Prokaryotic and Eukaryotic Community Structure in Field and Cultured Microbialites from the Alkaline Lake Alchichica (Mexico)

The geomicrobiology of crater lake microbialites remains largely unknown despite their evolutionary interest due to their resemblance to some Archaean analogs in the dominance of in situ carbonate precipitation over accretion. Here, we studied the diversity of archaea, bacteria and protists in microbialites of the alkaline Lake Alchichica from both field samples collected along a depth gradient (0–14 m depth) and long-term-maintained laboratory aquaria. Using small subunit (SSU) rRNA gene libraries and fingerprinting methods, we detected a wide diversity of bacteria and protists contrasting with a minor fraction of archaea. Oxygenic photosynthesizers were dominated by cyanobacteria, green algae and diatoms. Cyanobacterial diversity varied with depth, Oscillatoriales dominating shallow and intermediate microbialites and Pleurocapsales the deepest samples. The early-branching Gloeobacterales represented significant proportions in aquaria microbialites. Anoxygenic photosynthesizers were also diverse, comprising members of Alphaproteobacteria and Chloroflexi. Although photosynthetic microorganisms dominated in biomass, heterotrophic lineages were more diverse. We detected members of up to 21 bacterial phyla or candidate divisions, including lineages possibly involved in microbialite formation, such as sulfate-reducing Deltaproteobacteria but also Firmicutes and very diverse taxa likely able to degrade complex polymeric substances, such as Planctomycetales, Bacteroidetes and Verrucomicrobia. Heterotrophic eukaryotes were dominated by Fungi (including members of the basal Rozellida or Cryptomycota), Choanoflagellida, Nucleariida, Amoebozoa, Alveolata and Stramenopiles. The diversity and relative abundance of many eukaryotic lineages suggest an unforeseen role for protists in microbialite ecology. Many lineages from lake microbialites were successfully maintained in aquaria. Interestingly, the diversity detected in aquarium microbialites was higher than in field samples, possibly due to more stable and favorable laboratory conditions. The maintenance of highly diverse natural microbialites in laboratory aquaria holds promise to study the role of different metabolisms in the formation of these structures under controlled conditions

Relative Fall of Holocene Sea-Level and Coastal Progradation, Northeastern Spencer Gulf, South-Australia

The levels of intertidal environments of prograding Holocene coastal complexes of northeast Spencer Gulf have been precisely measured and their relation with the tidal range determined. Beach ridges and the top of subtidal Posidonia sea-grass deposits are the best indicators of relative sea level. C dating has been carried out on mollusc shells recovered from beach ridges and the base of the regressive intertidal facies above the subtidal sea-grass facies. A Holocene sea-level history has been constructed. A contributing cause of relative movement of sea level in the region may be continuing tectonic uplift. -from Autho

Contrasting Marginal Sediments of a Seasonally Flooded Saline Lake - Lake Eliza, South-Australia - Significance for Oil-Shale Genesis

Lake Eliza is a hypersaline coastal lake in a region of winter rainfall and summer drought. Two areas of the lake margin exposed in summer were studied. The western protected shore is an area of fine carbonate sediments with high organic content. The eastern, exposed shore is an area of moderately sorted quartz-carbonate sand of lower organic content. The sediments of Lake Eliza are similar to some of those described from the Wilkins Peak Member of the Green River Formation, USA, and a comparison suggests that the lamosite oil shales of the type found in the Green River Formation may not have been deposited on a fresh to brackish lake floor as has been supposed.-from Author