The production of diverse brGDGTs by an Acidobacterium allows a direct test of temperature and pH controls on their distribution

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 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. Consequently, culture-derived paleoclimate indices are in agreement with the global soil-derived MBT5ME (methylation index of C5-methyl brGDGTs) proxy for temperature but not the CBT5ME (cyclization index of C5-methyl brGDGTs) proxy for pH. Our findings provide important insights from a physiological perspective into the underlying mechanism of brGDGT-based proxies. Significance StatementBranched glycerol dialkyl glycerol tetraethers (brGDGTs) are biomarkers widely used for the quantitative estimation of past climatic changes due to their ubiquitous occurrence in the environment and the relationships between their distributions and temperature and pH. However, the ecophysiology of brGDGT-producing bacteria and the mechanistic basis for brGDGT-based climate proxies remain unknown. Here, we identify a brGDGT-producing Acidobacterium and present a physiological study of brGDGTs in response to cultivation variables, which provides pivotal insights into how brGDGT producers modulate methylation and cyclization under different culturing conditions. Our study represents a significant advancement in understanding the physiological role of lipid structures in microbial adaptation and helps us interpret the relationships between brGDGT-based proxies and environmental conditions of the geological environment.