UC Santa Cruz

Dinoflagellates employ a multitude of strategies to competitively assimilate nutrients. These include vertical migration for the assimilation of nutrients at depth and accessing dissolved organic phosphorus (DOP) even when easily accessible inorganic P is available. Identification of these strategies is helpful for characterizing phytoplankton communities and understanding the niche dinoflagellates occupy within that community. Phytoplankton maintain their P status by assimilating dissolved inorganic P (DIP), but when DIP is limiting, or as a preferential utilization, some phytoplankton are able to access dissolve organic P (DOP) by expressing alkaline phosphatase (AP), a metalloenzyme that uses zinc (Zn) as a co-factor.

The first objective of this dissertation was to identify AP activity (APA) in marine phytoplankton as a proxy for determining P-stress amongst communities in the nGoA and coastal Big Sur waters. To this end, a spectrofluorometric method was developed to identify APA in phytoplankton communities with low-cell abundance, like the samples from our study areas. P-stress was identified in dinoflagellates within the communities for both regions, resulting in subtle shifts in phytoplankton composition, even when the entire community was not exhibiting P-stress. In the coastal waters of Big Sur incubation experiments were conducted with the addition of Zn to further investigate the production of AP has on community structure. We found that the addition of Zn had no significant impact on biomass or growth, but may affect subtle changes in community phytoplankton composition. The second objective of this thesis was to investigate another competitive strategy that dinoflagellates employ: the assimilation of nitrate (N) and iron (Fe) from depth. Active vertical migration of Akashiwo sanguinea was observed in laboratory columns under N-replete and N-deplete conditions, as was the significant assimilation of stable isotopes of Fe and N from depth. These observations are the first reported Fe assimilation from depth for any phytoplankton species. Both of these competitive strategies identify tactics dinoflagellates use to access nutrients that are not accessible to other phytoplankton taxon, and the influence this could have on structured phytoplankton communities.