Adapted from Young (1992) by permission of the publisher.

Adapted from Young (1992) by permission of the publisher.

have developed between eukaryotes and prokaryotes that circumvent the eukaryotes' needs for an exogenous supply of fixed N. Bacteria that use N2 as the sole source of N are called diazotrophs. Although only a limited number of bacterial species fix N2, they represent a wide variety of phylogenetically and physiologically distinct types that occupy different ecological niches (Table 14.2). These bacteria use diverse energy sources including sunlight (phototrophs), reduced inorganic elements and compounds (lithotrophs), and a plethora of different organic substrates (heterotrophs). They are represented by obligate aerobes and facultative and obligate anaerobes. This metabolic diversity indicates that diazotrophs can contribute fixed N to other life forms in a wide variety of environments.

BNF is mediated by an enzyme complex called nitrogenase, which is composed of two proteins (dinitrogenase and nitrogenase reductase). There are three different forms of nitrogenase that differ in their requirements for molybdenum (Mo), vanadium (V), or iron (Fe) as a critical metallic component of the cofactor associated with the catalytic site. Most of the nitrogenases that have been studied extensively contain a Mo cofactor. N2 fixation is an energetically expensive process. Two ATP molecules are required for each electron transferred from nitrogenase reductase to dinitrogenase, which contains the catalytic site. Nitrogenase reductase is recharged with electrons provided by a protein called ferredoxin or flavodoxin. A total of 16ATP molecules are required to provide the six electrons necessary to reduce 1N2 molecule into 2NH3 because 25% of the energy used to reduce N2 is "lost" in the reduction of 2H+ to H2. One mole of H2 is formed per mole of N2

TABLE 14.3 Energy Costs for Biological N2 Fixation Relative to Assimilation of NHj-N"

Bacterial species

Energy source

N2-N fixed/g C source used (mg)

NH4-N assimilated/g C source used (mg)

Anaerobic growth: Clostridium pasteurianum Microaerophilic growth:

Azospirillum brasilense Aerobic growth: Azotobacter vinelandii

Sucrose 11

Malate 26

Sucrose 7

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