Nitrogen cycle dominated by Archaea
Not soil bacteria, but certain Archaea species probably dominate the nitrogen cycle of the biosphere and prepare the important nutrient element in a way that is suitable for plants, according to the results of soil investigations by Christa Schleper from the University of Bergen and her colleagues.
They found in individual soil types up to 3000-fold higher amounts of a certain archaea gene than the corresponding bacterial counterpart, both of which encode a subunit of the key enzyme ammonium monooxygenase (amoA). These quantitative differences were also confirmed by the high concentrations of crenarchaeol, a lipid found exclusively in certain archaea, and the activity of the nitrifying archaea actually measured in the soil.
In all samples of the twelve soil types from different regions of Europe, the amount of amoA of the Archaea exceeded that of the bacteria - regardless of the extraction depth of the substrate or the type and extent of artificial fertilization. In addition, in unfertilized soils, the number of nitrifying bacteria steadily decreased with increasing depth, while that of their peers remained more or less the same. These archaea are therefore likely to be adapted to a broader range of environmental conditions, the researchers say, making them arguably the numerically most important fraction involved in nitrogen uptake.
Nitrogen is an essential nutritional element as it is a component of amino acids in proteins, DNA or enzymes. However, 99 percent of the element is originally in the earth's atmosphere and therefore inaccessible to most living beings. At best, special bacteria can absorb and utilize it directly. It only becomes available to plants, animals and ultimately humans via detours – such as the symbiosis of plants with certain nodule bacteria on their roots. In addition to direct extraction from the air, the process of nitrification also plays a major role: the nitrogen compounds contained in dead organic substances are broken down in the soil to form ammonium, which in turn is used by bacteria living freely in the soil and water to generate energy, according to the previously common method Level of knowledge.
According to this thesis, specialized nitrite bacteria process this ammonium into nitrite to generate energy, which the microbes excrete from their metabolism. Then another helper fraction, the nitrate bacteria, takes on this nitrite and oxidizes it further to nitrate, which in turn is more easily digestible by plants and can be easily absorbed and processed by the vegetation as a mineral nitrogen fertilizer.
