Diatoms, a major group of marine and freshwater microalgae, are one of the most successful phototrophic organisms in the oceans. Their ubiquity and abundance pins them as important drivers of marine biogeochemical cycles and significant components of marine food webs. Diatoms exist within a complex web of interactions with other microbes, including equally ubiquitous marine bacteria. Interkingdom interactions between diatoms and bacteria are both diverse and numerous, and they underpin the success of diatoms through their resulting impacts on diatom physiology. Interactions between diatoms and antagonistic bacteria are increasingly recognised to significantly impact diatom growth and physiology, and potentially represent important biotic drivers of diatom bloom dynamics. Nevertheless, our understanding of the diversity, seasonal trends and regulation of antagonistic diatom-bacteria interactions in marine ecosystems remains a significant knowledge gap. This thesis set out to tackle this knowledge gap through a combination of laboratory and field-based approaches. In doing so, a robust environmental sampling pipeline was developed to systematically assess the diversity and seasonal trends of diatom-antagonistic bacteria in a highly productive ecosystem, the Western English Channel, utilising a range of bloom-forming diatom species. The result was a library of 18 potential diatom-antagonistic bacteria, abundance of which peaked upon the termination of a winter diatom bloom. Seven isolates were subsequently confirmed to exhibit significant growth inhibitory effects against a range of diatoms. However antagonistic behaviour was only observed when pre-cultivated on media derived of diatom necromass, highlighting cryptic antagonistic behaviour that is conserved across diverse bacterial lineages. Finally, the seasonal dynamics and global biogeography of confirmed antagonistic bacteria was assessed through metabarcoding, revealing confirmed antagonistic bacteria to co-occur with important bloom-forming diatom species both locally and globally. Together, this work provides an ecosystem scale assessment of antagonistic bacteria within a model ecosystem, and enhances our current understanding of the abundance, distribution and regulation of antagonistic diatom-bacteria interactions at an ecosystem scale. Building a comprehensive picture of these interactions in natural environments will allow us to better understand biotic drivers of diatom ecology, and thus marine food webs and biogeochemical cycling.

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