Influence of light intensity on methanotrophic bacterial activity in PetitSaut Reservoir, French Guiana

One year after impoundment in January 1994, methanotrophic bacteria in Peti t Saut Reservoir (French Guiana) sere active at the oxic-anoxic interface, This activity was revealed by the sudden extinction of diffusive methane em ission (600 metric tons of CH4 . day(-1) for the whole lake surface area, i .e., 360 km(2)). Lifting of inhibition was suspected. After reviewing the p otential inhibitors of this physiological guild (O-2, NH4+, sulfides) and c onsidering the similarities with nitrifiers, we suggest that sunlight influ enced the methanotrophic bacteria. On the basis of phospholipid analysis, o nly a type II methanotrophic community was identified in the lake. Both gro wth and methanotrophic activity of an enriched culture, obtained in the lab oratory, were largely inhibited by illumination over 150 microeinsteins . m (-2) . s(-1). These results were confirmed on a pure culture of Methylosinu s trichosporium OB3B. In situ conditions showed that water transparency was quite stable in 1994 and 1995 and that the oxycline moved steadily deeper until January 1995. Considering the mean illumination profile during this p eriod, are showed that removal of methanotrophic growth inhibition could on ly occur below a 2-m depth. The oxycline reached this level in October 1994 , allowing methanotrophic bacteria to develop and to consume the entire met hane emission 4 months later.