Several standard in vitro assays were performed in order to determine the potential antioxidant capabilities of purified aqueous extracts of the mycosporine-like amino acids (MAAs), porphyra-334 plus shinorine (P-334 + SH), isolated from the red alga Porphyra rosengurttii, asterina-330 plus palythine (AS-330 + PNE), from the red alga Gelidium corneum, shinorine (SH), from the red alga Ahnfeltiopsis devoniensis, and mycosporine -glycine (MGly), isolated from the marine lichen Lichina pygmaea. The scavenging potential of hydrosoluble radicals (ABTS + decolorization method), the antioxidant activity in lipid medium (β-carotene/ linoleate bleaching method) and the scavenging capacity of superoxide radicals (pyrogallol autooxidation assay) were evaluated. In terms of scavenging of hydrosoluble radicals, the antioxidant activity of all MAAs studied was dose-dependent and it increased with the alkalinity of the medium (pH 6 to 8.5). M-Gly presented the highest activity in all pH tested; at pH 8.5 its IC 50 was 8-fold that of L-ascorbic acid (L-ASC) followed by AS-330 + PNE while P-334 + SH and SH showed scarce activity of scavenging of hydrosoluble free radicals. AS-330 + PNE showed high activity for inhibition of β-carotene oxidation relative to vitamin E and superoxide radical scavenging whilst the activity of P-334 +SH and SH were moderate. According to these results, the potential of MAAs in photoprotection can be considered high due to a double function: (1) UV chemical screening with high efficiency for UVB and UVA regions of the solar spectrum, and (2) their antioxidant capacity.

The effects of increased CO2 levels (10,000 microl l(-1)) in cultures of the green nitrophilic macroalga Ulva rigida C. Agardh were tested under conditions of N saturation and N limitation, using nitrate as the only N source. Enrichment with CO2 enhanced growth, while net photosynthesis, gross photosynthesis, dark respiration rates and soluble protein content decreased. The internal C pool remained constant at high CO2, while the assimilated C that was released to the external medium was less than half the values obtained under ambient CO2 levels. This higher retention of C provided the source for extra biomass production under N saturation. In N-sufficient thalli, nitrate-uptake rate and the activity of nitrate reductase (EC 1.6.6.1) increased under high CO2 levels. This did not affect the N content or the internal C:N balance, implying that the extra N-assimilation capacity led to the production of new biomass in proportion to C. Growth enhancement by increased level of CO2 was entirely dependent on the enhancement effect of CO2 on N-assimilation rates. The increase in nitrate reductase activity at high CO2 was not related to soluble carbohydrates or internal C. This indicates that the regulation of N assimilation by CO2 in U. rigida might involve a different pathway from that proposed for higher plants. The role of organic C release as an effective regulatory mechanism maintaining the internal C:N balance in response to different CO2 levels is discussed.

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Measurements of photosynthesis, germination capacity and assessment of DNA damage were carried out in the laboratory to determine the effect of different conditions of ultraviolet (W) and photosynthetically active radiation (PAR) on zoospores of various large brown algae collected on Spitsbergen (Svalbard, High Arctic) and Tarifa (Cbdiz, southern Spain). Results were correlated to in situ light conditions and indicated that zoospores suffer photoinhibition of photosynthesis, loss of viability and DNA damage in relation to the growth depth of parental sporophytes. At both sites, germination capacity of zoospores in species collected in deep waters was more strongly mpaired after exposure to the same UV doses than in species from shallower waters. In general, zoospores exposed to PAR+UVA+UVB showed higher mortality rates than after exposure to PAR+UVA or PAR alone. For Larmnana digitata from Spitsbergen, it was found that the loss of zoospore viability is the result of DNA damage and photodamage of the photosynthetic apparatus. UVB irradiances occurring in southern Spain at water depths shallower than 7 m prevented the germination of spores of deep water Laminariales from this region.

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