Lake sediments are important archives of continental climate history, and their lipid biomarker content can be exploited to reconstruct paleoenvironmental conditions. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids widely used in paleoclimate studies to reconstruct past temperature. However, major gaps still exist in our understanding of the environmental controls on in situ (i.e. aquatic) production in lake systems. In Lake Chala, a permanently stratified tropical crater lake in East Africa, we determined the concentrations and fractional abundances of individual brGDGTs along depth profiles of suspended particulate matter collected monthly from September 2013 to January 2015 and in settling particles collected monthly at 35 m water depth from August 2010 to January 2015 and compared these brGDGT distributions with those in surficial lake bottom sediments and catchment soils. We find that brGDGTs are primarily produced within the water column and that their concentrations and distributions vary greatly with depth and over time. Comparison with concentration–depth profiles of the monthly distribution and abundance of bacterial taxa, based on 16S rRNA gene amplicon sequencing and quantification, indicates that Acidobacteria are likely not the main producers of brGDGTs in Lake Chala. Shallowing of the oxic–anoxic boundary during seasonal episodes of strong water column stratification promoted production of specific brGDGTs in the anoxic zone. BrGDGT distributions in the water column do not consistently relate with temperature, pH, or dissolved-oxygen concentration but do respond to transitions between episodes of strong stratification and deep (but partial) lake mixing, as does the aquatic bacterial community. Hence, the general link between brGDGT distributions and temperature in brGDGT-based paleothermometry is more likely driven by a change in bacterial community composition than by membrane adaptation of specific members of the bacterial community to changing environmental conditions. Although temperature is not the principal driver of distributional changes in aquatic brGDGTs in this system, at least not during the 17-month study period, abundance-weighted and time-integrated averages of brGDGT fractional abundance in the 53-month time series of settling particles reveal systematic variability over longer timescales that indirectly relates to temperature. Thus, although we do not as yet fully understand the drivers of modern-day brGDGT fluxes and distributions in Lake Chala, our data do support the application of brGDGT paleothermometry to time-integrated archives such as sediments.
Highlights.

• BrGDGTs in the tropical African lake Chala are produced in situ.
• Acidobacteria are not the dominant source of aquatic brGDGTs.
• Stratification and mixing drive aquatic brGDGT production and their signature.

中文翻译：

---

永久分层非洲火山口湖中brGDGT的季节性变化和来源

湖泊沉积物是大陆性气候历史的重要档案，其脂质生物标志物含量可用于重建古环境条件。支链甘油二烷基甘油四醚（brGDGTs）是广泛用于古气候研究以重建过去温度的细菌膜脂质。但是，在我们对湖泊系统原位（即水生）生产的环境控制的理解上，仍然存在重大差距。在东非一个永久分层的热带火山口湖查拉湖中，我们确定了从2013年9月至2015年1月每月收集的悬浮颗粒物的深度分布以及在从2010年8月至2015年1月水深35 m每月收集的颗粒沉降过程中各个brGDGT的浓度和分数丰度，并将这些brGDGT分布与表层湖底沉积物和集水土壤。我们发现brGDGT主要在水柱内产生，其浓度和分布随深度和时间变化很大。根据16S rRNA基因扩增子的测序和定量结果，与每月菌群的每月分布和丰富度的浓度-深度分布图进行比较，表明酸性细菌可能不是Chala湖中brGDGT的主要生产者。在强水柱分层的季节性事件中，有氧-缺氧边界的变浅促进了缺氧区特定brGDGTs的产生。在水柱中的BrGDGT分布与温度，pH或溶解氧浓度并不总是相关的，但是确实响应于强分层和深（但部分）湖水混合之间的过渡，水生细菌群落也是如此。因此，基于brGDGT的古温度计中brGDGT分布与温度之间的一般联系更可能是由细菌群落组成的变化驱动的，而不是由细菌群落特定成员对环境条件的膜适应所驱动的。尽管温度不是该系统中水生brGDGTs分布变化的主要驱动因素，至少在17个月的研究期间内，沉降颗粒在53个月的时间序列中brGDGT分数的丰度加权和时间积分平均值显示出与温度间接相关的较长时间范围内的系统变异性。因此，尽管我们尚未完全了解Chala湖中现代brGDGT通量和分布的驱动因素，但我们的数据确实支持将brGDGT古温度计用于时间积分档案（如沉积物）。
强调。

• 热带非洲查拉湖中的BrGDGT是原位生产的。
• 酸性细菌不是水生brGDGT的主要来源。
• 分层和混合驱动水生brGDGT的生产及其特征。