A carbon mass balance methodology employing minimal measurements was applied to heterotrophic and photoheterotrophic marine bacteria grown under constant dilution and exposed to 12-h intervals of light or darkness. Carbon mass balance calculations using measurements taken every 3 h closed to within 93-103% using dissolved organic carbon, biomass carbon and CO2 production data only, indicating that background interference from dissolved inorganic carbon variations in the amended seawater medium was not significant. Neither strain was observed to sustain a net CO2 fixation using paramagnetic measurement of oxygen uptake rates (OUR), indicating a need for more sensitive on-line measurement techniques for OUR. Photoheterotrophic growth demonstrated lower carbon-mole biomass yields (0.41+/-0.026 vs. 0.64+/-0.013 mol mol(-1)) despite higher specific glucose uptake rates (0.025 vs. 0.02 mol mol(-1) h(-1)), suggesting that bioreactor-based study of marine bacteria can present growth modes that are different from those encountered in the marine environment.