ABSTRACT: The marine photoheterotroph Erythrobacter longus Strain NJ3Y was grown in continuous culture under variable light:dark (L:D) regimes of 24:0, 12:12 and 0:24 h to examine the influence of photoperiod on pigmentation and metabolic efficiency. Average carbon mass balances for each light regime closed to within 5%, giving confidence in the measured variables as well as calculations of physiological parameters. Significant differences in photosynthetic pigmentation were observed for the experimental light treatments, with highest and lowest pigment concentrations measured in the 0:24 and 24:0 h L:D regimes, respectively, indicating that the photosynthetic pigments of E. longus Strain NJ3Y are preferentially synthesized in the dark. These differences are consistent with the strategy that photosynthetic pigments are produced during periods of darkness, for use in energy harvesting during subsequent light periods. In contrast, concentrations of the photoprotective pigment caloxanthin sulfate were highest for cells grown under a 12:12 h L:D cycle, indicating that this carotenoid is synthesized in the light and the dark. It is concluded that the 12:12 h L:D regime provides the maximum opportunity for E. longus Strain NJ3Y to derive a metabolic advantage from light availability (i.e. by enabling it to both synthesize and utilize its light harvesting apparatus). The corresponding carbon-specific glucose uptake rate was ~30% higher than that measured for the 0:24 and 24:0 h L:D regimes. It is hypothesized that light-stimulated rates of dissolved organic carbon uptake by bacteriochlorophyll a containing bacteria confer a slight competitive advantage over bacteria that lack this photosynthetic machinery. Lowest glucose-specific biomass yields (Y_x/s,g dry cell wt g^–1 glucose) were observed for the 12:12 h L:D treatment, suggesting that under the culture conditions used in this study, photosynthesis does not contribute significantly to the growth of E. longus Strain NJ3Y.