Efectos de la radiación solar sobre el fitoplancton de aguas antárticas y subantárticas
The increments in the ultraviolet radiation B (RUVB, 280-320 nm) resultants of the destruction of the ozone layer affect diverse echo-physiologic aspects of the marine phytoplankton. On this study were compared the responses of phytoplankton from an ambient Sub-Antarctic (Channel Beagle, 54º 52´S, 68º 18´W) and another Antarctic (Potter Cove, 62º 14´S, 58º 38´W) to the ultraviolet radiation (RUV). The decrease of the photosynthesis on short term (hours) was observed in both places, however inhibition thresholds were present. In the Beagle Channel, these were for RUVB and ultraviolet radiation A (RUVA, 320-400 nm), respectively, 0,2 and 6,2 Wm-2 for communities with more than 70% diatoms and 0,45 and 14,2 Wm-2, respectively, for communities with more than 70% phytoflagellates. MAA´s were synthesized by the diatoms in response to RUV. These compounds were absent in phytoflagellates. A smaller inhibition of the photosynthesis was observed with bigger concentrations of MAA´s. When the relative abundance of diatomeas was higher, it was determined that those smaller l from the RUVB were more effective in inhibiting the photosynthesis for energy unit. For the Antarctic communities (> 70% phytoflagellates) only was studied normal ozone conditions and it was not photosynthesis inhibition when the doses of RUVB and RUVA were lesser than 0.6 and 13 Wm-2, respectively. In the Channel Beagle there was significant differences when analyzing the photosynthesis inhibition during normal and reduced ozone conditions, being higher in this last. The results of the investigations clearly show a higher sensibility in the communities of ambient Sub-Antarctic regarding the RUVB compared with the Antarctic. The phytoplankton communities from both ambients they use different strategies to adapt to the exposure to RUVB and RUVA The long term effects of exposure in both ambients varied depending on the taxonomic initial composition of the community and the previous light history of the cells. However, a tendency existed to a change in the structure of the community like a decrease in the relative abundance of the phytoflagellates. Asteromonas sp. (phytoflagellate) it showed an important inhibition in their growth rate at the beginning of the exponential phase (180%), while inhibition was not observed in the diatom (Navicula sp.). The content of TBARS (oxidative stress index) in phytoflagellates cultures, exposed to RUVB and RUVA in Sub-Antarctic waters, presented a significant decrease the third day growth in coincidence with an increment in the content of a-tocopherol and ß-carotene. The maximum concentrations of a-tocoferol were 150 and 30 pmol 104 cell-1 for a phytoflagellate and a diatom (Thalassiosira sp.) respectively, coming from Antártida. This is approximately the 30 and 7% of the concentrations determined in the Channel Beagle, suggesting that the answers to the RUV are more marked in the phytoflagellates coming from Sub-Antarctic waters. On the other hand, inhibition was not determined in the growth of communities in the Antarctic. This could be explained like a better adaptation to the exposure to the RUV as for the content of antioxidants and MAA´s in antarctic diatoms. If it is compared with the initial time, the Sub-antarctic communities showed a content of MAA´s of 2% in comparison to the value found in Antártic, where maximum rates of accumulation of Shinorina and 334-Porphyrawere determined under the treatment UVB were 1,05 - 1,45 day-1, respectively in short term experiments. In spite of the initial reduction in the growth rate and in the so much accumulation of biomass of the communities like in the unialgal cultures, the final biomass was not different among the three treatments. This implies that the RUVB only slowed the process of accumulation, and the population was able to adapt efficiently to this radiation after several days exposure. With regard to the photosynthesis inhibition by the RUV, in different times along long term experiments, so much in ambient Antarctic as Sub-Antarctic was observed that after several days of exposure, this is annulled. In relation to the vertical mixture the results showed that for both ambients the percentage of photosynthesis reduction was significantly smaller in this condition when being compared with fixed incubations in surface. Also, the mixture showed to be more effective in providing fotoprotección to the exposure treatments to RUVB that to the other treatments, being this stronger effect under conditions of reduced ozone.