Sources and genetic controls of lipid biomarkers involved in paleotemperature proxies

Abstract

Lipids in marine sediments have been widely used in proxies to reconstruct Sea Surface Temperatures (SSTs). For example, the U_37^K proxy, based on the degree of unsaturation of long chain alkenones synthesized by haptophytes, the TEX86, based on glycerol dialkyl glycerol tetraethers (GDGTs) produced by Thaumarchaeota and the LDI, based on long chain diols by Eustigmatophyte algae. However, there is still some uncertainty regarding the biological producers of these lipids and their biosynthetic pathways. In this thesis, the pelagic and benthic archaeal community composition was investigated in multiple marine regions and compared it with the composition of GDGTs. This showed that Marine Group II Euryarchaeota, one of the most abundant marine archaeal groups, are incapable of producing these lipids. Studying three water columns in the Mediterranean Sea indicated the presence of deep-water Thaumarchaeota, which are potentially responsible for the overestimation of reconstructed SSTs in this region. Further, the biosynthetic pathway of long chain alkenones was investigated in a culture of the Haptophyte Emiliania huxleyi and identified a potential gene coding for the desaturase responsible for transforming C37:2 into C37:3 alkenones. In the African Lake Challa, a correlation was observed between the abundance of LCDs and uncultured Eustigmatophyceae algae. Besides, seasonal changes in LCD composition suggest multiple producers or changes in the LCD composition of the same producer within an annual cycle. The combination of omic (lipid and DNA) approaches in this thesis has proven to be key in increasing our knowledge on the biological producers of lipids involved in paleotemperature proxies.