A bioluminescence-based assimilable organic carbon (AOC) test was developed for determining the biological growth potential of seawater within the reverse osmosis desalination pretreatment process. The test uses Vibrio harveyi, a marine organism that exhibits constitutive luminescence and is nutritionally robust. AOC was measured in both a pilot plant and a full-scale desalination plant pretreatment.In the desalination industry, reverse osmosis (RO) membrane fouling from biological growth, i.e., biofouling, causes loss of productivity and increases operating and maintenance costs. Given that RO has generally been used in warm water, because of geography or colocation with power plants, biological growth has been considered "the Achilles heel of membrane processes" (2). Heterotrophic biological growth can be reduced by limiting energy sources, i.e., organic carbon, through coagulation and/or biofiltration. Having a straightforward measurement tool available will enhance an operator's control of limiting substrate during treatment. This article presents a marine AOC test for determining the biological growth potential of water within the RO desalination pretreatment process.The method has been adapted from a previously published freshwater AOC assay (5), except that the saltwater test uses the marine test organism Vibrio harveyi. The principle of the AOC test is based on the observed growth of a nutritionally diverse organism in the water sample. The amount of biomass produced can be directly related to the easily assimilable organic substrate. Because V. harveyi (ATCC 700106; American Type Culture Collection, Manassas, VA) exhibits constitutive luminescence, the amount of light produced can be directly related to biomass, which can be correlated to the AOC in the water.Bacteria were recovered from stocks frozen at Ϫ80°C in marine broth and 10% glycerol by streaking onto marine agar containing (per liter) 10 g peptone, 5 g yeast extract, 15 g agar (BD and Co., Sparks, MD), and 20 g sodium chloride (EMD Chemicals, Gibbstown, NJ) and incubated (30°C) overnight. To prepare the refrigerated stock, a single colony was inoculated into a saline buffer (1ϫ M9 salts [BD and Co., Sparks, MD] in 1,000 ml Milli-Q water with 2% sodium chloride, 0.1 mM CaCl 2 , and 1.0 mM MgSO 4 and adjusted to pH 7.2) fortified with 2 mg of acetate-carbon/liter. The inoculated buffer was incubated for 7 days at 30°C, and then the stock was stored at 4°C for up to 30 days for the inoculation of water samples. The bacterial titer was monitored by performing serial dilutions onto marine agar and was generally close to 7 ϫ 10 6 CFU/ml. AOC-free glassware preparation and sample collection have been described previously (5). A high-sensitivity, automated, and programmable photon-counting luminometer was used (SpectraMax L luminescence microplate reader; Molecular Devices Inc., Sunnyvale, CA) in conjunction with very-lowcross-talk 96-well white polystyrene microtiter plates (manufacturer catalog no. 7571; Thermo Scientific Corporation, Milford, MA). ...