Marine picocyanobacteria of the genera Synechococcus and Prochlorococcus are numerically dominant in vast tracts of the ocean. Using a combination of NaH14CO3 radiotracer and flow cytometric sorting studies we have evaluated the importance of each genera in marine CO2 fixation . Although a minor component numerically, pico-sized photosynthetic eukaryotes i.e. <2-3 µm in diameter, are also important marine primary producers, being responsible for up to 40% of total primary production at low latitudes such as the North-Eastern tropical Atlantic . These organisms encompass many uncultured lineages of the classes Prymnesiophyceae and Chrysophyceae, some of which are likely critical for controlling picocyanobacterial numbers in situ, given recent evidence showing the importance of mixotrophy by plastidic protists in oligotrophic gyre ecosystems [2, 3]. Biotic control of natural picocyanobacterial populations likely also includes viral lysis. We present new evidence that also suggests interactions between the grazing and viral lysis process.
As well as biotic control of cell abundance, natural picocyanobacterial populations are also structured genetically by abiotic factors such as temperature, nutrient availability and light intensity . Specifically for Synechococcus, we have developed a high resolution phylogenetic framework to fine tune the geographical partitioning of this genus in situ, which is particularly well typified along Atlantic Meridional Transects . In concert with in-depth genomic and metagenomic studies, we aim to uncover the specific adaptation mechanisms of the numerous Synechococcus phylotypes observed, that will help explain the successful colonization of this genus throughout the marine environment.
 Jardillier et al., (2010) ISME J 4: 1180-1192;  Hartmann et al., (2012) PNAS 109: 5756-5760;  Hartmann et al., (2013) Envion Micro Rep 5: 835-840  Scanlan et al., (2009) MMBR 73: 249-299;  Mazard et al., (2012) Env Micro 14: 372-386