Hydrogen (H_2) gas is an obligate byproduct of the N_2-fixing enzyme, nitrogenase, claiming about 33 percent of the reducing power and ATP that flows to the errzyme. In some legume symbioses, the bacteria also produce an uptake hydrogenase (HUP) that is able to oxidize the H_2 and thereby recover the reducing power used in H_2 production. However, many N_2 fixing legume nodules evolve H_2 due to the absence (HUP~-) or low activity of the uptake hydrogenase (Arp 1992). In a HUP" symbiosis, large amounts of H_2 can diffuse out of the nodule into the soil. For example, at peak growth every hectare of a N_2-fixing soybean field will produce about 5000 L H_2 d~(-1). This hydrogen evolution represents an energy equivalent to about 5 percent of the crop's net photosynthetic C gain for that day (Dong, Layzell 2001). It is interesting to note that the majority (>75 percent) of the rhizobia strains isolated from major soybean production areas in United States are HUP~-(Uratsu et al. 1982). Also, all knownclover and alfalfa symbioses are HUP~-.
展开▼
