STUDIES ON BLUE-GREEN ALGAE OF DESERT ALGAL SOIL CRUSTS

The predominant blue-green algae of (Chihuahuan) desert algal soil crusts were identified and characterized. In rehydrated crusts, a progression of algal species with time occurred with Microcoleus sp. appearing first, followed by Nostoc sp. and Scytonema sp.; The effects of various environmental factors on growth of unialgal cultures of the crustal algae and on their nitrogen-fixing capability were determined. The acetylene reduction assay was used as an index of nitrogen fixation.; Optimum growth for each alga occurred at pH 8.0 and over a temperature range of 21 to 30 C. Scytonema sp. demonstrated a high temperature tolerance. Increasing light intensity did not significantly influence growth. Nitrate- and ammonium-nitrogen supported growth while nitrite and urea were little utilized as nitrogen sources. Growth was influenced by water potential. Each species exhibited a greater sensitivity to a reduction in matric rather than osmotic potential. Scytonema sp. showed the greatest tolerance to increased water stress.; Acetylene reduction was optimum at pH 8.0 and temperature between 21 and 30 C for the diazotrophic algae. Ethylene production was not influenced by light intensity approaching 8100 lux. Low levels of combined nitrogen (100 mg/l), either as nitrate or ammonium ions, had no immediate effect on acetylene reduction. Acetylene reduction was more sensitive to a reduction in matric rather than osmotic water potential.; Heterotrophic acetylene reduction was absent from crust material. Dark acetylene reduction was observed but at a rate lower than that which occurred in light. In dehydrated crusts, ethylene formation decreased with 50% activity being reached at a water potential of -3.8 bars. Acetylene reduction was restored in three year-old desiccated crusts.; In situ acetylene reduction in desert crusts was greatest during early-morning hours when moisture, light, and temperature conditions were favorable.; A preliminary characterization of a Scytonema sp. extracellular substance revealed the compound to be organic and water soluble, with strong ultra-violet-absorbing properties.