Prochlorococcus is a cyanobacterium smaller than 1 μm that accounts for much of the primary production in nutrient-poor areas such as the North Pacific Subtropical Gyre (NPSG). In the transition from the NPSG to more productive coastal regions, there are fronts that have sharp changes in chemical, physical, and biological properties. In more coastal, nutrient-rich conditions, larger phytoplankton are more abundant, including Synechococcus and picoeukaryotes. Data previously collected on cruises going north from the NPSG (the Gradients cruises) were compared to data I collected on the TN398 cruise going east from the NPSG to the California coast. A SeaFlow flow cytometer measured small phytoplankton, including Prochlorococcus, Synechococcus, and picoeukaryotes. Prochlorococcus was most abundant in nutrient-poor conditions in the NPSG, reaching a concentration of 300 cells/μL, and larger phytoplankton, including Synechococcus and picoeukaryotes, were most abundant in the coastal ocean and subpolar region. The diameters of Prochlorococcus, Synechococcus, and picoeukaryotes varied on a diel cycle that was most strongly observed in the gyre. The average diameter of Prochlorococcus and Synechococcus increased by about 0.2 μm outside the NPSG, while the diameter of picoeukaryotes observed by SeaFlow decreased by about 0.4 μm. Prochlorococcus abundance was negatively correlated with nitrate and nitrite. In the future, these variables could be compared seasonally, annually, and across ocean basins to better understand how these populations are responding to climate change.

Global cycling of elements like carbon, nitrogen, and iron have key roles in maintaining the biosphere. These and other micro- and macro-nutrients undergo important reduction-oxidation and acid-base transformations in the environment. Biologically, iron (Fe) and manganese (Mn) are utilized by microbes as cofactors in many essential proteins and enzymes including nitrogenase, ferredoxin, peroxidase, cytochromes, and phosphotransferase. These elements (most notably Fe) can often limit microbial growth in large regions of the ocean because of their trace environmental concentrations, or due to structural bioavailability, or competitive microbial uptake and utilization. This can impact key ecosystem and cellular processes, including chemosynthetic carbon fixation at hydrothermal vents, nitrogen species reduction, and metabolic electron transport. Here we use flow cytometry measurements (FCM) to quantify bacteria and archaea from hydrothermal vent plumes along the Southern East Pacific Rise. We compare patterns in microbial abundance with total dissolvable Fe concentrations (predominantly Fe3+, including dissolved and labile particulate Fe) at the same locations. I find that bacterial abundance is most strongly related to trace-concentration of Fe below 400nM, and that similar relationships exist with trace methane (CH₄) and dissolved Mn concentrations. These findings suggest that microbial abundances in vent plumes could be partially explained by trace element and methane distributions, but further research is required to disentangle whether these important substrates are covarying with other biochemical factors impacting microbial growth and metabolism in these dynamic environments.