La dégradation du lactate par la bactérie photosynthétique Rhodobacter capsulatus cultivée dans un photobioréacteur en anaérobiose, à la lumière, en limitation d'azote, a été mesurée ainsi que la production d'hydrogène par la nitrogénase. En réacteur ouvert à faible taux de dilution (D = 0,04 h-1), le lactate initialement à 30 mM est dégradé à près de 90 % avec une faible accumulation (< 1-2 mM) des produits de dégradation (formiate et acétate). C'est dans ces conditions qu'on observe la meilleure production d'hydrogène (44 ml • h-1 • h-1). Lorsque la culture bactérienne devient trop dense en réacteur fermé ou an recyclage des bactéries, Il y a apparemment passage à un métabolisme de type fermentatif, chute de la production d'hydrogène et accumulation d'acides organiques (formique, acétique, propionique) provenant du lactate. Dans ces conditions, il n'y a pas diminution de la charge de carbone organique. Outre la présence de ces acides à courtes chaînes, les bilans carbonés; font apparaître la présence, dans le surnageant de la culture, de substances carbonées exocellulaires non identifiées pouvant représenter jusqu'à 50 % du flux carboné sortant.A 10 l photobioreactor, consisting of a PVC tubing, spiral-coiled so as to form a plane light captor (DELACHAPELLE et al., 1990), was used to study the degradation of lactate by the photosynthetic bacterium Rhodobacter capsulatus strain B10. The bacterial culture was continuously circulated in the reactor so as to maintain a homogeneous suspension, to optimize illumination of the cells, and make a well-mixed reactor for optimal nutrient transfers and degas ing of the medium. The bacterium was cultivated anaerobically under photoheterotrophic and N-limited conditions. The bioreactor was operated in three modes : as an open system (chemostat), as a closed system (batch), and as an open system with phases of bacteria recycling through the use of an ultrafiltration cell.When the bioreactor functioned as a chemostat, at low dilution rate (D < 0.04 h-1) and at relatively low bacterial density (A660 nm < 2.5), up to 90 % of the added lactate (initial concentration 30 mM) was degraded; the concentration of degradation products (formate, acetate) remained low (<2 mM). Under these conditions nitrogenase-mediated H2 production was maximal (44 ml • h-1 • l-1). In batch cultures or when the cells were « recycled » in the chemostat by ultrafiltration, the increase in absorbancy led to a decrease in cell illumination and the bacteria apparently switched from a photosynthetic of a fermentative type of metabolism; lactate was converted into formate + acetate + propionate, no H2 was produced. From the determination of the carbon balance between the influent and the effluent medium, the presence of non identified extracellular carbon compounds (up to 50 % of the C content of the effluent) was proved
Un réacteur de 10 litres, automatisé pour la culture continue en anaérobiose de bactéries photosynthétiques, a été réalisé et mis au point. Ce réacteur parfaitement agité a été utilisé dans différentes conditions de fonctionnement en système fermé (batch), en système semi-ouvert (atouts de substrats concentrés en discontinu), en système ouvert (chémostat) avec et sans recyclage de la biomasse, afin d'étudier la consommation, par les bactéries, d'un substrat carboné, le lactate. La production d'hydrogène par la bactérie photosynthétique Rhodobacter capsulatus, souche B10, résultant de la dégradation du lactate, est optimale pour des cultures en continu diluées, limitées en source azotée. Ainsi, à un taux de dilution de 0,04 h-1, avec 5 mM glutamate dans le milieu nutritif, la densité bactérienne étant de 2,1 è 660 nm, on a observé une production continue moyenne de 65 ml • h-1• l-1 pendant une période de 200 heures. Pour des concentrations bactériennes élevées, la limitation d'énergie lumineuse entrain une perte d'activité nitrogénase et, de ce fait, une chute de la production d'hydrogène.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 ope...
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