Microbial dolomite formation within exopolymeric substances

Dolomite (CaMg(CO3)2) is a common mineral in sedimentary rocks, whose origin has long been debated. Dolomite formation at low temperatures appears to occur in association with microbial activity. However, the exact role of microbes in the mineralization process remains poorly understood. Consequently, the interpretation of microbial dolomite in terms of a biomineral is controversial. Herein, this controversy was addressed through the study of microbe-mineral interactions occurring in the coastal sabkha of Abu Dhabi (UAE) and the evaluation of laboratory precipitation experiments with dolomite-mediating bacteria. In the studied area of the sabkha, the occurrence of dolomite has been documented in association with surface and buried microbial mats. Dolomite precipitates as a consequence of mineral nucleation and growth within the extrapolymeric substances (EPS) constituting the mats. EPS molecules act as an organic template, which infl uences the elemental composition of the precipitate leading to dolomite formation. Evidence for this process is observable in the surface microbial mat, but it is even more prominent in 1000 years old microbial mats, which are buried in the supratidal zone and no longer show signs of strong metabolic activity. Th ese observation led to the hypothesis that EPS, rather than an active microbial metabolism, is the major factor responsible for the mineralization process. Th is interpretation diff ers from current models, in which microbialdolomite formation is mainly linked with an increase in pH and alkalinity and consumption of sulfate due to metabolic activity. Th e key role of EPS in dolomite formation was tested using a set of laboratory culture experiments. Mg-calcite and Ca-dolomite were successfully precipitated at 30°C in association with freshly produced EPS, even in the absence of living bacteria, whose metabolism was inhibited with antibiotics. Combining confocal laser scanning microscopy and low-temperature scanning electron microscopy, it was possible to resolve important details concerning the role of sulfate-reducing bacteria in the production of dolomite and biogenic carbonates, in general. Th e results demonstrate that EPS is the main site of carbonate nucleation and growth. Th is phenomenon allow the bacteria to escape cell entombment, suggesting a self-preservation behavior. Th ese important observations explain why microbial relics are rarely found in ancient carbonates, even if they are of defi nitive microbial origin. Nanobacteria-like particles and grainy textures have been described in the geological record, as well as in the Martian meteorite ALH84001. Th ese microsedimentary structures were initially considered as possible evidence for past life on planet Mars, opening a still unsolved controversy. In this study, it was possible to unambiguously establish that these structures are produced during the early step of carbonate nucleation on EPS molecules. Th is observation provides a revised interpretation of such microstructures, which are possible indicators for ancient microbial activity and are commonly observed associated with modern and ancient microbial dolomite. EPS mineralization also produces microsedimentary structures consisting of mineralized alveolar microfabrics. Th ese structures, in combination with an unusual Mg-Si phase that characterizes the early stage of the mineralization process, is proposed as a new tool for the recognition of microbial dolomite in ancient rocks. A new strain of dolomite-mediating bacteria was isolated from a microbial mat growing in the intertidal zone of the Abu Dhabi sabkha. Th e complete 16S rDNA gene sequence of the isolate revealed a 99.6% identity with Desulfovibrio brasiliensis, a dolomite-mediating bacterium isolated from Lagoa Vermelha, Brazil. Th is fi nding establishes a link between two geographically distant natural environments, indicating that microbial mediation of dolomite may be a widespread mechanism, which is deeply related to the genome of particular microbes and, consequently, their ability to excrete specifi c EPS. In summary, this work shows that EPS play a key role in the nucleation of low-temperature dolomite, as well as in the formation of microsedimentary structures. Th ese results are helpful for identifying microbial mediated carbonate deposited throughout the geological record.