| Summary: | Microbes inhabiting surface waters of the Earth’s oceans are exquisitely adapted to their nutrient-poor environment. Marine phytoplankton, for example, are able to reduce their requirements for phosphorus by replacing membrane phospholipids with alternative non-phosphorus lipids. Heterotrophic bacteria, which can also thrive when phosphorus is scarce, had not, however, been shown to carry out this process – seemingly placing these organisms at a competitive disadvantage. In this thesis, I show that substitution of membrane phospholipids for a variety of non-phosphorus lipids is a conserved response to phosphorus deficiency amongst phylogenetically diverse marine heterotrophic bacteria. By deletion mutagenesis and complementation in the model marine bacterium Phaeobacter sp. MED193 and heterologous expression in recombinant Escherichia coli, I confirmed the roles of a phospholipase C (PlcP) and a glycosyltransferase in lipid remodelling. Analyses of two large collections of marine metagenomes, the Global Ocean Sampling (GOS) and Tara datasets, demonstrate that PlcP is particularly abundant in areas characterised by low phosphate concentrations. To better understand the lipids that potentially replace phospholipids during this remodeling process, I investigated a number of poorly-characterised aminolipids that are prevalent in the globally important marine Roseobacter group. I was able to identify two genes involved in the synthesis of one of these lipids, a glutamine lipid. Subsequent phylogenetic analysis of one of these genes revealed that the capacity to synthesise glutamine lipid appears to be virtually ubiquitous within the Roseobacter group. A further class of aminolipids was present in many Roseobacter strains, which I identified as a novel class of homotaurine-containing lipids using high-resolution, accurate mass spectrometry. These homotaurine lipids were detected in a battery of Roseobacter strains, enabling me to employ a comparative genomics approach to identify genes potentially involved in their biosynthesis. Surprisingly, neither of these aminolipids appeared to play an important role in the response to phosphorus limitation in the Roseobacter strains tested. Together, these results point to a key role for lipid substitution as an adaptive strategy enabling heterotrophic bacteria to thrive in vast, phosphorus-depleted areas of the ocean. Although phosphorus-free lipids play a crucial role in this process of adaptation, my work emphasises that many of these lipids are not simply substitutes for phospholipids but rather appear to have important roles in the cell in their own right.
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