Production and Consumption of Hydrogen in a Eutrophic Lake

Abstract

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 −1 ) in the water adjacent to the lake sediment. H 2 was apparently not released from the lake sediment. The highest H 2 concentrations (>4 μl of H 2 liter −1 ) 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 μm 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 μm. 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.