Spatially Resolved Genomic, Stable Isotopic, and Lipid Analyses of a Modern Freshwater Microbialite from Cuatro Ciénegas, Mexico

Abstract

Microbialites are biologically mediated carbonate deposits found in diverse environments worldwide. To explore the organisms and processes involved in microbialite formation, this study integrated genomic, lipid, and both organic and inorganic stable isotopic analyses to examine five discrete depth horizons spanning the surface 25 mm of a modern freshwater microbialite from Cuatro Ciénegas, Mexico. Distinct bacterial communities and geochemical signatures were observed in each microbialite layer. Photoautotrophic organisms accounted for approximately 65% of the sequences in the surface community and produced biomass with distinctive lipid biomarker and isotopic (d 13 C) signatures. This photoautotrophic biomass was efficiently degraded in the deeper layers by heterotrophic organisms, primarily sulfate-reducing proteobacteria. Two spatially distinct zones of carbonate precipitation were observed within the microbialite, with the first zone corresponding to the phototroph-dominated portion of the microbialite and the second zone associated with the presence of sulfatereducing heterotrophs. The coupling of photoautotrophic production, heterotrophic decomposition, and remineralization of organic matter led to the incorporation of a characteristic biogenic signature into the inorganic CaCO 3 matrix. Overall, spatially resolved multidisciplinary analyses of the microbialite enabled correlations to be made between the distribution of specific organisms, precipitation of carbonate, and preservation of unique lipid and isotopic geochemical signatures. These findings are critical for understanding the formation of modern microbialites and have implications for the interpretation of ancient microbialite records.

Metabolic potential of lithifying cyanobacteria-dominated thrombolitic mats

“…Similar trends have also been reported in the freshwater microbialites of Cuatros Ciénegas, Mexico (Nitti et al 2012). Results from these studies, which used a combined genomic, lipid and stable isotope analyses, also showed distinct spatial differences within the lithifying communities although no lithified layers were present (Nitti et al 2012) reinforcing the idea that the microbial communities can form discrete metabolic zones within unlaminated microbialite forming communities.…”

“…Similar trends have also been reported in the freshwater microbialites of Cuatros Ciénegas, Mexico (Nitti et al 2012). Results from these studies, which used a combined genomic, lipid and stable isotope analyses, also showed distinct spatial differences within the lithifying communities although no lithified layers were present (Nitti et al 2012) reinforcing the idea that the microbial communities can form discrete metabolic zones within unlaminated microbialite forming communities. Although the metagenomic survey correlated many of the specific pathways associated with the use of these different substrates, future work is required to delineate the specific expression of these molecular pathways to assess how the various metabolic activities are coordinated and regulated throughout the vertical profile of these lithifying microbial communities.…”

“…In microbialite forming mats the role of anaerobic respiration, specifically sulfate reduction, in promoting carbonate precipitation through remineralization has been well documented (Visscher et al 1998Paerl et al 2001;Baumgartner et al 2006;Nitti et al 2012;Gallagher et al 2013). Amplicon libraries of the 16S rRNA gene amplicon studies have previously shown the presence of several sulfate-reducing bacteria (SRB) mostly associated with the Desulfovibrionales in the thrombolitic mats (Mobberley et al 2012;Myshrall et al 2010).…”

Biogeochemistry of intertidal microbial mats from Qatar: New insights from organic matter characterisation

“…Boon et al, 1983;Boudou et al, 1986a,b;Grimalt et al, 1992;Bühring et al, 2009;Sherf and Rullkötter, 2009;Nitti et al, 2012;Pagès et al, 2014;Blumenberg et al, 2015). Lipid biomarkers were analysed for 5 out of the 7 sampled mat layers (Fig.…”

“…Shearman and Skipwith (1965) suggested that the mucilaginous jackets which surround most shallow-water carbonate grains could be the source of much of the kerogen in carbonate rocks. Although Nitti et al (2012) explored the vertical changes in the microbial community in lacustrine stromatolites, only a few studies have explored the vertical variation in bacterial and archaeal communities in zones of carbonate and evaporite precipitation (Kenig et al, 1990;Kendall et al, 2002). Kenig et al (1990), together with Baltzer et al (1994), analysed the OM in shallow-water lagoonal and intertidal sediments in Abu Dhabi.…”