Ultraviolet B-photoprotection efficiency of mesocosm-enclosed natural phytoplankton communities from different latitudes: Rimouski (Canada) and Ubatuba (Brazil).

Photoprotection against UV-B radiation (UVBR; 280-320 nm) was examined in natural phytoplankton communities from two coastal environments at different latitudes: temperate Rimouski (Canada) and tropical Ubatuba (Brazil). Mesocosm experiments were performed at these sites to examine the response of phytoplankton to increases in UVBR that corresponded to local depletions of 30% and 60% in atmospheric ozone levels (low and high UVBR treatments, respectively). A fluorescence method using a pulse amplitude modulation fluorometer (Xe-PAM, Walz, Germany) with selective UV filters was used to estimate photoprotection, and these results were compared with an index of mycosporine-like amino acid (MAA) concentrations determined using spectrophotometry of methanol extracts. The present study provided the first evidence, to our knowledge, of the suitability of this in vivo fluorescence method for the estimation of UV photoprotection efficiency in natural phytoplankton. No significant differences were found for most of the variables analyzed between the light treatments used at both sites, but differences were found between sites throughout the duration of the experiments. Vertical mixing, used to maintain cells in suspension, likely alleviated serious UVBR-induced damage during both experiments by reducing the length of time of exposure to the highest UVBR irradiances at the surface. In Rimouski, this was the main factor minimizing the effects of treatment, because optical properties of the coastal seawater rapidly attenuated UVBR throughout the water column of the ca 2 m deep mesocosms. In this location, synthesis of MAAs and photoprotective pigments likely contributed to the observed phototolerance of phytoplankton and, hence, to their growth; however, in a comparison of the UVBR treatments, these variables showed no differences. In Ubatuba, where nutrient concentrations were significantly lower than those in Rimouski, light attenuation was less than that in Rimouski and UVBR reached the bottom of the mesocosms. UVBR penetration and the forced vertical mixing of the cells, without the possibility of vertical migration below this photostress zone, resulted in photo-inhibition, because confinement in the mesocosms forced cells to remain constantly exposed to high levels of irradiance during the daytime. Hence, additional effects of UVBR were masked in this experiment, because cells were damaged too much and phytoplankton populations were rapidly declining. There was also an overall preservation of MAAs, in contrast with chlorophyll (Chl) degradation, in spite of the fact that this UV screening was not sufficient to counteract photo-inhibition, which suggests an important role for these molecules, either in the overall photoprotection strategy or in other physiological processes. Altogether, local water characteristics, namely attenuation, mixing, and nutrients concentration, can strongly modulate the photoprotection strategies used by natural phytoplankton populations in coastal environments.

UV effects on marine planktonic food webs: A synthesis of results from mesocosm studies.

UV irradiance has a broad range of effects on marine planktonic organisms. Direct and indirect effects on individual organisms have complex impacts on food-web structure and dynamics, with implications for carbon and nutrient cycling. Mesocosm experiments are well suited for the study of such complex interrelationships. Mesocosms offer the possibility to conduct well-controlled experiments with intact planktonic communities in physical, chemical and light conditions mimicking those of the natural environment. In allowing the manipulation of UV intensities and light spectral composition, the experimental mesocosm approach has proven to be especially useful in assessing the impacts at the community level. This review of mesocosm studies shows that, although a UV increase even well above natural intensities often has subtle effects on bulk biomass (carbon and chlorophyll), it can significantly impact the food-web structure because of different sensitivity to UV among planktonic organisms. Given the complexity of UV impacts, as evidenced by results of mesocosm studies, interactions between UV and changing environmental conditions (e.g. eutrophication and climate change) are likely to have significant effects on the function of marine ecosystems.

Effects of enhanced UV-B on pigment-based phytoplankton biomass and composition of mesocosm-enclosed natural marine communities from three latitudes.

A series of three outdoor mesocosm experiments was undertaken in Rimouski (Canada), Ubatuba (Brazil) and Ushuaia (southern Argentina) to examine the effects of lamp-enhanced UV-B (280-320 nm) on phytoplankton communities isolated from seawater at each site. Detailed pigment composition was used to identify these communities. Each experiment compared three replicated UV-B treatments, consisting of natural sunlight conditions (NUVB), low-level UV-B enhancement corresponding to local 30% ozone depletion (LUVB) and high-level enhancement corresponding to 60% ozone depletion (HUVB). Each mesocosm (ca 2 m deep) was mixed continuously (turnover time, ca 1.3 h) and samples were obtained daily over 7-10 days. In Rimouski a large diatom bloom occurred during the first week. Repeated-measures analysis of variance (RM-ANOVA), with time as the repeated factor, showed slight but statistically significant increases in the chlorophyll (Chl) a level with the HUVB treatment, which were especially obvious over the last 3 days of the experiment. A large decrease in grazers (ciliates) that was observed concurrently with this treatment is the most likely explanation for the increase in Chl a level. The lack of negative effect on algal biomass by enhanced UV-B is attributed to the mixing inside the mesocosms and to the relatively low UV-B penetration. In Ubatuba levels of most pigments decreased over time, particularly fucoxanthin, Chl c3 and alloxanthin. The RM-ANOVA showed no effect of the UV-B treatments, except for Chl c3, which had significantly lower concentrations under natural UVB conditions, indicating that enhanced UV-B directly or indirectly favored Chl c3 algae (likely prymnesiophytes). Although particulate organic carbon concentration was significantly larger during HUVB treatment than during the other treatments, Chl a was unaffected, suggesting that enhanced UV-B favored heterotrophs. Lack of algal growth during this experiment was attributed to low nutrient concentrations (which were the lowest of the three sites), high irradiances (which were the highest noon incident photosynthetically available radiation and UV of the three sites) and UV-B penetration down to the bottom of the mesocosms. In Ushuaia a small bloom took place over the first 5 days. The RM-ANOVA showed no overall effect of the UV-B treatments for any of the pigments examined but on the last 3 days of the experiment several green algae-type pigments, such as Chl b and siphonein, showed increased concentrations under the HUVB treatment. UV-B enhancement hence favored green algae, as seen from the stronger increase over time in the ratio of Chl b to Chl a associated with the HUVB treatment. UV-B enhancement also seemed to cause a slight decrease in physiological condition, because the relative concentration of chlorophyllide a and some pheophorbides that may be the product of dying algae increased during the HUVB treatments in Ubatuba and particularly in Ushuaia (where UV-B also penetrated to the bottom of mesocosms). For all three sites changes in phytoplankton biomass due to the UV-B treatments were minor, even though UV-B enhancement was important. This study indicates that effects of enhanced UV-B on the community structure of both phytoplankton and their grazers are potentially more important than effects on overall algal biomass.