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

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Abstract
In the Mammoth Springs (Yellowstone National Park) waters with near neutral pH and soluble sulfide (H2S, HS−, S2−) 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/m2/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 normal14C-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.