Exploring light controls on phytoplankton community structure and the biogeochemistry of the Ross Sea, Antarctica [electronic resource]
Keywords: Environmental Earth System Science

The Southern Ocean is one of the most important regions on Earth for absorption of anthropogenic carbon dioxide (CO2) from the atmosphere and long-term storage of that carbon in deep water and ocean sediments. While a significant amount of CO2 enters the deep ocean in this region along oceanographic fronts through the solubility pump, large seasonal phytoplankton blooms form on the Antarctic continental shelf and suggest that the biological pump also plays an important, and possibly underestimated, role in the oceanic sequestration of atmospheric CO2. This dissertation investigates the mechanisms by which light may control phytoplankton species distributions in one of the most productive areas of the Antarctic continental shelf, the Ross Sea. The Ross Sea is commonly dominated by two major phytoplankton species, diatoms, and the haptophyte, Phaeocystis antarctica. The distributions of these species are often correlated with different mixed layer environments, with diatoms dominating shallow mixed layers and P. antarctica dominating deeper mixed layers. Using a series of laboratory experiments, differences were assessed between P. antarctica and the common Ross Sea diatom, Fragilariopsis cylindrus, in their capacity for xanthophyll cycle photoprotection (Chapter 2). This was followed by chemical inhibition experiments that quantified the relative important of xanthophyll cycle photoprotection and the repair of photodamage for maintaining photosynthetic performance in each species. F. cylindrus produced significantly higher concentrations of xanthophyll cycle pigment and epoxidation of activated pigment (diatoxanthin epoxidation to diadinoxanthin) occurred much more slowly upon transition to low light than in P. antarctica. Although both species relied on xanthophyll cycle photoprotection to avoid photoinhibition and maintain maximal photosynthetic rates, P. antarctica was much more adversely affected when repair of photodamage was inhibited. Differences between species in strategies and rates of photoacclimation were also assessed (Chapter 3). F. cylindrus acclimated to shifts in irradiance by adjusting photosynthetic efficiency, with large changes in the functional absorption cross-section of photosystem two ([sigma]PSII) inferred from physiological measurements. P. antarctica exhibited significant changes in both photosynthetic efficiency and the maximum capacity for photosynthesis following shifts in irradiance. Changes in both [sigma]PSII and the number photosynthetic reaction centers or their maximum turnover rate were inferred from physiological measurements. Light was also found to play an important role in controlling elemental ratios in F. cylindrus and P. antarctica (Chapter 4). Particulate organic carbon to nitrogen to phosphorus ratios (C:N:P) varied as a function of growth irradiance in both species, but significant differences between species grown in identical conditions were also observed. F. cylindrus exhibited C:N:P ratios that were significantly lower than those of P. antarctica and often below the Redfield ratio, in agreement with observations from the Ross Sea. In contrast, P. antarctica exhibited ratios above the Redfield ratio when grown in all but very high light conditions. While protein, nucleic acid, and chlorophyll (Chl) concentrations explained the provenance of nearly 100% of particulate N in both species, nucleic acid concentrations were not sufficient to explain particulate P in either species. The remaining P could be partially accounted for if these species produce large concentrations of phospholipids, but storage of inorganic P most likely forms the largest cellular P-pool in nutrient replete cultures. Finally, data from the laboratory experiments were used to calculate phytoplankton growth rates in an ecosystem model of the Ross Sea to test the hypothesis that photophysiological differences between diatoms and P. antarctica can explain their distributions (Chapter 5). The phytoplankton growth model was modified from a previous steady-state model that included four physiological variables, the maximum quantum yield of photosynthesis ([phi]M), the irradiance at which [phi] = 1/2 [phi]M, the carbon to Chl ratio, and mean Chl-specific absorption. The parameters were allowed to vary as a function of mean mixed layer irradiance according to equations derived from laboratory data and acclimation rates measured in light shift experiments. Chl concentrations and distributions of P. antarctica and diatoms in the model agreed well with field observations, demonstrating that light is sufficient to explain phytoplankton community composition in the Ross Sea. These results also demonstrate that physiological information collected from ecologically relevant algal cultures can be used to understand and model phytoplankton dynamics in the natural environment.