Variations in dissolved O2 in a Chinese lake drive changes in microbial communities and impact sedimentary GDGT distributions

The production of diverse brGDGTs by an Acidobacterium allows a direct test of temperature and pH controls on their distribution

“…However, strain Ellin6076 is an obligately aerobic bacteria, indicating that at least a portion of brGDGTs in peats, soils, and lakes can be produced by aerobic Acidobacteria. This is consistent with one order of magnitude higher rates of brGDGT production in incubations with surface peat under oxic conditions than anoxic deeper peats (27), as well as the occurrence of abundant in situ brGDGTs produced in oxic lake water columns (29).…”

Near-universal trends in brGDGT lipid distributions in nature

“…Drawing analogies to other bacterial membrane lipid classes, early studies posited that the empirical relationships between brGDGTs and environmental parameters were physiological adaptations of source organisms to variations in external temperature and pH (24). Subsequent studies have since shown that changes in brGDGT distributions are also accompanied by shifts in the composition of the associated microbial community, indicating that the empirical correlations between the lipids and environmental parameters may be indirect in nature (25)(26)(27)(28)(29). Whether the variation in a brGDGT paleorecord is due to the direct physiological response of microbes to a changing environment or the indirect restructuring of a microbial community has important implications for its interpretation and fidelity (28).…”

“…These discrepancies have led to independent modern calibrations for different sample types [e.g., soils and peats (36), lacustrine sediments (37) or SPM (38), bones (39), speleothems (40), and marine sediments (23)] and regions [e.g., East Africa (41) and China (42)] and have necessitated much effort to disentangle the allochthonous versus autochthonous sources of brGDGTs to sedimentary archives (2,23,(43)(44)(45)(46)(47)(48). Further complicating the matter, brGDGT distributions are affected by a wide array of other environmental parameters, including oxygen levels (26,27,29,(49)(50)(51)(52)(53)(54), salinity/electrical conductivity (55,56), seasonality (27,36,46,57), nutrient availability (58,59), and soil chemistry (28), which can mask or override relationships with temperature or pH. Last, an observed bias in brGDGT-derived temperatures toward warmer seasons has proven difficult to quantify, with various studies finding summer air temperature (60), the mean air temperature of months above freezing (MAF) (36,37,55), growing degree days above freezing (61), or other temperature indices to provide the strongest correlations in modern training sets.…”

Calibration and Application of Branched GDGTs to Tibetan Lake Sediments: The Influence of Temperature on the Fall of the Guge Kingdom in Western Tibet, China

“…In our calibration, the MBT′ 6Me depends on tetramethylated and 6‐methyl brGDGT production, which occurs primarily under oxic conditions (Weber et al., 2018; Wu et al., 2021) and therefore should be independent of lake‐mixing type and lake depth. In contrast, previous calibrations relied on the 5‐methyl brGDGTs which are also produced under anoxic conditions (Weber et al., 2018; Wu et al., 2021), and thus might be produced in higher abundance in the bottom water of seasonally stratified lakes during the summer months. As our new calibration includes tetramethylated and 6‐methyl brGDGTs, it should be suitable for a wider range of lake‐mixing types and therefore provide a more reliable MAAT reconstruction without seasonal bias.…”

“…DO concentration may be another important factor when reconstructing temperatures for lacustrine brGDGTs (Weber et al., 2018; Wu et al., 2021; Yao et al., 2020). Loomis et al.…”