Environmental indications and controls of bacterial membrane lipids in a tropical peatland from Panama

• Bacterial biomarkers to trace environmental changes are abundant in tropical peats. • Hypothesised brGDGT precursor, iso -DA −only detected in upper ∼60 cm of tropical peat. • Occurrence of iso -DA might be influenced by the presence of oxygen in these settings. Lipid biomarkers are widely used to trace microbial activity and environmental changes, yet gaps remain in our understanding of their environmental occurrence, including in tropical peatlands. We analyzed the bacterial lipid abundance in a series of 2 m long peat cores from the Republic of Panama, focusing on the diether glycerol with C 15:0 n -alkyl chains (C 15:0 DEG), 13,16-dimethyl octacosanedioic acid ( iso -diabolic acid: iso -DA), and branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) and combined these with novel genomic data from the same cores. In addition to understanding these compounds’ environmental occurrence, we present the first environmental insights into the co-occurrence of brGDGT-Ia with its potential precursors C 15:0 DEGs and iso -DA. Our results show that iso -DA is found only in the occasionally dry top peat layer and is absent in water-saturated peat horizons. We suggest that the production of iso -DA in this peatland dependents on oxygen availability, likely due to the metabolism of the source organisms and consistent with limited available culture data of acidobacteria. In contrast, C 15:0 DEGs and brGDGTs (mainly in the form of brGDGT-Ia) persist across all depths with increasing concentrations at greater depths. Although these contrasting trends in abundance could reflect the greater persistence of ether lipids relative to acyl/acid lipids due to differences in preservation rates, they could also suggest that iso -DA is not a major precursor in the biosynthetic pathway of brGDGTs in these settings. When combined with novel genomic data, we show that acidobacteria, one of the few known iso -DA and brGDGT producers, were detected at all sites and depths, forming a stable proportion of the prokaryotic community. Their consistent presence aligns more with the brGDGT and C 15:0 DEG distribution than iso -DA. Combining the results from our study with experimental evidence, it appears that multiple pathways for brGDGT biosynthesis exist in tropical peats, with the phylogenetic and environmental control on their expression yet to be elucidated.