F. J. Hanus

Stimulation by nickel of soil microbial urease activity and urease and hydrogenase activities in soybeans grown in a low-nickel soil

SummaryThe concentration of nickel in some soils may be insufficient to meet the requirements of enzymes such as urease in soybeans and hydrogenase in Rhizobium. In an initial evaluation of nickel availability, several soils were examined for nickel content and microbial urease activity. Total and extractable nickel were determined by atomic emission spectrometry. Purified glucose and urea were added to soils to stimulate microbial growth and urease activity, the latter of which was monitored by the rate of decomposition of14C urea. Nickel also was added to some samples to determine if the indigenous supply was limiting. In one low-nickel soil (total Ni 13 ppm) urease activity increased 150% in response to additional nickel, while other soils (total Ni 22–3491 ppm) failed to respond to nickel. However, additional nickel did stimulate urease activity (up to 109%) in 3 out of 10 soils to which purified CaCO3 was added. Presumably the rise in pH associated with this treatment decreased nickel availability. Additions of Co, Mn, Fe, or Cu had no consistent effect on urease activity, thus indicating that the response to Ni was specific. Nickel fertilization increased leaf urease and nodule hydrogenase activity of soybeans grown in low-nickel soil, however, yield was not improved. These results may have practical implications in the nutrition of plants and micro-organisms that metabolize H2 and urea.

The importance of hydrogen recycling in nitrogen fixation by legumes

In vivo losses of H2 from legume nodules are often much greater than 25% of the nitrogenase electron flux. A minority of strains of Rhizobium form bacteroids with a capability to activate H2 by an hydrogenase and to oxidize this gas via the respiratory electron transport chain. A minority of R. leguminosarum strains but a majority of strains of Bradyrhizobium spp. (cowpea) possess H2 recycling capability. The genetic determinants for hydrogenase expression are present in the rhizobial cells but the effectiveness of the H2 uptake in nodules is influenced by the host and environmental conditions. The majority of trials in which Hup+ and Hup- inocula have been compared have shown growth increases from H2 recycling capability, but most of these comparisons have not utilized genetically defined strains. The genes for hydrogenase expression have been cloned and are being characterized. Progress in the transfer and expression of H2 recycling capability among strains of Rhizobium is discussed.

THE PRESENT STATUS OF HYDROGEN RECYCLING IN LEGUMES

ABSTRACT A brief discussion is presented of recent information concerning (a) factors influencing extent of N2, loss during N2 fixation by legumes; (b) electron carriers involved in the oxyhydrogen reaction of Rhizobium japonicum bacteroids; and (c) progress made in evaluating H2 recycling advantages. A major factor determining whether H2 is evolved from legume nodules is the presence of an active uptake hydrogenase which participates in the oxidation of H2 that is evolved as a by-product of the nitrogenase reaction. The extent of H2 evolution from the nitrogenase reaction is affected by those factors that influence the nitrogenase turnover rate. These include the supply of ATP and reductant and the ratio of the Fe protein to the MoFe protein component of nitrogenase. Oxidation of H2 in Rhizobium bacteroids is catalyzed by a series of enzymes located in bacteroid membranes. In addition to the hydrogenase per se, carriers so far shown to be involved in the process include cytochromes of the b and c types a...

Characterization, Significance and Transfer of Hydrogen Uptake Genes from Rhizobium Japonicum

Nitrogenases from different sources exhibit ATP-dependent formation of both NH4 + and H2. Many N2-fixing microorganisms are able to recycle the nitrogenase-mediated H2 using a membrane-bound hydrogenase, thereby recovering some of the energy utilized in the formation of H2. The physiology, biochemistry and energetics of H2 recycling have been extensively reviewed (Dixon, 1978; Robson, Postgate, 1980; Adams et al., 1981; Eisbrenner, Evans, 1983; Evans et al., 1985).

Chemolithotrophy in Rhizobium

The first pure cultures of Rhizobium species were described by Beijerinck in 1888 who surface disinfected nodules from Vicia, Lathrus and Trifolium and cultured the nodule endophytes on gelatin plates containing plant extracts, sucrose, and asparagine. Since this pioneering discovery, Rhizobium species have been considered chemoorganotrophs and always have been supplied with carbon substrates such as hexoses, pentoses, or complex carbohydrates (Vincent, 1978).