Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - WISLAS (Warmed Icelandic Soils, Lipids and Sequencing: towards a better understood climate proxy)
Teaser
To improve climate models that predict the sensitivity of our climate system, we need reconstructions of past climates. To quantitatively reconstruct temperature changes in the past, chemical fossils are a recent addition to the climate scientist toolbox. Chemical fossils are...
Summary
To improve climate models that predict the sensitivity of our climate system, we need reconstructions of past climates. To quantitatively reconstruct temperature changes in the past, chemical fossils are a recent addition to the climate scientist toolbox. Chemical fossils are molecular components that carry information on the environment in which they were produced, sometimes millions of years ago. The distribution of a group of 15 membrane lipids in soils, for instance, depends on the temperature and pH of the soils in which these organisms are living. They are conserved in several types of geological archives (marine sediments, lake sediments, soils) and their distribution has been used to reconstruct changes in the temperature of the past. However, even the most recent temperature calibration is not accurate enough to reconstruct absolute temperatures. In this research project we aim to improve this thermometer of the past, with geochemical and microbiological research. The thermometer is based on bacterial membrane lipids, but at the moment it is still unclear which bacteria produce them. Previous studies have indicated that Acidobacteria are probably the source organisms of this set of branched tetraether lipids, but only 1 of the 15 compounds that are frequently encountered in soils, has been recovered from an Acidobacterial culture. To shed light on the abundance and variability of the source organisms in soils, the branched tetraether distribution and the bacterial diversity along local environmental gradients (ForHot soils, REPEAT soils, Ossekampen soils) will be analyzed. This research will allow to gain a better insight in the environmental factors that influence the branched tetraethers and their bacterial source organisms.
The research performed in the WISLAS project has resulted in exciting new insights. Because of the dominant effect of the bacterial community composition on the brGDGT lipids, the dependency of these compounds with the environment is not linear. Instead, the linear dependency with temperature observed on a global scale, on which the calibration was based, contains several environmental threshold values. With this knowledge a new type of model can be designed to reconstruct past changes in soil temperature. This research has resulted in 3 manuscripts (2 submitted, 1 draft), and also in three oral presentations at international conferences (IMOG 2017, Goldschmidt 2017, AGU 2018 (invited)).
Work performed
Firstly, the branched tetraethers that are currently used in the existing climate proxies were measured in a set of Icelandic soils that are warmed by subsoil thermal heating, that thus show a gradient in temperature. As these soils have been studied extensively in the framework of the Forhot project (www.forhot.is), the lipid distribution can be linked directly to the in-situ measured temperature. From this dataset it became clear that soil temperature exerts a non-linear effect on the branched tetraether lipids, caused by an abrupt change in the community composition of the bacteria. This threshold effect was also observed on a global scale. Following up on this, the effect of soil pH on the branched tetraether lipids and the bacterial community was tested using soils retrieved from a long-term fertilization experiment (The Netherlands). This dataset revealed that large changes in the pH of the soil result in different environmental dependencies of the branched tetraether lipids, affecting the thermometer results. To determine potential influences of oxygenation state, another master variable influencing soil chemistry, a third dataset was collected under the umbrella of the ERA-NET Cofunds BiodivERsA3 funded REPEAT project (https://www.moorwissen.de/en/moore/tools/projekte/repeat/team.php). The collection of this dataset is still underway.
This research has been presented in oral talks at 3 international conferences and will result in 3 publications in peer-reviewed journals. The training aspect of the fellowship was also successful. The PLECO group got expertise in lipid extraction and I helped with establishing a PLFA extraction lab at Antwerp University. I diversified my skillset as a scientist. Building up on these results I have been able to obtain competitive funding for a 5 year post-doctoral position at ETH Zurich.
Final results
The research performed has therefore resulted in unique insights on the environmental dependencies on branched tetraether lipids and their bacterial producers in soils. In this project, brGDGT and bacterial DNA data were obtained from 3 independent datasets: ForHot (Iceland), Ossekampen (The Netherlands), Repeat (EU). In contrast to what was known before the start of this project, the linear dependency with temperature contains threshold values (P1, submitted). This new insight has spun the direction of the project more towards soil with different chemistries, evaluating the effect of soil pH (P2, submitted) and soil redox conditions (P3, draft).
Our knowledge on the production of brGDGTs in different environments has vastly exceeded the state-of-the-art knowledge. This has direct implications for the use of the paleoclimate proxy, and will thus impact the interpretation of existing and new paleoclimate reconstructions.