Effects of solar UV radiation on photosynthesis and enzyme activities (carbonic anhydrase and nitrate reductase) in marine macroalgae from southern Spain

The effects of solar ultraviolet (UV) radiation during daily cycles on phot osynthesis and two key enzymes involved in carbon incorporation, the carbon ic anhydrase, and in inorganic nitrogen reduction, the nitrate reductase, o f macroalgae from southern Spain are presented. During daily cycles, photoi nhibition in several inter-tidal macroalgae, expressed as decrease in the e ffective quantum yield from the morning to noon time, was linearly dependen t on the daily integrated irradiance. However, recovery, expressed as the i ncrease in the effective quantum yield from noon to the afternoon, presente d a different pattern; full recovery was found below daily integrated irrad iance of 1.0 x 10(4) kJ m(-2). However, recovery reached only 50 % at highe r irradiances. The existence of daily photoinhibition and full recovery in intertidal algae suggests that photoinhibition is a photoprotective mechani sm against high solar radiation as in higher plants. and that patterns of p hotoinhibition and recovery are affected by accumulative doses. Activities of carbonic anhidrase and nitrate reductase were determined in three marine macroalgae (Plocamium cartilagineum, Ulva rigida and Fucus spiralis) under full (PAR + UV-A + UV-B) and excluded UV solar radiation (PAR). Under PAR + UV-A + UV-B, peaks of enzyme activity were found in P. cartilagineum duri ng the evening, and accordingly to data previously published for other red macroalgae. This situation was modified by the absence of UV radiation sinc e the increase in the activities was delayed several hours. In the three ma croalgae and under full solar radiation, a significant and negative correla tion was found only when data from nitrate reductase activity was shifted i n time during at least four hours. This correlation is lost in Ulva rigida when UV radiation is excluded. The existence of these daily variations with a negative correlation of both enzyme activities could reflect a complex r egulatory link between carbon and nitrogen metabolism under solar radiation . Considering the absence of a significant correlation in U. rigida and the delay observed in maximal activities of P. cartilagineum in the absence of UV, it is suggested that UV radiation acts as an environmental signal invo lved in the control of cycles, The presence of feedback processes that cont rol nitrogen assimilation as a function of carbon content is proposed.