Azospirillum strains use phenolic compounds as intermediates for electron transfer under oxygen-limiting conditions

The effects of catechol, vanillic, caffeic (CAF), 2-hydroxyphenylacetic, 4-hydroxy- and 3,4-dihydroxybenzoic (3,4-DHBA) acids on the growth of a common rice rhizosphere inhabitant, Azospirillum lipoferum were studied. Two strains of this nonfermenting nitrogen-fixing bacterium were used: a motile strain (4B), and a nonmotile strain (4T). Under atmospheric conditions (pO₂ = 21 kPa), the growth of strain 4T was inhibited by catechol (0.1 mm) only. None of these compounds affected the growth of strain 413. Under 5 kPa O₂, no effect was observed on strain 413, whereas three of the six tested phenolics stimulated the growth of strain 4T; maximum effects were observed for 3,4-DHBA and CAF. As revealed by TLC and HPLC, under low oxygen, more new lipophilic compounds were formed from CAF by strain 4T, differing from CAF autooxydation products and from the products obtained under 21 kPa O₂. It was hypothesized that strain 4T had the ability to use an oxidized derivative of CAF as a terminal electron acceptor. This hypothesis was tested in experiments under nitrogen-fixing conditions, in the absence of oxygen, and in the presence of N₂O as a reoxidizing agent for CAF. Acetylene was used both as a substrate to measure nitrogenase activity (ARA) and to inhibit the biological transfer of electrons to N₂O. The addition of CAF in the presence of N₂O had the same effect on ARA rates as an addition of oxygen. It is concluded that the strain 4T of Azospirillum lipoferum is able to sustain some of its activities (e.g., N₂ fixation) using phenolics as alternative electron acceptors under low oxygen conditions.