7.4: Energy sources for phototrophs

Phototrophic reactions are generally limited to soils and aqueous environments that receive light from the sun. Light cannot penetrate deeply into soils. For example, in a study of 19 different soils with a range of textures by Ciani et al. (2005), the maximum light penetration depth measured was \(300 \ \mu \text{m}\). Thus, in soils, active microbial phototrophy would be limited to the upper surfaces of the soil. In contrast, light penetrates more deeply into aqueous systems, creating a large volume of potential habitat for phototrophs. In the oceans, for example, significant light can extend as deep as \(200 \ \text{m}\), and phototrophic microorganisms are abundant down to about \(100 \ \text{m}\) depth (Falkowski, 2012).

In addition to the sun, other sources of electromagnetic radiation have the potential to fuel phototrophs. Consistent with this possibility, Beatty et al. (2005) isolated an anaerobic green sulfur bacterial species from a deep-sea hydrothermal vent. The organism was obligately phototrophic and could grow on extremely low levels of light. The only source of light at the site is geothermal radiation, which included wavelengths that could be absorbed by the photosynthetic pigments of the organism. The organism was not found in surrounding waters, indicating that it uses radiation and reduced sulfur compounds from the vent to generate energy and fix carbon, respectively.

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