Phosphate uptake behavior of natural phytoplankton during exposure to solar ultraviolet radiation in a shallow coastal lagoon

The effect of solar UV radiation (UVR: 290-400 nm) on the P-32-phosphate uptake rates of natural phytoplankton from a southern Atlantic Ocean coastal lagoon was studied during two consecutive summers at one station located in the marine-influenced area. Due to the shallowness of this lagoon and also to the generally high UV water transparency in this area, phytoplankton are exposed to high UV irradiances. The P-32-phosphate uptake rates measured at several phosphate concentrations were inhibited up to 59.2% by UVR, although uptake stimulation was also observed in four of nine experiments (up to 28%). The effect of UVR on the apparent maximum velocity of P-32-phosphate uptake (V'(uptake)) ranged from an inhibition of 49% to a stimulation of 31%. Although the highest inhibition values were associated with the maximum registered incident UV irradiance, a significant correlation between these two parameters was not observed. Changes in microalgal community structure were not related to the observed UV effect; however, a significant relationship was found between the inhibition of P-32-phosphate uptake rates and V'(uptake) used as a proxy for phosphate deficiency. This relationship suggests that the phytoplankton phosphorus nutritional status modulates their sensitivity to UV exposure. Overall, our results suggest that solar UVR has the potential to affect phosphorus cycling.