Production and consumption of hydrogen in a eutrophic lake

The vertical distribution of hydrogen was measured in the Loclat, a eutrophic and holomictic lake near Neuchâtel, Switzerland, before and during summer stratification. H(2) concentrations decreased with depth in the anaerobic hypolimnion and were often below the detection limit (2.5 nl of H(2) liter) in the water adjacent to the lake sediment. H(2) was apparently not released from the lake sediment. The highest H(2) concentrations (>4 mul of H(2) liter) were observed in the aerobic water of the epilimnion and metalimnion. There, the H(2) concentrations changed with time, indicating a turnover of H(2). The H(2) production processes could not be studied in the laboratory since incubation of water samples in light or darkness did not result in H(2) production but rather always in H(2) consumption. The possible role of cyanobacteria and algae for H(2) production is discussed. Aerobic or anaerobic H(2) consumption activities were observed at all depths of the water column, with highest activities in the hypolimnion. Aerobic H(2) consumption activity was insensitive to azide inhibition, but sensitive to heat, mercuric chloride, or cyanide. It was restricted to a particle fraction of 0.2 to 3.0 mum in size, so that it must be due to single bacterial cells. Aerobic hydrogen bacteria, on the other hand, occurred in clusters of >3.0 mum. Therefore, the hydrogen bacteria could not have caused the H(2) consumption in lake water. The aerobic H(2) consumption activity followed Michaelis-Menten kinetics, with a K(m) of 67 nM H(2). This is an exceptionally low value compared with K(m) values of hydrogenases in hydrogen bacteria and other species, but is similar to that for H(2)-decomposing abiontic soil hydrogenases.