Relationships between geotaxis/phototaxis and diel vertical migration in autotrophic dinoflagellates

Marine dinoflagellate diel vertical migrations are often conceptually explained by a species' geotactic and phototactic preferences, but actual simultaneous measurements are rare. Newly collected simultaneous measurements on Heterocapsa (Cachonina) illdefina (Herman and Sweeney) and Gymnodinium breve (Davis) are combined with similar literature information on Amphidinium carterae (Hulbert), Peridinium faeroense (Paulsen) and Prorocentrum micans (Ehrenberg) to explore several examples of the actual relationships between diel vertical migration and geotaxis/phototaxis. Amphidinium carterae does not migrate, but it exhibits a negative geotaxis that may counter a small sinking velocity. The four other species all exhibit diel vertical migrations that yield surface aggregations during daylight, but the associated combinations of geotaxis and phototaxis precision (which is strongest when every cell in a population exhibits the same response to a stimulus and weakest when the response is random) and sign [which is positive (negative) when motion is toward (away from) the stimulus] are different in each case. These different taxis combinations may be related to species-specific sensor structure and/or placement. Furthermore, variations in the different biochemical pools over a species' cell cycle may contribute to structural/mechanical changes that influence how a given sensory array functions at a given time. If so, this coupling may be an important link in the growth optimization mechanisms and occasional bloom successes of different autotrophic dinoflagellate species under varying environmental conditions.