Nitrate respiration in chemoautotrophic symbionts of the bivalve Lucinoma aequizonata

Chemoautotrophic bacteria live symbiotically in gills of Lucinoma aequizonata, a n infaunal clam inhabiting a n oxygen-poor environment. These intracellular symbionts respire nitrate, i.e. they use nitrate instead of oxygen as a terminal electron acceptor in the respiratory chain. Nitrate is only reduced to nitrite and not further to nitrogen gas. Nitrate is respired by the symbionts under fully aerobic conditions at the same rate a s under anaerobic conditions. The bacterial symbionts contain a nitrate reductase that is associated with the membrane-containing fraction of the symbiont cell and that is sensitive to respiratory inhibitors; both features are consistent with the respiratory role of this enzyme. A review of nitrate reductase in chemoautotrophic syrnbionts suggests that nitrate respiration may be common among these symbioses. Symbiont nitrate reductase may be a n ecologically important factor permitting the survival of animal hosts in oxygen-poor environments.