Evidence for the involvement of azospirillum brasi lense and helicomyces roseus in the aerobic nitrogen-fixing/cellulolytic system from sugarcane litter

Abstract

A crude N 2-fixing-celluloytic culture from sugracane litter was enriched by inoculation onto filter paper then maintained on sterilized wheat straw under aerobic conditions. Attempts to reconstitute this system from assemblages of N 2 fixers, fungi and heterotrophic, non-N 2-fixing bacteria isolated from sugarcane culture were unseccesful, however, a variety of evidence indicates the ivolvement of the microerophilic N 2 fixer, Azospirillum brasilense, and dematiaceous fungus, Helicomyces roseus. While both Azotobacter and Azospirillum were isolated from field material, A. brasilense was the sole aerobic N 2 fixer to be isolated from culture maintained on straw and was obtained in high numbers (to 8 × 10 6 c.f.u. g −1 straw). Addition of a fungal inhibitor, cycloheximide, to sugarcane culture greatly reduced nitrogenase activity under air, except when malate or xylan (substrates used by A. brasilense) were added. Sucrose and glucose did not stimulate nitrogenase activity of cylclohexamide-treated culture under air. Onset of nitrogenase activity in straw or filter paper inoculated with sugarcane culture was invariably accompanied by darkening of this substrate and proliferaction of dematiaceous hyphae. H. roseaus was the only dimetiaceous fungus consistently isolated and its helical conidia were abundant shortly after straw was inoculated with sugarcane culture.