Bacteriohopanepolyols in a stratified cyanobacterial mat from Kiritimati (Christmas Island, Kiribati)

Bacteriohopanepolyols (BHPs) are lipids of distinct bacterial groups and are, along with geohopanoids as their diagenetic products, ubiquitous in microbial mats, sediments, soils and oil. Among BHP-producing bacteria, Cyanobacteria are of special interest since occurrences of C-2 methylated and other geohopanoids are interpreted as signals of oxygenic photoautotrophs in the early oceans. However, many questions, with respect to the source and function of hopanoids, remain open. Cyanobacterial mats are complex systems in terms of biogeochemical zones and hence, harbor phylogenetically and metabolically diverse photoautotrophic and heterotrophic microorganisms. Whereas Cyanobacteria are key microbial players for carbon fixation in these mat systems, most other bacterial and archaeal groups feed on their metabolic products and/or perform anoxygenic photosynthesis in deeper low-light regimes. e analyzed the abundance and distribution of BHPs in distinct layers of a stratified mat from a hypersaline lake on Kiritimati. Suites of BHPs were observed, with the majority probably originating from (proteo-)bacteria thriving in the deeper layers of the mat and not in upper layers where mat-forming Cyanobacteria are prevalent. Interestingly, the BHPs in the deepest layer include C-2 methylated structures, which likely do not originate from Cyanobacteria, but rather from α-Proteobacteria thriving at the redoxcline of this stratified microbial system.