Nitrate consumption of Chlamydomonas reinhardtii cells immobilized in calcium alginate beads was characterized and compared with nitrate consumption of freely suspended cells. In immobilized C. reinhardtii cells an apparent Ks for nitrate of 0.21 mM was calculated. The entrapped cells showed no nitrate consumption at concentrations lower than 0.14 mM whereas the suspended cells almost fully consumed it. Immobilized cells maintained high rates of nitrate uptake and viability (light-dependent oxygen measurements, LDOP) in a wider range of pH (5.5-8.0) and temperature (25-38°C) than the freely suspended cells, revealing a protective effect of the polymer on the biological activity. In immobilized C. reinhardtii cells a possible inhibition of nitrate consumption by nitrite was observed. Although in freely suspended cell cultures nitrate and nitrite were consumed simultaneously, in immobilized cells nitrate consumption proceeded last, and this could be of practical interest. To test the potential of immobilized C. reinhardtii cells in nitrate consumption processes, a discontinuous flow process was performed under the previously determined nitrate-uptake conditions. The average uptake rate over the studied period was 50 μmol NO3- per mg Chl per day, with a constant biomass concentration (cell load) of 130-140 μg Chl per g. The immobilized cells remained operative for at least 35 days.