Seasonal temperature dependency of aquatic branched glycerol dialkyl glycerol tetraethers: A mesocosm approach

Abstract

BrGDGTs, membrane-spanning lipids produced by bacteria, are at the basis of the MBT’_5ME, a biomarker ratio that has been used as a paleotemperature proxy. However, the response of the MBT’_5ME to temperature changes, particularly in freshwater systems, remains incompletely understood. In this study, oxic mesocosms are used to assess the temperature sensitivity of brGDGTs and their producers, sampled from a lake (Lake Rot) and a river (Sihl River) in three different seasons. Three temperature treatments are employed (10, 17.5, and 25 ℃), representing control (in-situ temperatures), cooling, and/or warming treatments, with GDGTs and the bacterial community measured at several timepoints (24 h, 1, 2, 3 and 5 weeks). The control experiments showed that this experimental approach could not replicate natural conditions exactly, with small changes in chemistry (pH, conductivity, alkalinity) and bacterial community composition. Still, our mesocosm setup yielded valuable insights into the temperature-dependent production of lacustrine brGDGTs and MBT’_5ME values, especially in warming treatments, while no response was observed in cooling treatments, potentially indicating limited sensitivity to cold temperatures. In the river mesocosms not the MBT’_5ME but the IR ratio showed a temperature dependency, potentially driven by small changes in the water pH. Coeval changes in the composition of the bacterial community and the MBT’_5ME and IR are determined to constrain potential GDGT producers. Although an increase in MBT’_5ME in response to some warming incubations is observed, the temperature-sensitivity of MBT’_5ME, as expected from GDGT studies on a global scale, is not supported by this experiment.