Photosynthesis can occur in the absence of sunlight. Beatty et al. (2005) discovered a species of phototrophic sulphur bacteria from a deep-sea hydrothermal vent located 2400 metres beneath the Pacific Ocean, off the coast of Mexico. At such depths, there is virtually no sunlight since sunlight can only penetrate down to a few hundred metres into the ocean. This species of bacteria is a type of green sulphur bacteria which performs photosynthesis using the faint light emitted by the hot hydrothermal vent. The green sulphur bacterium harvests the minuscule amount of light that is available to oxidize sulphur compounds to reduce carbon dioxide to produce organic material. It achieves photosynthetic growth at extremely low light intensities.
Cardenas et al. (2013) performed a quantitative assessment of the photosynthetic potential around deep-sea hydrothermal vents. Here, the photons of light required for photosynthesis comes from hot water. This is because anything with a temperature will emit radiation. For example, as the temperature of an object goes up, it begins to glow from dark red, to red, to orange, and so on; emitting more radiation as the temperature increases. Likewise, hot water spewing out from a hydrothermal vent will emit radiation, with most of it being infrared radiation as well as a tiny amount of optical radiation. Cardenas et al. (2013) show that the illumination from hot water around hydrothermal vents is sufficient for photosynthesis, albeit at rather low rates. Nevertheless, good rates of photosynthesis can be achieved if the organism can utilize infrared photons in addition to optical photos and/or if the water spewing from the hydrothermal vent is very hot.
The discovery by Beatty et al. (2005) shows that photosynthesis is not just limited to the surface of the Earth and photosynthetic growth can take place using light other than sunlight. For this reason, geothermally illuminated regions such as deep-sea hydrothermal vents are particularly interesting because it opens up the possibility that photosynthesis can occur on worlds in the solar system that are located much further from the Sun than Earth. A good example is Europa, a tidally-heated moon of Jupiter with a global ocean of liquid water beneath its frozen icy surface. It is reasonable to consider the presence of photosynthetic life living off the faint amount of light emitted by hydrothermal vents at the bottom of Europa’s dark ocean.
- Beatty et al. (2005), “An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent”, Proceedings of the National Academy of Sciences 102 (26): 9306-10
- Cardenas et al. (2013), “The potential for photosynthesis in hydrothermal vents: a new avenue for life in the Universe?”, arXiv:1304.6127 [astro-ph.EP]