Characterization of Pustular Mats and Related Rivularia-Rich Laminations in Oncoids From the Laguna Negra Lake (Argentina)

Abstract

Stromatolites are organo-sedimentary structures that represent some of the oldest records of the early biosphere on Earth. Cyanobacteria are considered as a main component of the microbial mats that are supposed to produce stromatolite-like structures. Understanding the role of cyanobacteria and associated microorganisms on the mineralization processes is critical to better understand what can be preserved in the laminated structure of stromatolites. Laguna Negra (Catamarca, Argentina), a high-altitude hypersaline lake where stromatolites are currently formed, is considered as an analog environment of early Earth. This study aimed at characterizing carbonate precipitation within microbial mats and associated oncoids in Laguna Negra. In particular, we focused on carbonated black pustular mats. By combining Confocal Laser Scanning Microscopy, Scanning Electron Microscopy, Laser Microdissection and Whole Genome Amplification, Cloning and Sanger sequencing, and Focused Ion Beam milling for Transmission Electron Microscopy, we showed that carbonate precipitation did not directly initiate on the sheaths of cyanobacterial Rivularia, which dominate in the mat. It occurred via organo-mineralization processes within a large EPS matrix excreted by the diverse microbial consortium associated with Rivularia where diatoms and anoxygenic phototrophic bacteria were particularly abundant. By structuring a large microbial consortium, Rivularia should then favor the formation of organic-rich laminations of carbonates that can be preserved in stromatolites. By using Fourier Transform Infrared spectroscopy and Synchrotron-based deep UV fluorescence imaging, we compared laminations rich in structures resembling Rivularia to putatively chemically-precipitated laminations in oncoids associated with the mats. We showed that they presented a different mineralogy jointly with a higher content in organic remnants, hence providing some criteria of biogenicity to be searched for in the fossil record.

Mineralized microbialites as archives of environmental evolution, Laguna Negra, Catamarca Province, Argentina

“…Although redox zonation of the lake sediment was not confirmed with pore water geochemistry during this study, the distinct smell of hydrogen sulfide is evidence that lake sediments are anoxic and likely host a metabolically active sulfate‐reducing community. The presence of sulfate‐reducing micro‐organisms within mat communities of Laguna Negra has been confirmed previously with 16S rRNA gene libraries (Mlewski et al., ).…”

“…Gomez et al, 2014), with very few occurrences of trapping and binding of detrital sediment. Microfabrics are summarized below and are consistent with the detailed descriptions in Gomez et al (2014) and Mlewski et al (2018)…”

“…() and Mlewski et al. (). A complex suite of petrographic microfabrics ranges from irregular micritic textures to regular isopachous fabrics.…”

“…The upper 1–3 mm of the mats, as revealed through microscopy, is dominated by a consortia of cyanobacteria, as well as pennate and centric diatoms. 16S rRNA gene sequence analysis shows that the undermat is composed of purple and green sulfur bacteria, and that the final bottom layer is a thick black horizon dominated by microbial sulfur reducers (Gomez, Mlewski, Boidi, Farías, & Gérard, ; Mlewski et al., ). In Zone 3 (Figure c,d), hydrological mixing occurs between brackish inlet waters and saline lake waters.…”

Biotic–Abiotic Influences on Modern Ca–Si-Rich Hydrothermal Spring Mounds of the Pastos Grandes Volcanic Caldera (Bolivia)

“…Black pustular mats are composed of bacterial filaments and associated coccoids. No specific characterization is available for microbial communities in Pastos Grandes, but the black pustular mats show similarities with those described at Laguna Negra in Argentina [66,67]. Diatoms are accumulated and associated with slime (EOM and organic matter) in shallow waters (>2 cm deep), and their frustules are observed in every active zone of the hydrothermal system.…”