Inhibition of Azotobacter salinestris growth by zinc under iron-limited conditions

The growth yield of Azotobacter salinestris is, a Na+-dependent, microaerophilic nitrogen-fixing bacterium, was inhibited more than 60% by 5 mu M Zn2+. This organism was much more sensitive to Zn2+ than the obligate aerobe Azotobacter vinelandii. Inhibition of A. salinestris was most evident in iron-limited cells and exogenously added Fe2+ was more effective than Fe3+ in preventing inhibition by Zn2+. While Zn2+ decreased the Fe content of the cells, decreased the activity of the soluble cytoplasmic ferric reductase, and altered the intracellular Fe2+/Fe3+ ratio, which in turn increased siderophore production, none of these effects appeared severe enough to account for growth inhibition. However, Zn2+ also was observed to be a powerful inhibitor of Fe-limited whole cell respiration. As the cells became more Fe sufficient, this inhibition of respiration was decreased. Growth of A. salinestris is also was inhibited by Cd2+ > Zn2+ > Cu2+ > Cr2+ > Ni2+ > Co2+, and inhibition by these ions also was reversed by exogenous Fe2+ or Fe3+. Examination of isolated cell membranes showed that the sensitivity of A. salinestris NADH oxidase activity to Zn2+ and other respiratory poisons changed as the cells became Fe sufficient, but a similar change did not occur in A. vinelandii. It is proposed that Fe-limited A. salinestris cells present a sensitive target for Zn2+ inhibition, possibly a sulfhydryl group in a terminal oxidase, but this target is lost or is of decreased importance in Fe-sufficient cells.