Putting marine microbes on the map: determining the global distribution of marine picophytoplankton using a combination of satellite and field data

Picophytoplanktonic cells (0.2-2 μm) are the dominant phytoplankters in the largest marine biomes on Earth: the subtropical gyres. The overaching aim of this thesis is to develop algorithms that use remote-sensing observables to map the distribution of the smallest and most abundant member of picophytoplankton, Prochlorococcus, and assess its contribution to the marine carbon cycle. To understand how the photoacclimatory status and growth of Prochlorococcus and its sister genera Synechococcus are influenced by light and nutrients, experiments were conducted in the South Atlantic Gyre (SAG). Results from the manipulation experiments show that, in the central region of the SAG, nutrient addition can induce marked changes in the optical properties of Prochlorococcus cells when subjected to saturating light levels, leading to a decrease in cell abundance, whereas at the gyre periphery no substantive changes in cell growth or optical characteristics were observed. Since light plays a central role in shaping the distribution of cyanobacteria, an empirical algorithm based on relationships between Prochlorococcus abundance and remotely-sensed observables was developed. The outputs were then used in a modified primary production model to predict the vertical distribution of carbon fixation by Prochlorococcus. The models estimate that ∼ 3.4 x 1027 Prochlorococcus cells in the global ocean fix 4.7 Gt C year-1. Most of the cell biomass and primary productivity is concentrated in the subtropical gyres and areas near the Equatorial Convergence, and 61% of the carbon fixation occurs in the upper water column (0-45 metres), where only 43% of the cells reside. However, in the gyres, carbon fixation is highest (62%) in deeper layers (45-200m), and both cell abundance and carbon fixation show marked seasonal patterns. The models developed in this study provide an unprecedented view of the vertical distribution of Prochlorococcus cells and their corresponding rates of carbon fixation in the global ocean.