The production of diverse brGDGTs by an Acidobacterium providing a physiological basis for paleoclimate proxies

Microbial lipid biomarkers preserved in geological archives can be used to explore past climate changes. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are unique bacterial biomarkers that have been used as molecular tools for the quantitative determination of terrestrial temperatures and the pH of depositional environments over a range of geological timescales. However, the exact biological source organisms – especially of the entire suite of brGDGTs found in the environment – remains unclear; by extension, so do the mechanisms that govern these proxies. Here, we identified a brGDGT-producing strain Candidatus Solibacter usitatus Ellin6076, by identifying archaeal tetraether synthase homologs in bacterial genomes. This strain synthesizes diverse brGDGTs, including regular C5-methylated and cyclic brGDGTs, and brGDGTs comprise up to 66 % of the major quantified lipids, far exceeding the proportions found in previous studies. The degree of C5-methylation in cultured strain Ellin6076 is primarily determined by temperature, whereas cyclization appears to be influenced by multiple factors, such as temperature, pH and oxygen availability. Consequently, culture-derived paleoclimate indices are in agreement with the global soil-derived MBT’5ME (methylation index of C5-methyl brGDGTs) proxy for temperature, albeit with a differed slope, but not the CBT5ME (cyclization index of C5-methyl brGDGTs) proxy for pH. Our findings provide insights from a physiological perspective into the underlying mechanism of brGDGT-based proxies.