Addendum

Review Lecture. The chemosynthetic support of life and the microbial diversity at deep-sea hydrothermal vents. Proc. R. Soc. Lond . B 225, 277-297 (1985). In this lecture, the chemosynthetic base of the food chain supporting rich deep-sea ecosystems around hydrothermal vents, was claimed to represent a primary production of organic carbon independent of sunlight. I received several comments criticizing this point of view for neglecting the fact that oxygen is the required electron acceptor in the metabolism of the eukaryotic part of the vent communities. I agree. The independence of light was, however, mentioned in connection with a catastrophic darkening of the globe’s surface. A temporary absence of photo­synthetic oxygen production might well be overcome for an extended period of time by the ‘aerobic’ deep-sea vent animals, given the minute consumption of oxygen relative to its huge total quantity available in deep ocean waters. In a permanent absence of light, however, the existence of eukaryotic organisms, as we know them, will depend on an oxygen-producing process such as photosynthesis. Populations of anaerobic bacteria, on the other hand, may well persist and differentiate into prokaryotic ecosystems in permanent darkness as long as the geothermal provision of H 2 and CO 2 continues. Physical chemists were troubled by the use of the term ‘source of energy’ for reduced inorganic compounds, such as H 2 S, in chemosynthesis because the actual amount of free energy available depends on the reaction with the oxidant. It is certainly true that the common equalization of the terms ‘electron donor’ and ‘energy source’ in microbial physiology does not take the specific type of electron acceptor into account. They are used as terms of convenience. In my discussion of deep-sea chemosynthesis as a form of primary production, the emphasis on terrestrial chemical ‘sources of energy’ was meant to illustrate the contrast to the use of solar energy which does not only supply oxygen as the most efficient electron acceptor but also the common electron donors, organic as well as inorganic, for all non-phototrophic life in surface waters and on the continents.