PRODUCTION Of NUTRIENT SOURCES FOR RHIZOBIUM

1.1.1� Carbon metabolism in Rhizobium species
A) Carbon nutrition in free living rhizobia
Many investigations have examined the nutritional diversity of carbon
utilization by rhizobia. Mannitol, glucose, sucrose and maltose were established early as useful carbon
sources for rhizobia (Fred, 1912; Ziefel, 1911, quoted in Stowers, 1985). One of the earliest
delineations of fast- and slow-growing rhizobia was based on carbon nutrition. Some evidences
summarized in Table 2, suggests that fast-growing rhizobia are capable of growing on a variety of
carbon substrates, whereas slow-growing rhizobia are more limited in their ability to use diverse
carbon sources.
Fast-growing rhizobia are able to use a broad range of hexoses, pentoses,
disaccharides, trisaccharides, and organic acids (Table 2). On the other hand, slow-growing rhizobia
are unable to use disaccharides, trisaccharides, and organic acids for growth (Table 2). Mannitol, the
traditional carbon source for rhizobia, gave highly variable growth responses with all rhizobia, but in
particular the slow growers (Elkan et al., 1968). Gluconate has been recommended for slow-growing
rhizobia, but has given poor growth responses with cowpea rhizobia. Glycerol is the most universally
used carbon sources among rhizobia (Arias et al., 1976, quoted in Stowers, 1985).
As stated in the previous section, rhizobia are able to use a broad range of
sugars, sugar alcohols, and organic acids (Table 2) as well as aromatic compounds for growth. A
broad distinction has been made between fast-and slow-growing rhizobia in their ability to use
disaccharides. Fast-growing rhizobia were able to use sucrose and other disaccharides, whereas slow
growers were unable to metabolize sucrose. Sucrose uptake by R. leguminosarum and R. trifolii has
been determined to be constitutive. Uptakes of sucrose by R. meliloti and a fast-growing cowpea
rhizobia isolate were determined to be inducible. Identical observations were reported for maltose
uptake. Lactose uptake was inducible with all fast-growing species. Evidence suggest that fastgrowing
rhizobia possess at least two transport mechanisms for disaccharides-one for sucrose,
maltose, and trehalose and another for lactose (Stower, 1985).
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