Long-term acclimation to UV radiation: effects on growth, photosynthesis and carbonic anhydrase activity in marine diatoms

Oyster-ponds are characterized by cyclical changes in the composition of the micro-algal population, and are mainly colonized by diatoms. Species living in oysterponds experience higher levels of UV-R than those living in the ocean because of the low turbidity and shallow depth in ponds ( where no stratification occurs). In artificial laboratory conditions, five marine Bacillariophyceae species commonly found in the oyster-ponds were acclimated to three artificial UV radiation (UV-R) treatments with differing durations (2.25 h, 5 h and 8 h) and intensities of exposure, but with the same daily UV-R dose (110 kJ m(-2) d(-1), unweighted) and UVB-R/ UVA-R ratio (4.5%), equivalent to that observed in the natural medium. Benthic species were able to sustain active growth, in contrast to the pelagic Skeletonema costatum that failed to acclimate. Amphora coffeaeformis was the only acclimated species that did not display any significant decrease, or even change in photosynthesis-related pigments and chlorophyll a (Chl a) content when acclimated to UV-R. The other acclimated species showed different patterns of pigment content change, depending on their ability to sustain an active growth rate under UV-R exposure: Haslea ostrearia accumulated photoprotective pigments and exhibited increased diatoxanthin-diadinoxanthin content. The pigment content of Entomoneis paludosa was only slightly modified by the longest UV-R exposure, and Phaeodactylum tricornutum exhibited a marked increase in Chl a turnover, coupled with accumulation of Chl c. In all acclimated species, photosynthesis-related parameters were differently, but in general moderately, affected by UV-R treatments, with a decrease of less than 50% in the maximum photosynthetic capacity. Some species exposed to UV-R exhibited similar or higher photosynthetic capacities than the controls. Carbonic anhydrase activity was strongly enhanced in UV-R tolerant species. Correlation between carbonic anhydrase activity and UV-R tolerance (i.e., ability to sustain active growth rate) in UV-R acclimated algae was established for all the acclimated species. This original finding is concordant with other results that suggest UV-R might play a role in the shift in species composition, since in oyster ponds, Skeletonema costatum, which is usually the dominant species, was the most sensitive diatom under the UV-R conditions tested here.