The ability of phytoplankton to cope with oxidative stress is one of the main factors that influence its survival in the marine environment, when senescence conditions prevail. In a first attempt to investigate the antioxidant strategies of different phytoplanktonic groups face to oxidative stress, the superoxide dismutase (SOD; EC 1.15.1.1) activity and photosynthetic pigment content along the growth curves of the dinoflagellate Lingulodinium polyedrum (Stein) Dodge, the prasinophycean Tetraselmis gracilis (Kylin) Butcher and the diatom Minutocellus polymorphus (Hargraves and Guillard) Hasle, von Stosch and Syvertsen were evaluated in batch-cultures. Total SOD activity was determined by an indirect method involving the inhibition of cytochrome c reduction. The contents of photosynthetic pigments were analysed by HPLC using a reverse phase column (RP-18), based on a ternary gradient. A peak of total SOD activity was detected at the beginning of the T. gracilis and M. polymorphus exponential growth. In L. polyedrum and M. polymorphus, SOD activity increased approximately three times by day 17 of growth, compared to the values obtained on day 3 (exponential phase) of the growth curve. All three species of microalgae had reduced SOD activity at the end of their growth. The levels of peridinin in L. polyedrum increased about 60% by day 17 of growth compared to the values obtained at exponential phase. Tetraselmis gracilis exhibited a remarkable increase (approximately 85%) in beta-carotene concentration after 10-14 days of growth whereas the beta-carotene levels in M. polymorphus decreased about 85% along its growth curve. These findings suggest that the antioxidant response during senescence in batch-cultures differ according to the species. Induction of SOD activity may occur either in the early exponential or stationary growth phases, which is important to prevent oxidative stress triggered by a number of factors that affects growth, such as nutrient and light availability.

To investigate adaptive responses to metal stress at the subcellular level, the oxidative balance in isolated chloroplasts was evaluated for the first time in the unicellular alga Gonyaulax polyedra exposed to the toxic metals Hg(2+), Cd(2+), Pb(2+), and Cu(2+). Different antioxidant responses were verified according to the metal and model of stress applied. Cells chronically exposed to metals exhibited high activity of the antioxidant enzymes superoxide dismutase and ascorbate peroxidase, high glutathione content, and decrease of peridinin levels, whereas no significant changes were detected for beta-carotene levels. In contrast, cells subjected to acute metal stress displayed twice as much beta-carotene but only a slight increase in superoxide dismutase and ascorbate peroxidase activities. The correlation of acute metal treatment and oxidative stress was inferred from the higher oxygen uptake and decreased reduced glutathione pool found in treated cells. In addition, increased oxidative damage to proteins and lipids occurred mainly in cells under acute stress. Pb(2+) was the most damaging toxicant, causing protein oxidation and lipid peroxidation even at chronic treatment. These results indicate that heavy metals are able to induce oxidative stress in chloroplasts of G. polyedra, particularly under acute conditions. Nevertheless, the maintenance of a high antioxidant capacity within chloroplasts seems to be an important strategy during acclimation of G. polyedra to chronic metal stress. By acting at the subcellular site, where oxidative stress is triggered, induction of such chloroplast antioxidants might be crucial for cell survival during exposure to heavy metals.

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