Characterization of Rivularia-rich laminations in oncoids associated with pustular mats from the Laguna Negra lake (Argentina)

MLEWSKI, C; PISAPIA, C.; GOMEZ, F.J.; LECOURT, L.; SOTO RUEDA, E.; BENZERARA, K., ; MENEZ, B., ; JAMME, F., ; REFREGIERS, M.

Frontiers in Microbiology

Frontiers in Microbiology

Año: 2018

Stromatolites are organo-sedimentary structures that represent some of the oldestrecords of the early biosphere in our Planet. Cyanobacteria are considered as a maincomponent of the microbial mats that are supposed to produce stromatolite-likestructures. Understanding the role of Cyanobacteria and associated microbes on themineralization processes is critical to better understand what is preserved in the typicallaminated structure of stromatolites. Given that the rock record is affected by diagenesisthat can obliterate biosignatures preservation, studying stromatolite formation inmodern environments is an useful approach.Here we study carbonate precipitation within stromatolitic oncoids andassociated microbial mats in a high-altitude hypersaline lake (Laguna Negra,Catamarca, Argentina). In particular we focuses on understanding carbonateprecipitation in the Black Pustular (BP) Rivularia mats and the possible link with theRivularia-like micro-fossils preserved in the associated oncoids. With this goal wecombine Confocal Laser Scanning Microscopy (CLSM), Raman spectroscopy,Scanning Electronic Microscopy (SEM), Laser Microdissection and Sanger sequencing,Focused IonBeam (FIB), Fourier Transform InfraRed spectroscopy (FTIR) andSynchrotron analysis to analyze to what extent Rivularia cyanobacteria and theassociated microbial communities are involved in calcium carbonate precipitation andmineralization lending to biosignature preservation. Our study shows that carbonateprecipitation does not initiate directly on the Rivularia sheath but within the associatedmicrobial consortia formed by diatoms and diverse heterotrophic bacteria, particularlywhere aerobic anoxygenic phototrophic bacteria (AAnPB) of the Roseobacter clade areobserved. Ureolysis and denitrification metabolisms, common in these groups, can thusbe the triggers of carbonate precipitation. The presence of epiphytic bacteria located onthe Rivularia sheaths can locally induce acidic conditions and this would be anadvantage for Rivularia by precluding precipitation on their own sheaths. Withprogressive precipitation Rivularia gets passively entombed producing organic-richlaminae even when the process was not triggered by itself. This can have implicationsfor understanding Rivularia-like filaments preservation in the ancient geological record.