Diel vertical migration in marine dinoflagellates

Abstract: Increasing precipitation and surface water temperature due to global change may strengthen stratification in coastal regions, which could influence the behavior of dinoflagellate diel vertical migration (DVM). DVM is a behavioral mechanism by which dinoflagellates can access photosynthetically active radiation near the surface, and nutrients at depth. During this process, cells may need to cross both salinity and temperature gradients (haloclines and thermoclines, respectively). My results show that different dinoflagellate species display unique DVM behaviors in stratified waters and increasing halo- and thermocline strength may act as barriers between the surface- and bottom water. There is, however, large variation in how dinoflagellates cope with stratification and even closely related species have different strategies. Different DVM strategies may lead to a niche separation among species, which was also observed in my research. Moreover, during powerful mixing of the water column i.e. during strong winds in situ, the continuous DVM behavior was disrupted.

In addition, as indicated by my experiments, dinoflagellates were influenced by the combination of salinity and temperature. An increase in temperature had no significant effect on growth rate if cells were grown in low salinity environments. The results indicate higher growth rates for two bloom-forming species when growing in salinities corresponding to bottom water conditions on the west coast of Sweden. Thus, there is a trade-off for dinoflagellates between low-salinity light-rich surface conditions and high-nutrient, low-light and high salinity bottom water conditions. If different species have different optimal growth conditions, a geographical separation among species is to be expected. Furthermore, the results indicate that the primary trigger for vertical migration is light in combination with an internal clock controlling the behavior. I show that there is a positive phototactic response to both white, blue and red light and demonstrate that the non-photosynthetic photoreceptor rhodopsin gene exists and is expressed in the cells.

Harmful algal blooms (HABs) affect nearly every coastal region of the world and dinoflagellate blooms is a major problem for the shellfish industry. Efforts are made into designing accurate models that predict harmful algae blooms and these models need to be derived from reliable experimental and observational data. High resolution sampling and repeated measurements in time is needed to be able to detect DVM behavior in the field and species-specific data may need to be coordinated and integrated in the models. To predict harmful algal blooms of vertically migrating species, the migration patterns and the growth rates in the natural environment should be further clarified for each species. If increasing precipitation and temperature strengthen the gradient in coastal regions, the nutrient-rich bottom water will be inaccessible to cells unable to migrate through the gradient. Thus, stronger stratification will benefit migrating species able to cross the gradient during DVM and generate more variability in were we can expect to find specific species in situ.