Pseudomonas saccharophila could fix N$ sb2$ under micro-aerobic conditions, heterotrophically and chemolithotrophically. Uptake hydrogenase activity under heterotrophic conditions had no effect on the O$ sb2$ sensitivity of nitrogenase. H$ sb2$ induced whereas sucrose and O$ sb2$ repressed hydrogenase synthesis. Sucrose and O$ sb2$ did not inhibit hydrogenase activity. Hydrogenase and urease were located in the membrane and soluble fractions, respectively. Nickel stimulated growth, hydrogenase expression, and nitrogenase activity under N-limited chemolithotrophic conditions. Hydrogenase synthesis specifically required nickel and its repression by O$ sb2$ was alleviated by increasing the nickel concentration. Incorporated $ sp{63}$Ni$ sp{2+}$ was about 3 times higher in the soluble than in the membrane fraction. The short-term uptake of nickel was energy-independent and had an apparent $K sb{m}$ of 31.7 uM and $V sb{max}$ of 3.8 nmol Ni$ sp{2+}$ (mg protein)$ sp{-1}$min$ sp{-1}$. / A counting method for heterotrophic and chemolithotrophic N$ sb2$-fixing H$ sb2$-oxidizing bacteria was developed. The white bean rhizosphere soil showed relatively high numbers of these bacteria.

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