We performed outdoor experiments to evaluate the effect of temperature on photoinhibition properties in the cosmopolitan diatom Thalassiosira weissflogii. Cultures were exposed to solar radiation with or without ultraviolet radiation (UVR, 280-400 nm), UV-A (320-400 nm), and UV-B (280-320 nm) at both 20uC and 25uC. Four possible cellular mechanisms involved in UVR stress were simultaneously addressed: carbon incorporation, chlorophyll a fluorescence of photosystem II, xanthophyll cycle activity, and ribulose-1,5-biphosphate carboxylase : oxygenase (Rubisco) activity and gene expression. Experiments consisted of daily cycles (i.e., the daylight period) and short-term incubations (i.e., 1 h centered on local noon). Samples incubated at 25uC had significantly less UVR-induced inhibition of carbon fixation and effective photochemical quantum yield compared to those incubated at 20uC. At 25uC Rubisco activity and gene expression were significantly higher than at 20uC. The higher Rubisco activity and gene expression were correlated with less dissipation of excess energy, evaluated via non-photochemical quenching, and the de-epoxidation state of the xanthophyll pigments, as more photons could be processed. An increase in temperature due to climate change would partially counteract the negative effects of UVR by increasing the response of metabolic pathways, such as those involved in Rubisco. This, in turn, may have important consequences for the ecosystem, as higher production (due to more Rubisco activity) could be expected under a scenario of global warming.

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In this article we review the existing knowledge about phytoplankton primary production (PPP) studies conducted in freshwater environments of Argentina, as well as the methodologies used to assess it. The large geographical and temporal differences in physical, chemical and biological characteristics of the aquatic bodies have resulted in a wide range of PPP values. This, together with the different methodologies and approaches used to assess PPP, precluded for the determination of spatial and temporal trends. In regard to PPP studies, and when looking at the water bodies in Argentina as a whole, it is evident that lakes have been scattered sampled; moreover, information about rivers and streams is scarce or virtually absent. These facts clearly highlight for the need of continuing (in some cases) or starting (in most of the cases) monitoring programs of PPP (as well as of related variables) to further assess the impacts of human activities on aquatic bodies i.e., as those occurring due to climate change.