A new model for silicification of cyanobacteria in Proterozoic tidal flats

Abstract

Microbial fossils preserved by early diagenetic chert provide a window into the Proterozoic biosphere, but seawater chemistry, microbial processes, and the interactions between microbes and the environment that contributed to this preservation are not well constrained. Here, we use fossilization experiments to explore the processes that preserve marine cyanobacterial biofilms by the precipitation of amorphous silica in a seawater medium that is analogous to Proterozoic seawater. These experiments demonstrate that the exceptional silicification of benthic marine cyanobacteria analogous to the oldest diagnostic cyanobacterial fossils requires interactions among extracellular polymeric substances (EPS), photosynthetically induced pH changes, magnesium cations (Mg2+), and >70 ppm silica.

Functional gene analysis and cultivation experiments predict the degradation of diverse extracellular polysaccharides by ubiquitous taxa in pustular microbial mats from Shark Bay, Western Australia

“…The elevated numbers of unique GT genes in the Cyanobacteria MAGs suggest that these cyanobacterial GTs could have a greater functional variety within their multifunctional families and/or greater functional redundancy than GTs in other organisms. Both interpretations are consistent with the cyanobacterial production of copious and complex EPS carbohydrates (18,19).…”

“…Planctomycetes were enriched in microbial communities grown on chondroitin sulfate and these communities clustered closest to the illuminated slurry in our PCoA. These observations support both the affinity of Planctomycetes for sulfated EPS and compositional similarities between the cyanobacterial EPS (18, 48) and chondroitin sulfate. Chondroitin sulfate, a sulfated animal polysaccharide that consists of an alternating chain of N-acetylglucosamine and glucuronic acid moieties decorated with sulfate groups, is not likely to be an abundant substrate in the mats because high salinity limits the animal community in the hypersaline Hamelin Pool (72).…”

“…CAZymes targeting other carbohydrates tended to be less numerous and more taxonomically restricted and appeared mainly in groups with high CAZyme counts and family diversity. Of particular interest, given the suggested roles in mineral precipitation (9,18,(38)(39)(40). The many anionic polysaccharide substrates predicted for CAZymes in our dataset.…”

“…Yet, the links between EPS degradation, carbonate precipitation, and microbial ecology in microbial mats remain poorly understood. Pustule-forming cyanobacteria, embedded in thick, multilayered envelopes, are the primary sources of sulfated and other EPS in Shark Bay pustular mats (18,19), but most organisms capable of degrading complex polysaccharides in EPS in these and other marine mats have not been identified. Bacteroidetes have been speculated to degrade HMW organic carbon in Shark Bay based on their role in other systems (14,15), but to our knowledge, their potential and actual contributions to polysaccharide degradation in hypersaline mats are yet to be demonstrated.…”

Biofilms as agents of Ediacara-style fossilization

“…These findings highlight that Ediacaran microbial mats not only played an integral ecological role in the development of complex Ediacara macrofaunal communities 32 , 70 but may have also contributed directly to Ediacara-style fossilization by serving as highly reactive loci for silica precipitation. We demonstrate that silica precipitation onto both biofilms and macroorganism tissues can easily occur under dissolved Si concentrations and pH values reconstructed for Ediacaran seawater 71 , and can even proceed after dissolved silica levels drop below the thermodynamic thresholds traditionally considered necessary to facilitate precipitation of amorphous silica 16 , 72 . The association of numerous Ediacara Biota fossil assemblages with textural and sedimentary evidence for widespread microbial mats indicates that this association may have been taphonomically, as well as ecologically, critical.…”