Microbialites are organosedimentary deposits formed by the interaction of benthic microbial communities with their environment (Burne & Moore, 1987) and provide the only continuous macroscopic record for life spanning its appearance in the Archean through to the present day (Riding, 2000). Biological, physical, and chemical processes combine to produce an internal structure characteristic of microbialites, which include laminated fabrics and clotted to unlayered fabrics (Kennard & James, 1986).
Microbial mats are organosedimentary structures organized as multilayered carpets of microbial communities. Within the microbial mat microenvironment, the occurrence of different metabolic processes can lead to local chemistry alterations, inducing carbonate precipitation. Carbonate accretion in lithifying microbial mats typically induces microbialite formation, a poorly understood process, but studies have suggested that taxonomic composition of lithifying mats and their predominant metabolic pathways contribute. In contrast to lithifying mats, non‐lithifying mats, which do not form microbialites, can sporadically trap carbonate sand grains that are actively bound to the microbial mat through the production of extracellular polymeric substances. Both mat types occur in hypersaline lakes at Rottnest Island (Western Australia) and are currently under threat due to pollution and thus are being managed by the relevant government agency. Characterizing the microbial communities and functional genes of both mat types may help to develop strategies to better manage their ecosystem and elucidate microbialite formation processes. Metagenomics was used to compare the taxonomic and functional diversity of both mat types to determine whether differences in their taxonomy and functional capacity may influence their ability to form microbialites. Results revealed that both mat types harbor taxa (e.g., Firmicutes and Archaea), and functional genes (e.g., associated with photosynthesis, carbon, and sulfur cycles) that are known to play important roles in microbialite formation. This suggests that although non‐lithifying mats are not accreting carbonate, they have the potential to form microbialites. Further investigation is needed to determine whether environmental factors could be inhibiting carbonate precipitation within these mats.
The siliciclastic ~1 Ga‐old strata of the Torridon Group, Scotland, contain some of the most exquisitely preserved three‐dimensional organic‐walled microfossils (OWMs) of the Precambrian. A very diverse microfossil assemblage is hosted in a dominantly phosphatic and clay mineral matrix, within the Diabaig and the Cailleach Head (CH) Formations. In this study, we report on several microfossil taxa within the CH Formation (Leiosphaeridia minutissima, Leiosphaeridia crassa, Synsphaeridium spp. and Myxococcoides spp.) that include populations of cells containing an optically transparent and highly refringent mineral, here identified using electron microscopy as anatase (TiO2). Most anatase crystals occur entirely within individual cells, surrounded by unbroken carbonaceous walls. Rarely, an anatase crystal may protrude outside a cell, interpreted to correspond to zones where the cell wall had broken down prior to anatase precipitation. Where an anatase crystal entombs an organic intracellular inclusion (ICI), the ICI is large and well preserved. These combined observations indicate that the intracellular anatase is an authigenic sedimentary phase, making this the first report of in situ precipitated anatase intimately associated with microfossils. The ability of anatase to preserve relatively large volumes of intracellular and cell wall organic material in these cells suggests that the crystallisation of anatase entombed cellular contents particularly quickly, soon after the death of the cell. This is consistent with the strong affinity of Ti for organic material, the low solubility of TiO2, and reports of Ti occurring in living organisms. With the data currently available, we propose a mineralisation pathway for anatase involving Ti complexation with organic ligands within specific cells, leading to localised post‐mortem anatase nucleation inside these cells as the complexes broke down. Further overgrowth of the anatase crystals was likely fuelled by very early diagenetic mobilisation of Ti that had been bound to more labile organic material nearby in the sediments.
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