Photosynthetic behaviors in response to intertidal zone and algal mat density in Ulva lactuca (Chlorophyta) along the coast of Nan'ao Island, Shantou, China.

Intertidal macroalgae suffer different environmental conditions and mat densities during growing period. In the present study, Ulva lactuca Linnaeus were collected from high, intermediate, and low tidal zones at Nan'ao Island, China. These algal photosynthetic pigments and photosynthesis behaviors with different mat densities were measured. The aim is to examine how the physiological responses and acclimation match the representative tidal distribution and algal mat density. The photosynthetic pigment (chlorophyll a and carotenoid) contents and irradiance-saturated maximum photosynthetic rates (Pmax) were greater in low zone-grown U. lactuca compared with the algae grown at high and intermediate zones. Under low algal mat density, the Pmax, apparent photosynthetic efficiency (α), and dark respiration rate (Rd) of U. lactuca grown at low zone were increased, whereas the irradiance saturation points (Ik) were decreased, compared with the algae grown at higher zone. However, the Pmax of high and intermediate zone-grown U. lactuca at high algal mat density were greater than at low density. Moreover, the pH compensation point of low zone-grown thalli (9.98) was lower than the higher zone-grown thalli (more than 10.15); however, the chlorophyll fluorescence parameters (reflect photosynthetic system activity) of the thalli collected from the three different zones were similar. Therefore, we proposed that the effects of varied densities on the photosynthetic rates of these three tidal zone-grown U. lactuca thalli were different, which might be related with different capacity of HCO3- utilization of macroalgae at their zonations.