The H-2-sensing system of Rhodobacter capsulatus was engineered to elicit a fluorescent response upon cell exposure to H-2. The system is surprisingly sensitive to H-2 and is capable of detecting levels of H-2 down to 200 pM in solution, which approximates the background concentration of H-2 in water exposed to the earth's atmosphere. The response was roughly linear between 0.3 and 300 ppm V of added headspace H-2 and gave a K-app of 142 nM H-2. when cells were grown anaerobically for 12 h in the presence of H-2. Hydrogen-sensing R. capsulatus cells were grown fermentatively in the dark in co-culture with Chlamydomonas reinhardtii on microtiter plates and the bacteria fluoresced in proportion to H-2 production by the algae. This represents a promising, high-throughput assay for H-2 production in algal libraries, and an enhanced capability for developing H-2 as a clean and renewable fuel. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.