Food from chemical energy: how deep-sea communities function

The energy lies in chemical disequilibrium

Where fluids emerge from the seabed, different chemical environments meet. Substances such as hydrogen sulphide or methane can serve as starting points for energy-yielding reactions in microorganisms. This energy can be used to build organic matter. Chemosynthesis therefore does not describe a metabolism without an energy source, but a way of obtaining energy that is not directly based on light absorption. Hydrothermal vents and cold seeps differ in temperature, geological driving forces and chemical composition. They should therefore not be treated as a uniform habitat. It is also crucial that suitable reactants become available together. Chemical energy does not simply reside in a single substance, independently of its surroundings. It becomes usable through a possible reaction between substances in different chemical states, whose supply helps determine the activity of the community concerned.

Symbioses bring microbes to favourable locations

Many conspicuous animals in these habitats have close relationships with chemosynthetic microorganisms. Their partners may live in or on the animal and benefit from access to suitable substances. In return, the animal receives organic compounds or uses the microbes as a food source in other ways. Such relationships are not universally identical exchanges; their precise organisation depends on the species. Researchers therefore investigate anatomy, microbial composition and metabolism together. The mere presence of a bacterium does not prove what contribution it makes to nutrition. This requires additional evidence, for example of substances actually metabolised and the transfer of carbon. The central role of chemosynthetic microbes in many vent and seep communities is established. The details of a particular partnership, however, remain the subject of targeted biological and geochemical investigations.

Not all of the deep sea lives off vents

The impressive communities at vents and seeps must not be equated with the entire deep sea. Large areas receive organic material from upper water layers, while other sources, such as sunken animal carcasses or wood, can become important locally. Even systems shaped by chemosynthesis are connected to their surroundings. Certain reactions, for example, can use oxygen that reaches deep water through large-scale processes. This finding therefore does not contradict the importance of photosynthesis for the rest of the ocean. It broadens the picture to include additional pathways of biological primary production. Research vessels, submersibles and sample analyses each reveal only portions of these hard-to-reach places. A complete annual budget therefore cannot be derived from a single expedition. Findings become particularly reliable when chemical measurements, microbial analyses and repeated observations independently support the same functional explanation of the community.

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