Environmental controls on the distribution of bacterial membrane lipids

Abstract

Since their discovery in ancient sediments, hopanes and their biological precursors, bacteriohopanepolyols (BHPs), have been of great interest for their potential to serve as proxies for bacteria in the geological record. The validation of these and other biomarkers have implications for understanding the coevolution of organisms and the environment throughout Earth's history. 2-Methylhopanoids are of particular interest because their occurrence may be confined to cyanobacteria and alphaproteobacteria. Similarly, a stereoisomer of bacteriohopanetetrol (BHT), BHT II, has been identified exclusively in anaerobic ammonium oxidation (anammox) bacteria. However, the interpretation of sedimentary hopanoids is presently limited by an incomplete understanding of their phylogenetic associations, biological functions, and spatial and temporal disposition throughout diverse environments. I address some of these shortcomings through lipid biomarker characterization of water column and benthic microbial mat samples collected across geochemical and physiochemical gradients in the Eastern Tropical South Pacific oxygen minimum zone off the coast of northern Chile and in two ice-covered lakes of the McMurdo Dry Valleys, Antarctica. The results will enhance our ability to interpret the molecular fossil record, particularly those biomarkers that might preserve evidence of ancient marine and glacial environments. I will provide evidence that supports the use of BHT II as a biomarker for suboxia/anoxia, confirm the biosynthesis of 2- MeBHT in cyanobacteria as a response to photosynthetic stress, and describe improved methods that allow us to better detect, quantify, and interpret these markers in modern environments.