GENERAL MICROBIOLOGY - T.P. Pirog - 2004

15. ELECTRON TRANSPORT UNDER ANAEROBIC CONDITIONS (ANAEROBIC RESPIRATION)

15.1. DENITRIFICATION AND NITRATE REDUCTION

Microorganisms use nitrate:

1) for the synthesis of nitrogen-containing cellular components. Such assimilatory nitrate reduction can occur under both aerobic and anaerobic conditions;

2) for nitrate Respiration (dissimilatory nitrate reduction), where nitrate serves as the terminal hydrogen (electron) acceptor under anaerobic conditions.

In both cases, nitrate is first reduced to nitrite by means of nitrate reductase.

During assimilatory nitrate reduction, the resulting nitrite is reduced to ammonia by nitrite reductase, a process requiring six electrons. The electrons are supplied by NAD(P)H (in Fungi and Bacteria) or ferredoxin (in plants and certain bacteria). Ammonia is then utilized for the synthesis of cellular components.

Denitrifying bacteria are capable of reducing nitrate via nitrite to gaseous nitrous oxide (N₂O) and molecular nitrogen (N). This denitrification process has been identified exclusively in facultative anaerobes. Some denitrifiers can grow using not only nitrate, but also nitrite and sometimes even nitrous oxide as a terminal hydrogen acceptor. Examples of denitrifying bacteria include soil bacteria, such as bacilli and Paracoccus denitrificans.

Nitrogen depletion of soil due to denitrification. Temporary losses of nitrogen in localized soil areas are associated with The activity of denitrifying bacteria. This process occurs most actively when anaerobic conditions develop in the soil (e.g., waterlogging), especially when organic fertilizers and nitrates are applied during this period. Nitrite, formed from nitrate during denitrification, accumulates in wastewater and may also contaminate drinking Water.

The Significance of denitrification in nature. Denitrification is the sole biological process by which bound nitrogen is converted into free molecular nitrogen. From a global perspective, this process is crucial for the sustenance of life on land. In normally aerated soils and water bodies, nitrate is the end product of mineralization. Due to its high water solubility and weak binding to soil particles, nitrate ions would otherwise be leached from soils and accumulate in seawater. This would ultimately lead to a depletion of molecular nitrogen in the atmosphere, eventually halting Plant Growth and biomass production on land.