World Journal of Microbiology and Biotechnology, 8, r51-154 Acetylene reduction by Rhodospirillaceae from the Aswan High Dam Lake A.A.M. Shoreit,* M.H. Abd-Alla and M.S.A. Shabeb Potential nitrogen-fixing freshwater photosynthetic bacteria of 32 isolates from the Aswan High Dam Lake were investigated using the acetylene reduction technique to ascertain the magnitude of their contribution of fixed nitrogen to the ecosystem. The superior nitrogen-fixing Rhodospirillaceae were Rhodocyclus gelatinosus and Rhodomicrobium vannielii followed by Rhodobacter capsulatus, Rhodopseudomonas viridis and Rhodopseudo- monas palustris. High and moderate nitrogenase activities (534 to 1528 nmol ethylene produced/4 ml/h) were found in 56% of the isolates, while the remainder showed low activities (76 to 477 nmol ethylene produced/4 ml/h). Key words: Aswan High Dam Lake, nitrogen-fixation, Rhodospirillaceae. The Aswan High Dam Lake is one of the greatest man-made lakes in Africa, extending for about 300 km in southern Egypt and joining with the 180-km long Lake Nubia in northern Sudan. Populations of N2-fixing and photosynthetic bacteria are found in great numbers in ponds, lakes and estuarine lagoons where there is an active breakdown of organic matter (Stewart et al. 1971; Sherman-Torrey & Lee 1976; Bergstein et al. 1981; Guerrero et al. 1985; Eichler & Pfennig 1988; Shoreit el aI, 1989). The ability of bacteria to fix nitrogen was first demonstrated in 1949 in studies on H 2 production by the purple non-sulphur bacterium, Rhodospirillum rubrurn (Karnen & Gest, 1949). This was soon followed by reports on nitrogen fixation in other phototrophic bacteria, e.g. Rhodopseudomonas sphaeroides, Rhodopseudomonas palustris, Rhodobacter capsulatus, Rhodocyclus gelatinosus, Chromatium vinosum, Rhodomicrobium vannielii and Chlorobium Iimicola (Lindstrom et aI. 1949, 1951; Stewart et al. 1971; Madigan et aI. 1984). Siefert (1976) tested 52 strains and found that among the three families studied, 24 of 26 Rhodospirillaceae, 16 of 19 Chromatiaceae and 13 of 17 Chlorobiaceae exhibited nitrogen fixing activity. The present investigation aimed to isolate, identify and test some strains of Rhodospirillaceae isolated from different A.AM. Shoreit, M.H. Abd-Alla and M.S.A. Shabeb are with the Department of Botany, Faculty of Science, University of Assiut, Assiut, Egypt. *Corresponding author. ~) 1992 Rapid Communications of Oxford Ltd locations in the Aswan High Dam Lake for their ability to fix nitrogen under phototrophic conditions. Materials and Methods Collection of Samples Water and mud samples were collected from different locations in the Aswan High Dam Lake in clean sterile polyethylene bottles using a water sampler (Reversing Nasen bottle) as described by Colwell et al. (1975). The samples were transferred to the laboratory within 6 h. Physico-Chemical Analysis Water temperature was measured in the field and pH values were determined with a pH meter. Total soluble salts was calculated from the weight of dry residue left after evaporating a known volume of filtered water. The organic matter content was determined as mg1-1 by the Walkely and Black method recommended by Jackson (1958). The dissolved oxygen was determined according to Winlder's method (WinNer I962). Nitrate and sulphate were determined spectrophotometrically by the standard method of American Public Health Association (1981). Enrichment and Isolation Thirty-two isolates of Rhodospirillaceae were isolated from water samples and mud according to the modified general enrichment protocol described by Madigan el al. (I984). Positive enrichments were red, brown, or purple-red after about I week. Pure cultures were obtained by repeated sub-culturing in agar shake tubes (Van Niel 1931). Isolates were identified to species level by microscopy and pigmentation according to the scheme of Pfennig & Triiper (I974) and Triiper World Journal of Microbiology and Biotechnology, Vol 8, 1992 15 1