Carbon and nitrogen allocation

There is evidence that UVR, especially UV-B, affects carbon allocation in aquatic autotrophic organisms. This has important consequences for food web dynamics, as these changes will affect growth and, consequently, the availability of food for other trophic levels, such as bacteria and heterotrophic microorganisms (see Chapters 5 and 15). Changes in lipid, protein, polysaccharide, and fatty acid levels due to UVR have been determined in some phytoplanktonic and MPB organisms [47,161-167]. These studies have especially highlighted the variations in responses, according to the specific sensitivity of the organisms. For example, Buma et al. [168], working with three marine diatoms, found a significant increase in cell protein content when cells were exposed to low UV-B doses, whereas the opposite occurred at higher doses. Veen et al. [169], working with a chlorophyte, demonstrated an increase in cell protein levels when cells were exposed to UVR. Skerratt et al. [167] exposed the diatom Odontella to UV-B radiation and found a reduction in lipid content whereas in Chaetoceros an increase was found. Goes et al. [162,163], working with diverse phytoplanktonic species, found changes in the rates and sizes of storage and structural carbohydrates and polyunsaturated fatty acids when exposed to UV-B. Moreover, Dohler [170,171] found UVR-induced changes in pool sizes of diverse amino acids of several Antarctic and temperate marine phytoplankton species with UV-A causing, in general, an increase in their levels, whereas UV-B produced the opposite effect. These results agree with those obtained by Goes et al. [172], who also showed that UV-B caused changes in amino acid concentrations within the cell. Finally, studies performed to determine UVR effects on the ATP content of Antarctic phytoplankton showed a reduction in this component when cells were exposed to UVR [173], but Dohler and Biermann [161], working with a marine diatom, did not find any effect.

Studies performed with MPB communities in sediments have also demonstrated changes in carbon allocation as a result of UVR exposure. The most frequently observed change was a larger relative allocation to proteins at UV-B exposure [29,64,68]. This can be interpreted as a larger proportion of fixed carbon spent on growth when carbon fixation decreases, as microalgal cells tend to retain synthesis of proteins rather than storage products under adverse environmental conditions [174]. Other UV-B effects on carbon allocation were related to polar lipids, which were lower under enhanced UV-B [47]. In macro-algae it was also found that UV-B radiation aifects carbon and nitrogen allocation, although very few studies have been done in this regard. For example, Altamirano et al. [54] found that more than 78% of the seasonal changes in the internal content of carbon and nitrogen in the green macroalga Ulva rigida were explained by seasonal changes of UV-B.

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