Geotaxis/phototaxis and biochemical patterns in Heterocapsa (=Cachonina) illdefina (Dinophyceae) during diel vertical migrations

Two separate experiments with Heterocapsa (= Cachonina) illdefina Herman et Sweeney, one with and the other without water volume replacement, were per formed in a 250-L laboratory mesocosm (45-cm diameter x 150-cm height) to e xamine how diel vertical migration (DVM) relates to taxis sign and strength and to cellular biochemical state. Although only the cell population grown with water volume replacement maintained a division per day over the cours e of the experiment, periodic measurements during both experiments demonstr ated that cells aggregating at the surface during the light period generall y were deficient in all measured biochemical constituents compared to cells obtained from a midcolumn depth. More specifically, H. illdefina cells tha t aggregated at the surface during the Light period in both experiments exh ibited weakened positive geotaxis but strengthened positive phototaxis and were very deficient in lipid and free amino acid compared to midcolumn cell s. Cells sampled at midcolumn during the light period exhibited similar but weaker taxes changes compared to surface samples, and geotaxis strength wa s inversely correlated with cell, diameter, cellular DNA and protein conten t, and RNA/DNA ratio. In comparison, published data on Gymnodinium breve Da vis, a harmful algal bloom species, showed that cells aggregating at the su rface during the light period generally exhibited weakened negative geotaxi s and strengthened positive phototaxis and were very deficient in lipid and chi a compared to midcolumn cells. Although the persistent tendency toward negative geotaxis was weaker in midcolumn subpopulations throughout the da y, its strength was inversely correlated with cell diameter and cellular Li pid content. The combined results for both species support a revised concep tual model of optimized DVM in autotrophic marine dinoflagellates incorpora ting generalized expressions of taxis and biochemical state of individual c ells.