Evaluation and application of microbial membrane lipids as (paleo)environmental proxies in lacustrine settings – ALPINE

A better understanding of past climate variations and their interactions with geosphere and biosphere is essential to understand future climatic changes. Most of the available paleoenvironmental proxies were developed for and applied to oceanic environments. However, it is essential to develop new proxies applicable to continental environments to assess climatic variability over the continents and improve our understanding of past global environmental changes. Membrane lipids produced by certain microorganisms can be used to achieve this goal. Microorganisms are able to adjust their membrane composition in response to the prevailing environmental conditions to ensure the optimal state of the cellular membrane. Analysis of bacterially-produced branched GDGTs (brGDGTs) in soils, peats and lakes distributed worldwide showed that their chemical structure varies mainly with air temperature and to a lesser extent soil pH, making them increasingly used as paleoproxies since more than fifteen years. BrGDGTs are the only microbial organic proxies available for temperature reconstructions in both aquatic and terrestrial settings. Nevertheless, corresponding paleoenvironmental data have to be interpreted with care, as these compounds may have allochthonous and autochthonous sources in aquatic settings and brGDGT source microorganisms remain unknown. The development of environmental proxies independent and complementary to brGDGTs is crucial to improve the reliability and accuracy of continental paleoreconstructions.

The ambition of the ALPINE project is to propose a new environmental proxy applicable to lake environments, based on 3-hydroxy fatty acids (3-OH FAs), membrane lipids produced by Gram-negative bacteria. These organic compounds were recognized as potential temperature and pH proxies after their analysis in a large set of soils distributed worldwide. Nevertheless, the influence of environmental parameters on 3-OH FA distribution in lacustrine environments and the applicability of 3-OH FAs as temperature and pH proxies in such settings has not been investigated in detail yet, despite the high sensitivity of lacustrine archives as recorders of past environmental conditions. It is now essential to obtain accurate information on the adaptation of 3-OH FA source microorganisms to temperature/pH changes in lakes before potentially developing robust and universal (paleo)environmental proxies applicable to both aquatic and terrestrial settings.

The main objectives of this project will be (i) to investigate the applicability of 3-OH FAs as new temperature and pH proxies in lakes and (ii) to concomitantly constrain the limits and conditions of use of existing GDGT-based proxies in such settings. To this aim, the source(s) of microbial lipids in lakes will first be assessed. Then, the effect of temperature and pH on lacustrine microorganisms and their membrane lipids will be investigated by combining field and laboratory experiments – lake sediment microcosm incubations with 13C- and 2H-labeled substrates and isolation cultures. The degradability of 3-OH FAs in lacustrine sediments under biotic and abiotic conditions will be concomitantly examined. We then envision to develop calibrations between temperature/pH and distribution of microbial lipids in sediments from lacustrine sediments collected worldwide. Last, these calibrations will be applied to long-term paleoenvironmental reconstructions from lacustrine cores.

This interdisciplinary project represents a unique opportunity to gather together researchers with complementary expertise for the development of an environmental tool. It will be based on an integrated approach coupling state-of-the-art organic geochemistry molecular biology, electron microscopy, and isotope techniques, applied to samples from present and past times.