"Cyanobacteria of the genus Synechococcus have a ubiquitous distribution in oceanic waters and are responsible for around a quarter of the primary production. The genus is genetically diverse, with at least ten phylogenetically distinct lineages or clades. The in situ community structure of these organisms is complex, with the specific lineages occupying different niches to populate the world’s oceans. Whilst such molecular ecological studies are effectively mapping the spatial distributions of specific genotypes, but the factors that dictate this global community structure, and the relationship between niche and genetic potential, are still poorly defined. This is important because changes in dominant picocyanobacterial lineages indicate major domain shifts in planktonic ecosystems, allowing us to assess changes in the rates of biogeochemical cycles. We propose here to undertake a molecular approach to assess specifically how regulation of specific gene sets defines the ecological ‘distinctness’ of these lineages. We propose to focus on the key nutrient regulon of iron (Fe) since this limiting element for primary production in many oceanic environments varies both spatially and temporally with obvious ‘differences’ in more stable oligotrophic open ocean systems compared to more unstable coastal waters. Hence, there is strong reasoning to expect that differences in regulatory capacity exist between lineages occupying contrasting niches, and that such regulatory ‘constraints’, or indeed lack of constraints, facilitate occupation of specific niches (specialists) or overlapping niches (opportunists). Thus, this project will be set out to obtain a comprehensive understanding of the Fe regulon facilitating its acquisition in organisms we consider to be either specialist oligotrophs, specialist mesotrophs or opportunists i.e. with differing lifestyles, which we hypothesise is key to their successful colonization of vast tracts of the world oceans."