The Nitrogen Cycle

This is my first attempt at using Research Blogging, hope I don't mess this up.

At any rate I've been reading a number of articles, for a paper I'm putting together, and I came across this review which I think is pretty good. It goes over the nitrogen cycle, of which I'm primarily concerned with nitrification and denitrification. Nitrification is the biological degradation of ammonia to nitrite and nitrates. Denitification takes nitrite and nitrate and reduces it to (hopefully) dinitrogen gas. I've illustrated (and simplified) the process in the following diagram. I'll also explain the "hopefully" comment towards the end of this entry.



Nitrification, since it is an oxidation reaction, requires oxygen (as one would surmise). Denitrification typically occurs under anaerobic conditions. As a result, the major players for both reactions are typically different, though there is some overlap. There are other processes involved in the nitrogen cycle, such as ANAMMOX (Anaerobic Ammonium Oxidation), Nitrifier Denitrification, and Aerobic Bacterial Denitrification, which have all gained increasing examination amongst scientists but have a long way to go before being as well understood as the classical systems mentioned above.

The field has also revealed that these processes are no longer the domain of bacteria, as once thought, but that the fungi and archaea also play a role in these processes. In fact, fungi in denitrification, and archeae in nitrification and denitrification, may be major players if not THE major players in particular environs.

What has genomics told us? Genome sequencing has definitely sped up the process of understanding, and more importantly, identifying the organisms responsible for these processes. These studies have also shed light on the evolutionary paths these organisms have undergone as well. One such example is that of Nitrosomonas europaea (Ne). This intricate organism lacks all but one single, rudimentary synthesis pathway for siderophore production (citrate). Siderophores bind to iron, which are then picked back up by the organism via a receptor. With few exceptions, iron is a nutrient essential for life, which is why at first glance it makes little sense for Ne to not produce siderophores. However, Ne does encode receptors for many different siderophore types. What does this mean? It means it scavenges off of the siderophores of other organisms. This allows Ne to devote most of its energy into other pathways without having to worry about synthesizing means to acquire iron.

So back to the "hopefully" comment I made earlier. Why are nitrification and denitrification so important? Well, there are several reasons, one of the major ones being that nitrous oxide is a major green house gas (GHG). Nitrous oxide actually has 296 times more of an impact on global warming than carbon dioxide, which sort of makes it a "Big Deal". Nitrous oxide is produced when denitification occurs "incompletely". This incomplete denitrification occurs naturally in the environment. Some organisms don't have the genes which encode for the final step of denitrification, the conversion of nitrous oxide to dinitrogen gas. Sometimes the reason is regulatory, in that the environment is such that nitrous oxide is the final end product. Understanding how and why this occurs is the focus of current research by many individuals.

Hope this sheds some light on these two steps of the nitrogen cycle. I hope to post more on the matter in the future.

References

Hayatsu, M., Tago, K., Saito, M. (2008). Various players in the nitrogen cycle: Diversity and functions of the microorganisms involved in nitrification and denitrification. Soil Science and Plant Nutrition, 54, 33-45. Article here.

Chain, P. (2003). Complete Genome Sequence of the Ammonia-Oxidizing Bacterium and Obligate Chemolithoautotroph Nitrosomonas europaea. Journal of Bacteriology, 185(9), 2759-2773. DOI: 10.1128/JB.185.9.2759-2773.2003

Resources to Learn More About Nitrogen Cycle
Online University Rankings
University of Missouri