Lacustrine brGDGT response to microcosm and mesocosm incubations

Branched dialkyl glycerol tetraethers (brGDGTs) are increasingly used as a paleotemperature proxy, yet the microbial producers of these compounds and the way they respond to environmental perturbations remain poorly understood. In this work, we used mesocosms to track the evolution of brGDGTs over several months, as well as an array of microcosms to study microbial brGDGT response to oxygen availability and carbon source. Overall our results confirm that brGDGTs are produced in the water column and respond to environmental perturbation. brGDGT concentrations increase in response to low oxygen, in agreement with empirical observations, but respond to added carbon only when ambient nutrients are low. Slow adjustment to temperature changes from the field to the laboratory appears to have influenced the relative methylation and isomerization of brGDGTs in the mesocosms. Overall our work is consistent with the inference that the producers are facultative aerobic heterotrophic organisms. Our results demonstrate that the use of micro- and mesocosms is a viable option to study brGDGTs in real time and help better understand this biomarker as a paleoproxy.