Nitrate is not easily removed by conventional water treatment. Biological nitrate removal using aerobic microalgae offers some advantage over anaerobic, microbial denitrification since both ammonia and nitrate nitrogen are readily removed and the process is less complicated (Vı ´lchez et al., 1997). Microalgae play a dominant role in stabilizing earthen pond water quality. However, the main disadvantage of using micro- algae is that algal cells are not easily removed from the culture system. If algal cells are not removed, nitrogen compounds are released back to the water. Moreover, high microalgae concentrations can cause dissolved oxygen depletions during the night due to high respiration rates. Microalgal immobilization has been used for wastewater treatment in whatever entrapment (Vı ´lchez and Vega, 1994; Kaya et al., 1995) or attachment conditions (Garbisu et al., 1991; Gil and Serra, 1993), but these techniques are expensive and not practical on large scales with aquaculture ponds. Moreover, immobilization using carrageenan or alginate can be easily dissolved in seawater which contains salts and other ions such as phosphate and EDTA