Nitrogenase activity requires large amounts of ATP and reductant from aerobic respiration (Hunt 1993), but nitrogenase is sensitive to inactivation by oxygen. Gluconacetobacter diazotrophicus (Yamada 1997), an aerobic, N2-fixing bacterium that inhabits intercellular spaces of sugarcane (Dong 1994), has the unusual ability to fix nitrogen on solid media under relatively high (21 kPa) partial pressures of atmospheric oxygen. This bacterium does not form special structure with its host (e.g. nodules) to limit exposure to oxygen. On solid agar media G. diazotrophicus produces lens shaped, mucilagenous colonies. In a series of three experiments, we tested the hypothesis that colony structure provides a suitable 02 environment for nitrogenase activity in G. diazotrophicus.

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