Distinct sources of bacterial branched GMGTs in the Godavari River basin (India) and Bay of Bengal sediments

Branched glycerol monoalkyl glycerol tetraethers (brGMGTs) are a group of membrane-spanning lipids produced by (yet) unidentified bacteria. They are characterized by a C-C bond connecting the two alkyl chains, which is thought to enhance membrane stability at higher temperatures. So far, they have been found in peats, lakes, and marine sediments, where their abundance relative to that of branched glycerol dialkyl glycerol tetraethers (brGDGTs) increases with temperature. However, the preferred niche(s) for production and the origin of brGMGTs in the terrestrial and marine realm remain unknown. Here we explore the occurrence of brGMGTs in soils, suspended particulate matter (SPM) and riverbed sediments in the Godavari River basin, and compare these with brGMGTs in a Holocene sediment core from the Bay of Bengal close to the Godavari River mouth. BrGMGTs are mostly detected in agricultural and/or regularly inundated soils, and in the river at sites with standing water and/or agricultural/wastewater effluents, as well as in the delta, where low oxygen conditions and/or high nutrient levels prevail. In contrast, brGMGTs are continuously present in the marine sediment core, but with a different isomeric composition than in the terrestrial realm, indicating a primarily marine source. The stable brGMGT distribution downcore and brGMGT-inferred temperature estimates which resemble the bottom water temperature, may suggest marine brGMGT production in the deep water column and/or sediments. However, to establish the proxy potential of brGMGTs for paleoreconstructions in the terrestrial and marine realm, brGMGT sources and environmental controls require further study.