Microbial Nitrogen Cycle: Determination of Microbial Functional Activities and Related N-compounds in Environmental Samples
D. Correa-Galeote, G. Tortosa and E.J. Bedmar
from: Metagenomics of the Microbial Nitrogen Cycle: Theory, Methods and Applications (Edited by: Diana Marco). Caister Academic Press, U.K. (2014)
Nitrogen (N) is part of essential compounds such as proteins, nucleic acids, hormones, etc. Although N makes up to about 80 per cent of the Earth's atmosphere, it is not readily available for plant and animal consumption. Free-living and symbiotic microbes contain the enzyme nitrogenase which initiates the N-cycle in the biosphere by reducing dinitrogen gas to bio-available ammonia, a process called nitrogen fixation. Ammonia is subsequently oxidized to nitrate by nitrification, a two-step aerobic pathway during which ammonia is oxidized to nitrate and nitrite by the enzymes ammonia monooxygenase and nitrite oxidoreductase, rexpectively. Finally, nitrate is reduced to dinitrogen gas by denitrifying microorganisms, thereby closing the N cycle. Denitrification is carried out by the sequential activity of the enzymes nitrate-, nitrite, nitric oxide and nitrous oxide-reductase, respectively. Ammonia can also be incorporated into cellular biomass via the glutamine synthetase-glutamate synthase and glutamate dehydrogenase pathways to form amino acids and other nitrogen compounds. After cellular death, organic nitrogen compounds are released to the environment to be mineralized by microbial activities. Widely-used procedures for determination of microbial functional activities of the nitrogen cycling microorganisms and of N-compounds produced during the redox reactions of the cycle will be addressed. In addition, we will consider new methodologies being developed for further understanding of the N-cycle read more ...