Effect of near infrared (NIR) light on oxygenic photosynthesis in hypersaline microbial mat

Oxygen profiles and gross oxygenic photosynthesis were measured in hypersaline microbial mats under light conditions with and without near infrared (NIR) illumination. During illumination without NIR light oxygen concentrations and oxygenic photosynthesis rates were increased compared to incubations with full artificial light or artificial light with added NIR light to simulate sun light. Photosynthetic communities in these mats consist of two major groups. Oxygenic phototrophs using light at wavelengths below 700 nm producing oxygen on the one hand and anoxygenic phototrophs with absorption maxima in the near infrared (NIR), above 700 nm, which oxidize reduced sulfur on the other hand. The absence of NIR light might directly stimulate oxygenic photosynthesis, but to our knowledge no such effect has been reported from culture studies. Alternatively, the effect of increased photosynthetic activity under no NIR conditions might be indirect by eliminating either a competition between oxygenic and anoxygenic phototrophs for a common resource or an inhibitory effect of the action of the anoxygenic phototrophs on the oxygenic phototrophs. As the effect is most pronounced in the low sulfate treatment, competition in question might be for a reduced sulfur species.