Photoacclimation of antarctic marine diatoms to solar ultraviolet radiation

The present study was carried out at Palmer Station (64.7 ° S, 64.1 ° W), Antarctica, during the austral spring-summer of the years 1993 and 1994. Two centric diatom species ( Thalassiosira sp. and Corethron criophilum Castracane) and two pennate species ( Pseudonitzschia sp. and Fragilariopsis cylindrus (Grunow) Krieger) were isolated from natural phytoplankton assemblages and exposed to solar radiation to study long term (more than 1 week) photoacclimation to ultraviolet radiation (UVR). At the beginning of the experiments, three of the cultures had relatively low concentrations of UV-absorbing compounds (i.e., mycosporine-like amino acids) and photosynthetic rates were significantly inhibited by UVR. At the end of the experiments (8–12 days), however, the two centric diatom species had high contents of mycosporine-like amino acids (MAAs) and did not show any significant differences in photosynthetic rates when exposed to either UVR + PAR or just to PAR. The synthesis of MAAs was slightly less when samples were exposed only to PAR than when exposed to UVR in addition to PAR. The rates of synthesis of MAAs, relative to phytoplankton carbon, for the two centric diatoms were 0.001 and 0.008 μg MAAs · (μg C) −1 · day −1 for shinorine and porphyra-334, respectively. The concentrations of MAAs in Pseudonitzschia sp., and Fragilariopsis cylindrus at the end of the experiments were much lower (less than one tenth) than that in the centric diatoms and the cultures were still inhibited by UVR. In the pennate diatoms MAAs increased in concentration as a response only to UVR and not to PAR. The loss rates of MAAs in Thalassiosira sp. after transferring the culture from high (1200 μE · m −2 · s −1) to low irradiance (250 μE · m −2 · s −1) were 0.0002 and 0.0023 μg MAAs · (μg C) −1 · day −1 for shinorine and porphyra-334, respectively. These results provide further evidence that MAA compounds are synthesized in response to high light conditions and that they do decrease the photoinhibitory effects of UVR.