Susceptibility to ultraviolet-B radiation of the dinoflagellate Alexandrium catenella Kofoid Balech, and the diatom Phaeodactylum tricornutum Bohlin

Differential susceptibility to ultraviolet B (UV-B, 280 - 320 nm) radiation among microalgae generates patterns of dominance in phytoplankton assembla ges. However, despite some events of harmful algal blooms (HAB's) of the di noflagellate Alexandrium catenella have been coincident with high UV-B radi ation episodes in the southern region of Chile, a causal relationship has n ot been found among these processes. Through the population parameters, int rinsic growth rate (mu) and carrying capacity (K), the UV-B effect over the population dynamic of A. catenella cultures was determined, and compared w ith the demographic response of cultures of the UV-B-susceptible diatom, Ph aeodactylum tricornutum. Results showed that both species exhibit a signifi cant decrease in mu faced with an increased UV-B radiation. Nevertheless, w hile response of A. catenella exhibited a threshold dose after which growth decreased down to 0.03 d(-1), mu-values in P. tricornutum showed a proport ional decrease down to 0.34 d(-1). Notwithstanding, the parameter K showed a similar decline in both species under a RUV-B gradient, A. catenella exhi bited a significant inhibition at 2.9 KJ m(-2) d(-1), meanwhile in P. trico rnutum K was affected under doses equal to or higher than 4.1 KJ m(-2) d(-1 ). Contrary to prediction, results indicated that with an increase in the U V-B radiation dose, A. catenella presented a higher susceptibility compared with P. tricornutum, granting a week causality to this factor in determini ng the dominance pattern of A. catenella during HAB's.