Linkages between biochemical fluxes and migration behavior in populations of the red tide dinoflagellate, Karenia brevis
Karenia brevis (K. brevis) , as other dinoflagellates, undergoes diel vertical migration. Cells usually aggregate at the surface during daytime and descend or disperse into the water column at night. It is believed that the diel vertical migration behavior is associated with the initiation, growth and maintenance of the K. brevis bloom. Prior laboratory studies and modeling have explored the possibility that the migratory behavior of K. brevis is beneficial to cell's growth. The goal of this study was to examine the linkage between migration behavior and cellular internal biochemical states, which were mediated by external environmental factors such as light and nutrients. Migrating populations during bloom events in Florida coastal waters in Oct 2000 and 2001 were followed to determine whether there were systematic differences in biochemical fluxes in different stages of migration. The incorporation of photosynthetically fixed inorganic 14 C into major subcellular end products, low molecular weight materials (LMW), lipid, carbohydrate+nucleic acids, and protein was measured, and biochemical indices (prot/carb, C/N assimilation ratio and glutamane/glutamate) were determined. The biosynthetic patterns were different in populations at different migrating stages. Surface populations exhibited higher incorporation into carbohydrate while populations at depth were characterized by higher incorporation into protein. Nutrient enrichment experiments and biochemical indices support the view that surface and deep populations differed in nutritional status, populations at depth having higher intracellular nitrogen-status. Differing responses of the Gln/Glu ratio to nutrient enrichment suggested nitrogen assimilation pathways were different for bloom poplulations in 2000 and 2001. In order to further examine the migration behavior and how both migration and biosynthesis were mediated by light, nutrient availability, as well as the differences between strains of K. brevis , two mesocosm experiments were conducted. Light experiment showed that both behavior and biosynthetic patterns were affected by light. Higher lipid synthesis was observed under higher surface light intensity (960 μmol quanta m-2 s-1 ), and higher protein synthesis under lower surface light intensity (250 μmol quanta m-2 s -1 ). Two strains, APA and MAN exhibited differences both in migration patterns and biosynthetic patterns. APA descends and ascends earlier and exhibited bioconvection in the mesocosm, which was not seen in MAN . Higher lipid synthesis was found under nutrient replete conditions, and higher RNA synthesis in nutrient depleted conditions. Overall, results were consistent with the view that patterns of behavior and biosynthesis were interrelated. A conceptual model was developed to illustrate how such relationships may serve to optimize growth.