BrGDGTs, membrane-spanning lipids produced by bacteria, are at the basis of the MBT’, a biomarker ratio that has been used as a paleotemperature proxy. However, the response of the MBT’ 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’ 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’ 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’ and IR are determined to constrain potential GDGT producers. Although an increase in MBT’ in response to some warming incubations is observed, the temperature-sensitivity of MBT’, as expected from GDGT studies on a global scale, is not supported by this experiment.

中文翻译：

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水生支化甘油二烷基甘油四醚的季节温度依赖性：介观方法

BrGDGT 是细菌产生的跨膜脂质，是 MBT' 的基础，MBT' 是一种生物标志物比率，已被用作古温度替代指标。然而，MBT 对温度变化的响应，特别是在淡水系统中，仍不完全清楚。在本研究中，使用含氧中生态系统评估 brGDGT 及其生产者的温度敏感性，在三个不同季节从湖泊（罗特湖）和河流（锡尔河）取样。采用三种温度处理（10、17.5 和 25℃），分别代表对照（原位温度）、冷却和/或升温处理，并在几个时间点（24 小时、1、2、24 小时）测量 GDGT 和细菌群落。 3 周和 5 周）。对照实验表明，这种实验方法无法准确复制自然条件，化学成分（pH、电导率、碱度）和细菌群落组成的变化很小。尽管如此，我们的中生态系统对湖泊 brGDGT 和 MBT' 值的温度依赖性产生产生了有价值的见解，特别是在变暖处理中，而在冷却处理中没有观察到任何反应，这可能表明对低温的敏感性有限。在河流中生态系统中，IR 比而非 MBT' 显示出温度依赖性，这可能是由水 pH 值的微小变化驱动的。细菌群落组成以及 MBT' 和 IR 的同期变化被确定为限制潜在的 GDGT 生产者。尽管观察到 MBT' 因某些变暖的孵化而增加，但该实验并不支持全球范围内 GDGT 研究预期的 MBT' 的温度敏感性。