The Possible Photosynthetic Use of Sulfide by the Filamentous Phototrophic Bacteria of Hot Springs1

Filamentous gliding bacteria containing bacteriochlorophylls a and c occur abundantly in hot springs of alkaline pH and usually arc in direct contact with blue‐green photoautotrophs even though the bacteria may have a higher growth temperature limit. In contrast, hot springs with at least 0.2 mg liter−1 of soluble sulfide in water of 60–70°C support orange to green mats of these bacteria upstream from and independent of the presumed organic donation of the blue‐green algae. In these cases sulfide seemed the only plausible source of reductant in phototrophism and CO2 the only likely carbon source. In the field and in culture, the stimulatory effect of sulfide on CO2 photoincorporation was only a small (1.2–3.4‐fold) increase over the nonsulfide rate. Anaerobic illuminated cultures showed an increase in dry weight and pigment in basal autotrophic medium only when sulfide was added. However, small amounts of yeast extract resulted in far greater yields. The combined observations and results suggest that photoautotrophy, with sulfide as electron donor, is a possible alternative to photoheterotrophy in this newly discovered group of phototrophic bacteria.