Climate Models
What drives the distribution of chemical fossils? Influence of microbial community dynamics on bacterial membrane lipid signatures. [MiCoDy-Lipids]
Reconstructing past continental temperatures is essential for developing climate models that can accurately predict temperature changes expected in the future. BrGDGTs (Branched Glycerol Dialkyl Glycerol Tetraethers) are bacterial membrane-spanning lipids preserved in a wide variety of geological archives (e.g. lake sediments, marine sediments, paleosoils) that find an application as climate (temperature) and environmental (pH) proxies. Using calibrations that are based on modern settings (soils or lakes), these lipid biomarkers allow the quantitative reconstruction of temperature and pH over time.
However, it remains unclear which mechanism underpins their observed variation with climatic gradients in modern environments, i.e. whether the lipids follow a shift in the community composition of their bacterial producers (indirect environmental effect), or whether the cell membrane composition varies directly with the environment (direct environmental effect). My recent work shows that nonlinear shifts in the microbial community composition determine a large part of the brGDGT temperature sensitivity along a temperature gradient in soils. Furthermore, the lipid fingerprint from different communities was shown to react in a separate way to the environment. Accounting for the community effect (indirect effect) thus allow to determine which environmental factors have a direct influence on the brGDGT lipids.
BrGDGT lipids produced in freshwater environments have been shown to follow a different dependency to environmental factors (temperature, pH, lake depth), compared to the brGDGTs produced in soils. Because of this, different calibrations were developed for brGDGTs produced in soils or in lakes. In addition, brGDGTs found in lake sedimentary archives that reflect a mixture of soil and lacustrine sources cannot be used to reconstruct changes in past temperatures. I propose to solve this problem by studying the brGDGTs variation encountered on a watershed scale (i.e. soils with a different soil water moisture content, rivers, lakes water column and lake sediments).
I will determine the influence of indirect environmental effect, caused by shifts in the bacterial community composition and separate this from the direct environmental effect, the physiological plasticity of the cell membrane. Sampling at high spatial (watershed soils, lacustrine sediments) and temporal (river and lake water) scale in two Swiss watersheds (Sihl River and Lake Rot) will allow tracing those environmental changes that are known to influence the composition of the bacterial community (i.e. type and concentration of cations and anions, nutrients, alkalinity and pH, oxygen content, organic carbon source). BrGDGT production and conservation will be determined in sediment cores from 4 lakes. The bacterial community composition will be determined by Illumina Miseq analysis of the 16S rDNA fragment. The fossil and the recently produced brGDGT lipids (core and intact polar lipids respectively) will be analyzed using HPLC-MS.
The proposed research will significantly improve our understanding of the environmental controls on brGDGT production, and the influence of the community composition. By accounting for the indirect environmental effect, I will aim to construct a “universal brGDGT calibration”, targeting the environmental parameters temperature, pH, and moisture, that will be valid for brGDGTs produced in soils, rivers and lakes. With this, I will increase the confidence in and accuracy of continental paleoclimate reconstructions, both within Switzerland and on a global scale.