Regulation of photochemical activity in cultured symbiotic dinoflagellates under nitrate limitation and deprivation

The effects of nitrate limitation and nitrate starvation on the photochemical efficiency of photosystem II (Fv/F m) were examined in batch cultures of two species of symbiotic dinoflagellates, Symbiodinium kawagutii and S. pilosum. F v/F m values were determined along growth curves and show that the F v/F m values are negatively correlated with external nitrogen concentrations in cultures of both species. Changes in growth irradiances in the batch cultures due to increments of the cell densities were estimated S. kawagutii cultures showed a negative correlation between F v/F m and growth irradiance. These results indicate that F v/F m is dependent on the light history of the cultures and on the individual sensitivity of each species, and independent of their nutrient status. Nitrate starvation was analyzed by measuring changes in the quantum yield of fluorescence (Fv/F m), electron transport rate (ETR) and non-photochemical quenching (NPQ) at five time points along the growth curves under three conditions: control (C), without nitrogen (N-), and with ammonia (N+) as a nitrogen source sufficient to meet daily nitrogen requirements. Cells collected during the exponential growth phase and exposed to N- and N+ showed significant reductions in their maximum ETR relative to controls (20% in S. pilosum and 40% in S. kawagutii). The loss of electron transport capacity is consistent with a sink limitation rather than the result of nitrogen starvation. Under nitrate-starvation, the induction of NPQ resulted in effective protection against photosystem II damage in S. pilosum. In contrast, S. kawagutii cells failed to induced NPQ resulting in a concomitant increase in the excitation pressure over photosystem II leading to damage. Collectively the data indicate that F v/F m is not a robust indicator of nitrogen limitation in symbiotic dinoflagellates and that protection against photosystem II damage under sink limitations, is largely dependent on the differential capacities of each species to induce NPQ.