Understanding Vertical Mixing and Photoacclimation Processes in the Surface Oceans Using Single Cell Analysis.

Prepared in cooperation with Massachusetts Institute of Technology, Cambridge.

Vertical mixing in the surface layer of the ocean will affect phytoplankton growth by changing the light field to which cells are exposed. Conversely, indicators of phytoplankton photoacclimation should be diagnostic of mixing processes. A combination of laboratory and field experimental work, field observations, and theoretical models were used to quantify the relationship between vertical mixing and photoacclimation in determining the time and space evolution of single cell optical properties for the photosynthetic picoplankton, Prochiorococcus spp. Diel time-series observations from the Sargasso Sea revealed patterns in single-cell fluorescence distributions within Prochiorococcus spp. populations which correspond to decreasing mixing rates and photoacclimation during the day, and increased mixing at night. Reciprocal light shift experiments were used to quantify the photoacclimation kinetics for Prochicrococcus spp. fluorescence. In addition, a continuous culture system was developed which could simulate the effects of mixing across a light gradient at the level of the individual cell. When this system was operated at four different simulated diffusivities, Prochiorococcus marinus strain Med4 fluorescence distributions showed distinct patterns in the mean and higher moments which are consistent with a simple quasi-steady turbulent diffusion-photoacclimation model. Daytime photoacclimation drove the development of a gradient in mean fluorescence, a decrease in variance overall, and skewing of distributions away from the boundaries. These results suggest that picophytoplankton single-cell fluorescence distributions could prove to be a useful diagnostic indicator of the mixing environment.