Photoacclimation to long-term UVR exposure of natural Sub-Antarctic

MARCELO HERNANDO, IRENE SCHLOSS, SUZANNE ROY AND GUSTAVO FERREYRA

PHOTOCHEMISTRY AND PHOTOBIOLOGY

Año: 2006 p. 923 - 935

0031-8655

Solar ultraviolet B radiation (UVB, 280-320 nm) is known to produce detrimental effects in marine phytoplankton. Associated with the seasonal ozone hole in Antarctica, stratospheric ozone depletion occasionally influences the Sub-Antarctic (Beagle Channel, Argentina) region, enhancing levels of UVB. The main objective of this work was to study the effects of several (i.e., 6-10) days of exposure to UVB on the taxonomic composition and photosynthetic inhibition of local phytoplankton communities. For different light treatments, fixed-depth incubations placed in an outdoors water tank were compared to incubations in well mixed 1900 L mesocosms, where vertical mixing was present. Phytoplankton growth was inhibited by UVR in fixed-depth experiments but not in the mixed mesocosms. Under both, fixed and mixed conditions, photosynthesis was significantly inhibited by UVB at the beginning of the experiment but no longer after several days of exposure, suggesting acclimation of cells to radiation conditions. There was a change in species composition in response to ultraviolet radiation (UVR) exposure in both experiments which likely explained acclimation. In the community exposed to fixed conditions this change was from a phytoflagellate-dominated assemblage to a community with high relative abundance of diatoms after 6 days of exposure. The UVA was responsible for most of the observed growth inhibition; however, the reduction in photosynthesis was produced by UVB. The reasons behind this variability in response to UVR are associated to the specie-specific sensitivity and acclimation and the previous light history of cells. In the community inside the mesocosms, an assemblage co-dominated by phytoflagellates and diatoms was observed at the beginning of the experiments. After 10 days of exposure, green algae (Eutreptiella sp.) had increased and phytoflagellates were the dominant group. The synthesis of mycosporine-like amino acids (MAAs), antioxidant enzymes and antenna pigments in relation with repair and protection processes may explain the reduced inhibition of both growth and photosynthesis that was observed in the phytoplankton community after several days of exposure. For environments like the Beagle Channel, seasonally exposed to the ozone hole, the results obtained from the fixed-depth experiments show that species can cope with UVR by means of MAA synthesis, while mixing would mainly promote a change in species composition and defence strategies