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

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.