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The effect of NaCl in the culture medium on growth, photosynthesis and cell content of chlorophyll, K+, Na+, Ca2+ and Mg2+ in Euglena gracilis was studied. O2 production, quantum yield of photosystem II (PSII), the non‐photochemical quenching of chlorophyll fluorescence (qN) and the chlorophyll alb ratio all diminished by 0.2 M NaCl. Respiration and chlorophyll a and b increased, whereas the photochemical quenching (qp) of chlorophyll fluorescence was not affected by 0.2 M NaCl. Salt stress also induced an increase in cell volume and in K+ and Na+ concentrations, but decreased the concentrations of Ca2+ and Mg2+. Except for a protective effect on O2 production, additional Ca2+ in the culture medium did not attenuate the salt effect on the parameters measured. The addition of HCO3− restored the PSII quantum yield of O2 production in cells grown in high salt. Salt stress promoted a decrease in the apparent rate of quinone A (QA) reduction and an apparent obstruction of QB reduction, which were not prevented by excess HCO3−; the addition of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) did not increase chlorophyll fluorescence in salt‐grown cells. These results indicate that photosynthesis in Euglena grown under salt stress exhibits: (1) diminution of the HCO3− dependent water‐splitting activity of PSII; (2) inhibition of the electron transfer at the quinone pool level; (3) probable increase in thylakoid stacking (as indicated by the effect on the chlorophyll alb ratio); and (4) dissipation of the H+ gradient across the thylakoid membranes (as indicated by the decrease of qN).

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