Direct Evidence of 6-methyl Branched Tetraether Production in Freshwater

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids produced by bacteria in soils and peats worldwide. The distribution of brGDGTs relates to ambient environmental conditions, and the changes in their downcore distributions preserved in coastal marine sediments are used to reconstruct past temperatures of adjacent river catchments. However, brGDGT signatures in lakes and rivers deviating from those in soils suggest in situ production of brGDGTs, which can alter the original soil signal. In this study, we incubated river water with and without soils with ¹³C-labeled glucose addition to identify characteristics of brGDGTs produced in situ. We observe that the brGDGT signature shifts from more 5-methyl to more 6-methyl brGDGTs in river water, while the river water with soil remains dominated by 5-methyl brGDGTs during the incubation. Furthermore, the 6-methyl brGDGTs in river water show an uptake of ¹³C-labeled glucose added to the experiment, providing direct evidence of their production in an aquatic environment. However, label uptake by brGDGTs in experiments with soil addition was minimal, suggesting that brGDGTs produced in aquatic environments only alter the initial soil-derived brGDGT signal at times when soil input is relatively low.