Role of the denitrifying Haloarchaea in the treatment of nitrite-brines.

Abstract

Haloferax mediterranei is a denitrifying halophilic archaeon able to reduce nitrate and nitrite under oxic and anoxic conditions. In the presence of oxygen, nitrate and nitrite are used as nitrogen sources for growth. Under oxygen scarcity, this haloarchaeon uses both ions as electron acceptors via a denitrification pathway. In the present work, the maximal nitrite concentration tolerated by this organism was determined by studying the growth of H. mediterranei in minimal medium containing 30, 40 and 50 mM nitrite as sole nitrogen source and under initial oxic conditions at 42 degrees C. The results showed the ability of H. mediterranei to withstand nitrite concentrations up to 50 mM. At the beginning of the incubation, nitrate was detected in the medium, probably due to the spontaneous oxidation of nitrite under the initial oxic conditions. The complete removal of nitrite and nitrate was accomplished in most of the tested conditions, except in culture medium containing 50 mM nitrite, suggesting that this concentration compromised the denitrification capacity of the cells. Nitrite and nitrate reductases activities were analyzed at different growth stages of H. mediterranei. In all cases, the activities of the respiratory enzymes were higher than their assimilative counterparts; this was especially the case for NirK. The denitrifying and possibly detoxifying role of this enzyme might explain the high nitrite tolerance of H. mediterranei. This archaeon was also able to remove 60% of the nitrate and 75% of the nitrite initially present in brine samples collected from a wastewater treatment facility. These results suggest that H. mediterranei, and probably other halophilic denitrifying Archaea, are suitable candidates for the bioremediation of brines with high nitrite and nitrate concentrations.