Ecophysiological strategies in response to UV-B radiation stress in cultures of temperate microalgae isolated from the Pacific coast of South America

Marine microalgae exposed to ultraviolet radiation (UV) have complex adapti ve responses provided by a series of protection and repair mechanisms. Inte rspecific differences in UV sensibility could result in differential select ion of the more tolerant species, having consequences for the structure of phytoplankton assemblages. The relative importance of protection and photor epair mechanisms of microalgal cells exposed to potential UV-B stress was s tudied in monocultures with different-taxonomic, ecological and size charac teristics obtained from the Chilean coast. Differences in photosynthesis an d growth rates were predicted, since the ability to effectively acclimate t o UV is not universal between microalgal species. The dinoflagellate Alexan drium catenella Whedon et Kofoid Balech, the diatom Phaeodactylum tricornut um Bohlin, the chrysophyte Aureococcus sp. and the cyanobacterium Spirulina subsalsa Oersted were acclimated during exponential cell growth under PAR + UV-A radiation (365 nm, 140-240 kJ m(-2)d(-1)) and thereafter exposed 2 h d(-1) to high and low UV-B radiation (312 nm, maximum 3.1 kJ m(-2)d(-1)) a t the center of the 16 h light period. Measured parameters were growth rate s (mu), in vivo spectral absorption, cellular fluorescence capacity, pigmen t concentration, photosynthesis and photoreactivation during three cycles i n controls and treatment samples. Growth rates diminished less than 35 % in Phaeodactylum and Aureococcus compared to 80-100 % decrease in Alexandrium and Spirulina. In these two last species, a significant increase in UV abs orbing substances was observed, probably related to the presence of mycospo rine-like aminoacids (MAAs) and scytonemin, respectively, and also lower ph otoreactivation efficiency compared to Phaeodactylum and Aureococcus, The a nalysis of photosynthetic performance under different PAR/UV-A ratios for A lexandrium and Phaeodactylum. could also explain the differences in mu Thes e results suggest that in time, species with high rates of photorepair migh t be more tolerant to UV-B than those species. which depend on the synthesi s of UV absorbing compounds as their principal protection mechanism.