Hydrogen production in bioreactor by a photosynthetic bacterium Rhodobacter capsulatus 1. Photobioreactor and optima conditions of hydrogen production
A photobioreactor was set up to cultivate a photosynthetic bacterium in continuous cultures. The bioreactor was designed so as to 1) allow the capture of light energy by bacteria through a spiral transparent flexible tube placed under the light, in a water bath maintaining the growth temperature at 30 °C; 2) male the suspension of bacteria circulate continuously in the reactor with a volumetric pump to maintain the medium homogeneous; 3) allow degassing of the suspension in a degassing chamber; 4) feed the culture with nutritive media, add neutralizing solution (pH 7) and withdrax aliquots white maintaining constant the volume of the culture; 5) recycle the bacteria by filtration when the bioreactor was used as e closed system (batch). The photosynthetic bacterium was Rhodobacter capsulatus strain B10 is known to lie a good H2 producer [Hillmer and Gest (1977) J. Bacteriol. 129, 724-731]. The bioreactor was run using 10 l of a synthetic medium containing lactate as carbon source and glutamate as nitrogen source. It was studied for its capacity to degrade lactate. Glutamate was the growth-liliting substrate allowing a maximum derepression of nitrogenase, the enzyme catalysing the reduction of protons to H2. The bacterial suspension was continuously circulated in the photoreactor, conceived as a plane light captor of 1 m2, to avoid bacterial self-shading and allow regeneration of ATP by photophosphorylation at high rates. The circuit was tightly closed to avoid air entry, which would prevent H2 production due to respiration of the bacteria. To run it under automated conditions, the bioreactor was equipped with two temperature sensors, two pH electrodes, a water level detector, a manometer and a computer-controlled electric valve. The bioreactor, of the well-mixed type, was used under various working conditions, namely as a closed (batch) system, as a fed-batch system (discontinuous additions of concentrated substrates), and as an open system (chemostat) with or without biomass recycling. Control of key parameters (pH, temperature, dilution rates) allowed us to define the culture conditions producing maximal amounts of molecular hydrogen. The production of H2 accompanying lactate degradation was maximal in diluted nitrogen-limited continuous cultures. It was observed at a dilution rate of 0.04 h-1 with 5 mM glutamate in the influent medium, the optical density of the culture being 2.1 at 660 nm. Under these conditions an average production H2 of 85 ml • h-1 • l-1 was observed over a 200 h period. At higher bacterial concentrations, the limitation of light energy resulted in a decrease in nitrogenase activity and therefore in a drop in the production of hydrogen. The interdependence of various parameters (pH, dilution rates, N and C sources light intensity) renders the system complex and not easily controlled by computer. Indeed, we observed that during recycling of the bacteria by the use of an ultrafiltration cell, the bacteria became a fermentative-type of metabolism accompanied by a decrease in nitrogenase activity and therefore in a drop in the production of hydrogen.