The effect of sulfide on the blue-green algae of hot springs II. Yellowstone National Park.

In the Mammoth Springs (Yellowstone National Park) waters with near neutral pH and soluble sulfide (H2S, HS(-), S(2-)) of over 1-2 mg/liter (30-60ΜM) are characterized by substrate covers of phototrophic bacteria (Chloroflexus and aChlorobium-like unicell) above 50‡C and by a blue-green alga (Spirulina labyrinthiformis) below this temperature.Synechococcus. Mastigocladus, and other blue-green algae typical of most hot springs of western North America are excluded, apparently by sulfide. The sulfide-adaptedSpirulina photosynthesized at maximum rates at 45‡C and at approximately 300 to 700ΜEin/m(2)/sec of "visible" radiation. Sulfide (0.6-1.2 mM) severely poisoned photosynthesis of nonadapted populations, but those continuously exposed to over 30ΜM tolerated at least 1 mM without inhibition. A normal(14)C-HCO3 photoincorporation rate was sustained with 0.6-1 mM sulfide in the presence of DCMU (7ΜM) or NH2OH (0.2 mM), although both of these photosystem II inhibitors prevented photoincorporation without sulfide. Other sulfur-containing compounds (S2O3 (2-) SO3 (2-), S2O4 (2-) thioglycolic acid cysteine) were unable to relieve DCMU inhibition. The lowering of the photoincorporation rate by preferentially irradiating photosystem I was also relieved by sulfide. The most tenable explanation of these results is that sulfide is used as a photo-reductant of CO2, at least when photosystem II is inhibited. It is suggested that in some blue-green algae photosystem II is poisoned by a low sulfide concentration, thus making these algae sulfidedependent if they are to continue photosynthesizing in a sulfide environment. Presumably a sulfidecytochrome reductase enzyme system must be synthesized for sulfide to be used as a photo-reductant.