CO₂ fixation in halobacteria

Abstract

Seven strains of extremely halophilic bacteria (Halobacterium spp., Halococcus spp., and Haloarcula sp.) fixed CO₂ under light and dark conditions. Light enhanced CO₂ fixation in Halobacterium halobium but inhibited it in Halobacterium volcanii and Haloarcula strain GN-1. Propionate stimulated ¹⁴CO₂ incorporation in some strains, but inhibited it in others. Semi-starvation in basal salts plus glycerol induced enhanced CO₂ fixation rates. ¹⁴CO₂ fixation in semi-starved cells was stimulated by NH ⁺₄ or pyruvate and inhibited by succinate and acetate in most strains. No possible reductant was found. In cell-free extracts of H. halobium, NH ⁺₄ but not propionate stimulated ¹⁴CO₂ fixation. No RuBP carboxylase activity was detected. The main ¹⁴C-labeled α-keto acid detected after a 2-min incubation with ¹⁴CO₂ and pyruvate was pyruvate. Little or no α-ketobutyrate was detected among the early products of propionate-stimulated CO₂ fixation. Glycine was the major amino acid synthesized during a 2-min incubation with NH ⁺₄ , propionate, and ¹⁴CO₂. Propionate-stimulated CO₂ fixation was sensitive to trimethoprim and insensitive to avidin. A novel pathway for non-reductive CO₂ fixation involving a glycine synthase reaction with CO₂, NH ⁺₄ , and a methyl carbon derived from the β-carbon cleavage of propionate is tentatively proposed.