Some aspects on the role of bacteria in deep water ecosystems

Summary

Some remarks on the role of bacteria in deep lakes will be presented with an emphasis on their functioning in the carbon cycle in such ecosystems.

The metabolic intensity of most lakes is regulated to a large extent at the primary producer level. Aerobic and anaerobic decomposition by heterotrophic bacteria of the unstable ultimate products of photosynthesis results in the production of carbon dioxide and methane. Bacterial growth occurs at the expense of energy released by the flow of electrons from donors to acceptors. Typical electron acceptors for bacterially mediated reactions are oxygen, nitrate, sulphate and carbon dioxide. When oxygen is used as electron acceptor, the highest amount of energy is released, while the lowest is released when carbon dioxide is used. These reactions are mediated biologically, and the chemical reaction sequence is paralleled by an ecological succession of microorganisms: aerobic heterotrophs, denitrifiers, fermenting bacteria, sulphate reducers, and methane producers. The presence of oxygen is inhibitory to the organisms mediating the last reactions (STUMM, 1966; McCARTY, 1972), and this explains the succession of micro-organisms concomittant with the decrease in redox potential.

Both labile and refractory fractions of the pelagial dissolved organic matter can directly be utilized largely by heterotrophic bacteria. At the onset of the summer stratification of Lake Vechten a high concentration (numbers ≥10⁹ bact/L) of heterotrophic bacteria has been detected in the lower water layers (CAPPENBERG, 1972). The reason for this increase may be a release of nutrients from the mud. During stratification the water layers above the mud become anaerobic owing to the metabolic activity of the bacterial flora and the chemical oxygen demand of the mud. In summer time the highest numbers of heterotrophic bacteria are found in the metalimnion, where organic matter accumulates due to a lower relative rate of sedimentation caused by an increase in viscosity and density.

Furthermore it was found that sulphate-reducing bacteria which are capable of reducing sulphates to sulphides using sulphate as terminal electron acceptor, were observed in the hypolimnion only at the time of maximal stratification. During stagnation, gradually decreasing sulphate concentrations are found in the hypolimnion, correlated with increaseing cell numbers of sulphate reducers. As no sulphate could be detected in the mud of Lake Vechten as well, we may conclude that the sulphate concentration limits the number of these bacteria, as can be predicted for similar aquatic environments.

Usually the sulphate reducers are found in bottom deposits and are an important group of bacteria of the non-methanogenic populations in mud. Summarizing the biological methane production and its subsequent oxidation by methane-oxidizing bacteria, we may conclude that these processes can be important factors functioning in the carbon cycle in deep fresh-water ecosystems.