Adverse effects of strong light and nitrogen deficiency on cell viability, photosynthesis, and motility of the red-tide dinoflagellate Karenia mikimotoi

Abstract

:The dinoflagellate Karenia mikimotoi is a red tide–forming alga that causes serious damage to aquaculture in coastal areas around the world. Its eco-physiological characteristics have been investigated extensively but conditions associated with declines of red tides of K. mikimotoi remain unknown. In the present study, we investigated the effects of light and nutrients on the viability, activity of photosystem II (PSII), and motility of K. mikimotoi. Cells were incubated under low (100 μmol photons m−2 s−1 on a 12:12 light:dark cycle of illumination) or high irradiance (1000 μmol photons m−2 s−1) for 4 h in the middle of the photoperiod, in different media. Under low light, most cells survived for about 1 month in media depleted of nitrogen (N) and phosphorus (P), similar to their survival pattern in complete medium. However, when cells that had been pre-incubated in N-depleted medium for 7 d were incubated under high light in an N-depleted medium, cell density and PSII activity declined within a few days. In P-depleted and complete media, there were no significant differences in cell density and PSII activity between low and high light, at least for 10 d. Furthermore, PSII activity was much more sensitive to photoinhibition in N-depleted than in complete medium. The rhythm of diurnal vertical migration of K. mikimotoi was observed in very low light (10 μmol photons m−2 s−1) in different media. The rhythm became obscure in N- and P-depleted media; in fact, most cells stayed in the surface layer all day in N-depleted medium. Moreover, cells accumulating in surface layers in N- and P-depleted media descended very little, even when irradiated with high light. These observations suggest that N-deficiency might trap cells in the surface layer and increase their susceptibility to photoinhibition of PSII, possibly leading to cell death. The combination of N-deficiency and high light thus appears to be an important factor that determines the decline of red tides of K. mikimotoi.