Opening up the black box of marine phototroph-heterotroph interactions

Although marine microorganisms drive the major biogeochemical cycles in marine ecosystems, there is a dearth of information on interactions between phototrophic and heterotrophic organisms co-occurring in oceanic waters. The aim of this project was to study these interactions using Synechococcus sp. as the model phototroph – a cosmopolitan and highly abundant member of the picophytoplankton. Heterotrophic bacteria most-frequently present in non-axenic Synechococcus sp. cultures, were identified by PCR screening using primers targeting the 16S rRNA gene. Members of the Nitratireductor, Rhodobacteraceae, Muricauda and Phyllobacteriacae genera were present in more than half of all the cultures tested (Chapter 3). Using a member of the Rhodobacteraceae as the model heterotroph, specific metabolites present in axenic cultures and co-cultures were analysed (Chapter 4). Much lower concentrations of these specific metabolites were present in the milieu of Synechococcus – Roseobacter co-cultures compared to axenic Synechococcus cultures as discovered by LC-MS. Natural product database searches suggest that these may be a group of novel compounds. A Synechococcus sp. WH7803 null mutant in the gene encoding a type III polyketide synthase was constructed (Chapter 5). A targeted exometabolomic analysis showed a decreased production of the metabolites identified above in the mutant strain compared to the wild type. Growth was considerably affected in the T3 PKS mutant and T3 PKS mutant culture supernatants had a stronger negative growth effect on a range of picocyanobacteria and green algal species than the wild type extract. Further research is required to establish the precise biological function of the observed molecules, their biosynthetic pathway and their function in the natural environment. Improving our understanding of interactions between environmentally important microorganisms not only helps us to learn more about how biogeochemical cycles in the ocean function, but can also provide new natural products for use in the pharmaceutical industry