Our results indicate that 200 mM NaCl, 0.2 mu M methyl viologen, 4 mM arsenite or 300 mu M of cadmium in the culture medium are highly toxic for Chlamydomonas reinhardtii productivity. All of them inhibit the photosynthetic-electron flow, while cyanide-sensitive respiration remains high in the alga. The addition of NaCl, 200 mM to the culture medium immediately blocks the photosynthetic activity of the alga which is partially recovered, after 1 h of treatment, remaining high during the following 24 h. At short time (4 h) methyl viologen (MV) inhibits the photosynthetic activity of the alga, while respiration inhibition was only evident at 0.5 mu M of MV. After 24 h treatment, 0.2 mu M MV inhibits drastically the photosynthetic activity of C reinhardtii, while photosystem I was still functional. At short time, cadmium and arsenite inhibit the alga photosynthetic activity, while only arsenite 4 mM inhibits both photosynthesis and respiration in 24 h-treated C. reinhardtii cells. Catalase activity of C. reinhardtii was stimulated by abiotic stress, being NaCl the most efficient inductor of the enzyme. After 24 h treatment with NaCl 200 mM, the intracellular catalase activity of the alga reaches 337.5 +/- 2.8 mu mol O-2 mg(-1) Chl min(-1), which means a 20-fold higher level than in the control cells. The degree of catalase stimulation correlates the NaCl concentration in the culture medium. These data indicate that abiotic stress induces in C. reinhardtii an increase of intracellular peroxide, which parallel a significant inhibition of the photosynthetic-electron flow. (c) 2006 Elsevier Inc. All rights reserved.