Abstract
The marine bivalve Lucinoma aequizonata has intracellular chemoautotrophic symbionts residing in the gill tissue. These bacteria are capable of nitrate respiration even under fully saturated oxygen conditions. Nitrate reductase in the symbionts of L. aequizonata appears to be constitutively expressed and without significant regulation by oxygen or nitrate. We discuss the stationary-phase growth state of the symbionts as an explanation for the lack of enzyme induction.