MIXOTROPHY IN THE TERMITE GUT ACETOGEN, SPOROMUSA-TERMITIDA

Cell suspensions of H-2/CO2-grown Sporomusa termitida catalyzed an H-2-supported synthesis of acetate from CO2 at rates of about 1-mu-mol acetate x h-1 x mg protein-1. Cells pre-grown on methanol, mannitol, lactate, or glycine also displayed H-2-supported acetogenesis from CO2, although at rates 5-85% that of H-2/CO2-grown cells. With methanol-grown cell suspensions: the presence of methanol greatly stimulated the rate of H-2-supported conversion of (CO2)-C-14 to C-14-acetate (which became labeled mainly in the COOH-group); and likewise the presence of H-2 stimulated the conversion of (CH3OH)-C-14 + CO2 to C-14-acetate (which became labeled mainly in the CH3-group). Analogous stimulatory effects were observed for cell suspensions pre-grown on methanol + CO2 + H-2. Furthermore, when H-2 (+ CO2) was included as a growth substrate with either methanol or lactate: both substrates were used simultaneously; there was no diauxie in the growth of cells or in acetate production; and the molar growth yield of S. termitida was close to that predicted from summation of the yields observed when grown with each substrate alone. These data indicated that S. termitida can grow by mixotrophy, i.e. by the simultaneous use of H-2/CO2 and organic compounds for energy. Results are discussed in light of the ability of H-2/CO2 acetogens to outprocess methanogens in H-2 consumption in the hindgut fermentation of wood-feeding termites.