Effect of Dissolved Oxygen Tension on Production of Carotenoids, poly- -hydroxybutyrate, Succinate Oxidase and Superoxide Dismutase by Azospirillum brasilense Cd Grown in Continuous Culture

Abstract

Azospirillum brasilense strain Cd was grown in a medium containing NHZ in a chemostat at a range of constant dissolved oxygen tensions (d.0. t.) (0-0074.18 atm). Poly-P-hydroxybutyrate (up to 12% of the cell dry weight) increased under oxygen limitation and moderate dilution rate ( D = 0.14 h-l). The highest carotenoid content was observed at high d.0.t. and dilution rates up to 0.12 h-l. The amount of protein varied with d.0.t. from 0-29 mg protein (mg dry wt)-' at 0.007 atm to 0.54 mg at 0-18 atm. The yield efficiency and respiration rate were highest at low d.0.t. and decreased significantly at a d.0.t. of 0.18 atm. Succinate dehydrogenase and malate dehydrogenase activities increased 2.5-fold at 0.1 0-0-18 atm 0, , whereas succinate oxidase and NADH oxidase activities increased consistently with increasing d.0.t. Azospirillum brasilense showed a low specific activity for catalase; the specific activity of superoxide dismutase increased sharply above 0.1 6 atm 02. INTRODUCTIONAlthough capable of aerobic growth, rhizosphere bacteria of the genus Azospirillum are capable of fixing molecular nitrogen only under low p 0 2 (0.005 atm), probably because of the inactivation of nitrogenase by oxygen and its radicals (Okon et al., 1976a; Day & Dobereiner, 1976;Nelson & Knowles, 1978).In recent studies (Nur et al., Okon et al., 1980), it was found that although A. brasilense was capable of growing at faster rates at high pOz in the presence of NH4C1, if given the choice and in spite of the presence of combined nitrogen, A . brasilense actively sought microaerobic conditions, i.e. it developed as a pellicle below the surface in semi-solid medium or it entered capillaries containing only water or phosphate buffer, forming a band which moved towards a decreasing PO, gradient (aerotaxis to low p02). When grown on N 2 at low PO,, A . brasilense strain Sp7 formed poly-P-hydroxybutyrate (PHB) which amounted to about 30% of the cell dry weight, whereas in cells growing under high PO, in the presence of NH4C1, PHB formed less than 1 % of the cells weight (Okon et aZ., 1976 b). Nur et al. (198 1) reported that red-pigmented A.brasilense strain Cd produced carotenoids, apparently involved in the protection of A . brasilense Cd nitrogenase from 0 2 .Carotenoid synthesis started in liquid static cultures only after the concentration of NH4Cl in the medium had decreased and N,-fixation became evident. Carotenoid synthesis did not occur under microaerobic conditions. When carotenoid synthesis was specifically inhibited in the presence of diphenylamine, the rate of acetylene reduction in strain Cd decreased by 50%.Carbon-limited chemostat cultures of A . brasilense Sp7 have bean studied on N-free medium at various dissolved oxygen concentrations (Nelson & Knowles, 1978). In this work we report the behaviour of A . brasilense Cd in the presence of NH4C1 in a chemostat, under different dissolved O2 concentrations and different dilution rates. (NBS model M1016-0208). The oxygen probe was calibrated initially by passing pure nitrogen an...

Wheat Root Tips as a Vector for Passive Vertical Transfer of Azospirillum brasilense Cd

“…All Azospirillum species are microaerophilic (Barak et al, 1982;Hurek et al, 1987;Nelson & Knowles, 1978;Nur et al, 1982). The colonization of root tips is of advantage for these cells because, when the roots penetrate deeper into the soil layer, the oxygen supply is lower and competition with the aerobic bacterial population of the rhizosphere is therefore reduced.…”

An extra-cytoplasmic function sigma factor and anti-sigma factor control carotenoid biosynthesis in Azospirillum brasilense

“…Besides the enzymes that detoxify superoxide and hydrogen peroxide, carotenoids in A. brasilense strain Cd are implicated in protection against oxidative damage owing to their ability to quench singlet oxygen and possibly oxygen radicals (Hartmann & Hurek, 1988;Nur et al, 1981Nur et al, , 1982. The fact that nitrogen fixation is inhibited in A. brasilense Cd at high dissolved oxygen concentrations upon inhibition of carotenoid synthesis by diphenylamine suggests that carotenoids protect nitrogenase from oxidative damage (Nur et al, 1981).…”

Physiological properties of Azospirillum brasilense sp 7 in a malate limited chemostat.

“…Due to its economic importance, work had been done previously on the physiology of A. brasilense in batch culture systems (S-7) and continuous culture systems (8)(9)(10). Carbon-limited continuous cultures of A, brasilense however have not yet been studied on N-free medium (8).…”

“…For this reason, studies on the physiological properties of this organism in a carbon limited chemostat were undertaken. The effect of oxygen on the nitrogen fixing ability and the efficiency of the nitrogenase was the main objective of the studies (8)(9)(10).…”