UVR effects on aquatic photosynthesis conclusions and future research

UV can reduce photosynthetic rates of both micro- and macroalgae by direct effects on the photosynthetic apparatus through (1) pigment photobleaching in the photosynthetic antenna, (2) reduction of proteins in photosystem II, (3) decrease of enzyme activity in the Calvin cycle, and (4) inhibition of carbonic anhydrase activity, as well as via indirect effects, such as DNA damage. Few studies have analyzed the effects of UVR on photosynthetic responses other than carbon assimilation, for example nitrogen or phosphorus assimilation. Thus, analyzing the effects of UVR on such integrated metabolic processes should become an experimental effort of high priority.

A fact that complicates the study of UVR effects on photosynthetic organisms is that ozone depletion is occurring parallel to other global environmental changes, such as the increase in C02 and temperature, as well as the increasing eutrophication and acidification of natural waters. In order to predict the effects of increased UV-B radiation on aquatic ecosystems it is necessary to take into account the changes in other environmental factors. The increase in UV-B levels does affect algal physiology and ecology, including biogeochemical cycles in the coastal zone and enhanced radiation may have a significant global-scale climatic impact [225] (see also Chapters 5 and 17). Changes in productivity or diversity of aquatic primary producers due to elevated UV-B levels are likely to bring about alterations on several trophic levels of coastal marine food webs. Therefore, changes in community structure and ecosystem function can be expected.

Even ambient UVR levels can have a significant effect on benthic and water-column algal communities. For example, a general feature of MPB response is that ambient UV-B levels can exert a selective pressure on early successional stages on both hard and soft substrata. However, systems appear to vary greatly in susceptibility, depending on climate, the availability of refuges and nutrients, as well as the level of productivity and structure of the food webs. The local ecological implication of the initial selective pressure during early colonization will thus depend on the general importance of colonization events in relation to UV-B exposure. In the case of macroalgae, UV-B may also function as a selective pressure at the time of early colonization and recruitment. Consequently, macro-algal zonation can be determined by the different resistance against UV-B

radiation of spore germination and growth of young plants [223].

On a longer time scale, algal resistance, shielding properties of the habitat, and trophic cascades may counteract UV-B effects on the primary producer level. For example, increasingly more studies on already established benthic microal-gal and phytoplankton communities suggest a UV-B- effect minor to that first expected from short-term experiments. Thus, it is still very difficult to draw predictive conclusions about a general, long-term effect of UV-B on aquatic primary producers. This appears to apply particularly to systems where primary production depends mainly on benthic microalgae. Despite the fact that these communities consist of small organisms, with a rapid turnover, we perhaps still need to address system-level responses to UV-B on even longer time scales (years), such as in experiments conducted on terrestrial communities [60]. However, even results from these terrestrial field experiments show that observed effects are not unambiguous, particularly as effects can be largely modified by other environmental factors (temperature and nutrient availability), which may even reverse the initial UVR effects.

We are now at the stage where we can conclude that UVR affects all types of aquatic primary producers, although the long-term response at the community level may be highly variable and modified by both environmental and biological factors. Future experimental approaches must include the interaction between different environmental factors in the scenario of ozone depletion. Only then can we expand our knowledge on the effects of increased UV-B, not only at organism level, but also at the community and ecosystem level, which is crucial for understanding the consequences for aquatic biodiversity and productivity.

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