AZOSPIRILLUM STRAINS USE PHENOLIC-COMPOUNDS AS INTERMEDIATES FOR ELECTRON-TRANSFER UNDER OXYGEN-LIMITING CONDITIONS

The effects of catechol, vanillic, caffeic (CAF), 2-hydroxyphenylaceti c, 4-hydroxy- and 3,4-dihydroxybenzoic (3,4-DHBA) acids on the growth of a common rice rhizosphere inhabitant, Azospirillum lipoferum were s tudied. Two strains of this nonfermenting nitrogen-fixing bacterium we re used: a motile strain (4B), and a nonmotile strain (4T). Under atmo spheric conditions (pO(2) = 21 kPa), the growth of strain 4T was inhib ited by catechol (0.1 mM) only. None of these compounds affected the g rowth of strain 4B. Under 5 kPa O-2, no effect was observed on strain 4B, whereas three of the six tested phenolics stimulated the growth of strain 4T; maximum effects were observed for 3,4-DHBA and CAF. As rev ealed by TLC and HPLC, under low oxygen, more new lipophilic compounds were formed from CAF by strain 4T, differing from CAF autooxydation p roducts and from the products obtained under 21 kPa O-2. It was hypoth esized that strain 4T had the ability to use an oxidized derivative of CAF as a terminal electron acceptor. This hypothesis was tested in ex periments under nitrogen-fixing conditions, in the absence of oxygen, and in the presence of N2O as a reoxidizing agent for CAF. Acetylene w as used both as a substrate to measure nitrogenase activity (ARA) and to inhibit the biological transfer of electrons to N2O. The addition o f CAF in the presence of N2O had the same effect on ARA rates as an ad dition of oxygen. It is concluded that the strain 4T of Azospirillum l ipoferum is able to sustain some of its activities (e.g., N-2 fixation ) using phenolics as alternative electron accepters under low oxygen c onditions.