Solar radiation leads to hydrogen peroxide (H202) accumulation in shallow intertidal surface waters dunng daytime tidal emersion periods. The lugworm Arenjcola marina irrigates its sedimentary tubes with surface water containing variable H 2 0 2 concentrations. We studied aspects of the antioxidant status of the intertidal polychaete A. marina in response to seasonal variations of oxidative stress in its environment. Antioxidant enzyme activities [superoxide dismutase (SOD), glutathione reductase (GR) and catalase] and v~tamin E were concentrated chiefly In the chloragog and to a lesser extent in body wall tissue. Response to experimental H 2 0 2 exposure (5 1.lmol I-') was confined to the chloragog tissue and consisted of elevated catalase activity. On a subcellular level, the major part of enzymatic antioxidants examined was found in the cytosolic (77 % of SOD activity, 60% of GR activity, 41% of catalase activity, 87% of total glutathione) and the peroxisomal (56% of catalase activity) fractions, whereas the same antioxidants were virtually absent in mitochondria. Gradual acclimation of winter A. marina (5°C) to higher ambient temperatures (20°C) over 20 d resulted in a doubling of chloragog SOD activities, while catalase activity was not affected. Elevated in situ concentrations of photoproduced H202 during summer (1.7 pm01 1-l) coincided with a significant increase of chloragog catalase activities in young (winter-317.97 + 78.3 U mg-' protein, summer: 783.41 + 192.7 U mg-' protein, means * SD) and in adult (winter: 480.09 * 160.1 U mg-' protein, summer: 1165.5 * 207.5 U mg-' prote~n) lugworms. Seasonal differences in the SOD activities of adult worms (winter: 16.06 i 4.69 SOD units mg-' protein; summer: 23.29 * 3.31 SOD units mg-' protein) were attributed to the elevated sediment temperatures in intertidal areas during summer.

The energetic consequences of strict oxyconformity in the intertidal worm S. nudus were studied by characterizing the Po dependence of respiration in mitochondria isolated from the body wall tissue. The contribution of the rates of mitochondrial ROS production and proton leakiness to mitochondrial oxygen consumption and uncoupling was investigated by using oligomycin as a specific inhibitor of the ATP synthase. The maximum contribution of oligomycin independent respiration to state 3 respiration remained below 6% and showed a minor, insignificant increase at elevated Po , at a slope significantly lower than the increment of state 3 respiration.2 Therefore, Po dependent mitochondrial proton leakage or ROS production cannot explain oxyconformity. In 2 conclusion Po dependent state 3 respiration likely relates to the progressive contribution of an alternative oxidase 2 Ž . cytochrome o , which is characterized by a low affinity to oxygen and an ATPrO ratio similar to the branched respiratory system of bacteria. The molecular nature of the alternative oxidase in lower invertebrates is still obscure. ᮊ