vermelha sediments. These cultures produced carbonate minerals that were studied to provide insight into the initiation of the fossilization process. In mixed culture experiments, bacterial colonies became calcified, whereas in pure culture experiments, single bacterial cells were associated with dolomite surfaces. Dolomite nucleated exclusively in bacterial colonies, intimately associated with extracellular organic matter and bacterial cells. Electrophoretic mobility measurements of the bacterial cells in electrolyte solutions demonstrated the specific adsorption of Ca 2 + and Mg 2 + onto the cell surfaces, indicating the role of the bacterial surface in carbonate nucleation and bacterial fossilization. The affinity of the cells for Mg 2 + was related to the capability of the strains to mediate dolomite formation. Combined with sulphate uptake, which dissociates the [MgSO 4 ] o ion pair and increases the Mg 2 + availability, the concentration of Mg 2 + ions in the microenvironment around the cells, where the conditions are favourable for dolomite precipitation, may be the key to overcome the kinetic barrier to dolomite formation. These results demonstrate that bacterial fossilization is a consequence of the cell surface involvement in carbonate precipitation, implying that fossilized bacterial bodies can be used as a tool to recognize microbially mediated carbonates.

Microbial fossilization in carbonate sediments: a result of the bacterial surface involvement in dolomite precipitation

Lithifying microbial mats in Lagoa Vermelha, Brazil: Modern Precambrian relics?

Sulfate reduction has been suggested as a mechanism to induce precipitation of calcium and magnesium carbonates in marine sediments and microbial mats through most of Earth’s his-tory. However, sulfate reduction also causes a drop in pH that favors dissolution rather than precipitation of carbonates. Model results obtained in this study show that in modern seawa-ter, modern hypersaline water, and assumed Precambrian alkaline seawater, sulfate reduction initially lowers the saturation of carbonates due to a rapid decrease in pH. With continu-ing sulfate reduction, the pH stabilizes between 6.5 and 7, and carbonate saturation slowly increases as a result of increasing dissolved inorganic carbon concentration. However, sulfate reduction in surface microbial mats is not sufﬁ cient to cause such an increase in saturation. With increasing salinity, sulfate reduction becomes even less efﬁ cient to induce carbonate pre-cipitation. In an alkaline Precambrian ocean, where large amounts of carbonate were formed, induction through sulfate reduction was entirely ineffective. Other metabolic pathways or abi-otic factors must be responsible for inducing carbonate formation in microbial mats through Earth’s history.

Two opposing effects of sulfate reduction on carbonate precipitation in normal marine, hypersaline, and alkaline environments

Modern lacustrine microbialites: towards a synthesis of aqueous and carbonate geochemistry and mineralogy

In order to evaluate vloatile compounds, especiallly sesquiter-penoids, in environmental and geological samples a method was penoids, in environmental and geological samples a method was required to characterize these components when they occur at low concentrations. After fractionation of the total sample extracts by TLC the aliphatic fractions were analyzed by HRGC AND THE kovats indices of all resolved peaks were calculated and comkpared with indices of all resolved peaks were calculated and compared with those obtained for commercial standards. As final step, the fractions were analyzed by HRGC-MS and structures confirmed by comparison of their mass spectra with those of certified sesuiterpene standards. Additionally sesquiterpenoids present in the dissolved phase in water samples were concentrated by solid phase extraction using XAD-2 resin. In this initial study, recovery tests with the resin were carried out in order to evaluate its performance rather than validating the methodology.

Analysis of volatile sesquiterpenoids in environmental and geological samples

Biogeochemistry of a hypersaline lagoon east of Rio de Janeiro, Brazil

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Biogeochemistry of a hypersaline lagoon east of Rio de Janeiro, Brazil