"Adapted from Hill (1992) by permission of the publisher.

transformed to 2 NH3. Considerable interest has been shown in those diazotrophs that contain an enzyme called uptake hydrogenase, which reoxidizes H2 to protons and electrons and salvages the reductant lost in nitrogenase-dependent H2 formation.

The high energy costs of BNF can also be illustrated by comparing the amount of reduced C oxidized g-1 of N assimilated from either N2 or NH4-N (Table 14.3). It can be seen that the cost of assimilating N2-N is about twice that of assimilating NH4 -N. Several factors contribute to this high energy cost. In addition to the high ATP cost of reducing N2 to NH3, at least 20 genes and their products are needed for synthesis of a completely functional N2-fixing enzyme system (Table 14.4). The genes involved collectively in synthesis of nitrogenase and the catalytic process of N2 fixation are called nif genes. Accessory genes are called fix genes, and they are also necessary for the function and regulation of nitrogenase in micro-aerobic and aerobic diazotrophic bacteria. The catalytic process of N2 fixation is remarkably slow and inefficient. The two proteins must come together and dissociate eight times to reduce one molecule of N2 to two NH3. To compensate for this sluggish mechanism, nitrogenase might account for ~10% of total cell protein in a diazotrophic microorganism. Finally, many of the nif gene proteins are denatured by oxygen and turnover quickly in an aerobic environment. Replacement of these denatured proteins constitutes an additional energy expense.

Because of the extreme sensitivity of nitrogenase to irreversible denaturation by O2, aerobic N2-fixing bacteria have developed various O2 protection mechanisms. For example, some diazotrophic O2-evolving cyanobacteria produce specialized cells called heterocysts under N2-fixing conditions (Fig. 14.2). Heterocysts do not divide like their neighboring vegetative cells, do not evolve O2 or fix CO2, and use light energy to provide the energy needed for BNF. A combination of respiration and thick cell walls excludes O2 sufficiently well to protect the dinitrogenase enzyme complex. In some N2-fixing nonheterocystous cyanobacteria, temporal separation of N2 fixation and O2 evolution results in BNF occurring primarily during darkness in the absence of O2 production. At least under C-rich laboratory conditions, aerobic, N2-fixing bacteria such as Azotobacter express a high respiratory rate that

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