Microbial glycerolipids in the global ocean: environmental controls and sinking flux dynamics

Henry Cameron Holm, Ph.D., 2024
Benjamin Van Mooy, Advisor

The composition of glycerolipids from a range of ocean environments were analyzed to better understand the biochemical adaptations and remineralization processes of planktonic biomass. Glycerolipids compose the cell membranes and energy stores of ocean microorganisms. As a biochemical group they present a useful target for tracking a wide range of environmental stress responses. In Chapter 2 of this work, I employ high resolution mass spectrometry to assemble a global dataset of planktonic lipidomes. Using this dataset of samples mostly from the epipelagic, I find that water temperature is a major controlling factor on fatty acid saturation state and hypothesize this derives from known biophysical constraints on cell membranes. I extend this analysis further to investigate links between long-chain essential fatty acids (LCEFA) and temperature. My analysis shows that the abundance of one of these LCEFAs (eicosapentaenoic acid, 20:5 n-3) is well correlated with temperature. Using this relationship, I project temperature-based declines in its abundance due to climate change over the next century. In Chapter 3, using samples from the West Antarctic peninsula, I assess the pigments and glycerolipid compositions of microbial communities within seasonal pack ice floes to gain insights on growth in an extremophile environment. The caloric content of this glycerolipid biomass is additionally calculated to assess its value to higher trophic level consumers within the sea ice ecosystem. I find that glycerolipids’ physical distribution within sea ice core to be calorically dense but highly geographically and physically heterogenous. I additionally show evidence of a new biomarker (fatty acid hydroxy fatty acid triacylglycerols, FAHFA-TAG) which have not been detected before in an ocean system. I further evaluate the possible biochemical sources of these novel biomarkers within sea ice. Lastly, I examine the glycerolipids present in sinking material from two separate studies of carbon flux in the upper mesopelagic. These compositions revealed both sources and remineralization processes influencing lipid carbon. I find evidence that lipid biomass is mostly lost non-selectively from particles, differing from previous reports of selective degradation of lipids. However, I find compositional differences between surface-suspended and sinking material possibly pointing to selection mechanisms at sinking material conception. I use glycerolipids to create a new source metric predicting the relative amounts of sinking material derived from fecal pellets verse aggregates. The findings of this thesis extend our understanding of microbial biochemical adaptations to their environment and reveal controlling factors on glycerolipid cycling in the ocean system.