Particles in nepheloid layer water from four stations in the Sargasso Sea (25#x2013;55 m depth) were gently concentrated by reverse flow filtration and tested for their micro#x2010;bial hydrogen production potential. After the addition of a reducing agent and nutrient supplements, the samples were incubated in the dark, under argon. Controls were prepared by filtering the concentrate through 0.2#x2010;#x3BC;m membrane filters to remove particulates and microorganisms. The total bacterial population ranged from 7.3 #xD7; 104to 1.8 #xD7; 105cells ml#x2212;1. Hydrogen production occurred in the experimental vials from all stations; hydrogen in the control vials was 0.1 of that in the experimental vials. Samples were fractionated into 3#x2010;#x3BC;m, 3#x2013;30#x2010;#x3BC;m, and 30#x2010;#x3BC;m particle size fractions; all fractions produced hydrogen upon incubation. Maximum accumulations of hydrogen reached 3 #xD7; 105to 106nl per 25#x2010;ml sample. These results suggest that fermentative, hydrogen#x2010;producing microorganisms are abundant in the nutrient#x2010;poor Sargasso Sea and, given suitable in situ conditions, are a potential source of dissolved hydrogen.
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