Among the three factors the experiment identified as being critical in determining the process, the ratio of the two oxidized forms of fixed nitrogen, nitrate (NO3-) or nitrite (NO2-), is the most basic because when nitrite is more abundant (nitrate can change to nitrite and vice-versa), the process that dominates is inevitably denitrification.
"Nitrite accepts three electrons in denitrification and six in ammonification," Strous pointed out. "In both cases, the supply is there, but the difference between the way the denitrifiers respire them is that out of each electron accepted they are able to conserve more of the energy less energy gets lost as heat, per electron."
When nitrite is more abundant, the denitrifying bacteria dominate because they are more efficient and faster. The fundamental advantage of the speed and efficiency of the denitrification process also contributes in part to the second critical factor the study found microbial generation time. Regardless of the ratio of the fixed nitrogen supply, when conditions encouraged rapid regeneration (generation time of less than 1.7 days), then the denitrifying bacterial groups came to dominate (either rapidly or eventually), but slow growth conditions favored ammonifying bacteria.
The same kind of fundamental chemical energy reasoning seems to help explain the third critical factor the team identified the carbon to nitrogen ratio.
"As has already been proposed in the literature, the carbon to nitrogen ratio is an important factor because, based on the bioenergetics, carbon limitation will favor denitrification denitrifying organisms will have higher productivity under those conditions. Whereas the ammonifying microorganisms will have higher productivity when
|Contact: James Hathaway|
University of North Carolina at Charlotte