A test designed to quantify the richness of soil by creating statistical models from soil samples taken from Alaskan and Minnesota found 5,000 and 2,000 different bacterial species per 0.5 grams of soil, respectively. Only 20% of the bacteria appeared to be endemic to both soils which speaks to the diversity, complexity and the vast number of interrelationships and processes in the bacterial communities and organisms responsible for sustaining all of life.
Only a tiny fraction of these organisms can be cultivated upon laboratory culture media. Scientists have yet to provide appropriate culture conditions for the vast majority of soil microbes. Many live in complex communities in which individuals cross-feed one another in a manner that cannot be replicated when the microbes are placed in artificial culture. The microbial activity of soil is severely underestimated using artificial culture. An estimate of the microbial activity of soil is further influenced by the fact that many soil bacteria and fungi are present as dormant spores. These may germinate when brought into contact with a rich artificial growth medium.
Soils contain many aerobic and facultative organisms and, because of the microbial manipulation of microenvironments, soils may harbour a large number of obligate anaerobes. Bacteria are the largest group of soil microbes, both in total number and in diversity. Indeed the presence of bacteria gives freshly dug soil its characteristic ‘earthy’ smell. The odor is that of geosmin, a secondary metabolite produced by streptomycete bacteria.
Both bacteria and fungi provide an abundant source of food for soil protozoa. The most commonly encountered soil protozoa include flagellates and amoebas. The abundance of such creatures depends upon the quantity and type of organic matter present in the soil sample. Protozoa play a key role in the regulation and maintenance of the equilibrium of soil microbes. Whereas many microbes obtain their nutrients from solution, protozoa are frequently found to be of a scavenging nature, obtaining their nutrients by devouring other microbes.
Biologic Nitrogen Cycle
Inorganic nitrogen compounds such as nitrates, nitrites and ammonia are converted into organic nitrogen compounds such as proteins and nucleic acids in the process of nitrogen assimilation. Many bacteria reduce nitrates to nitrites and some bacteria further reduce nitrites to ammonia. Ammonium salts may then be incorporated into organic polymers in the process of assimilatory nitrate reduction. Ammonia is primarily fixed into organic matter by way of amino acids such as glutamate and glutamine. Other nitrogen compounds can be made from these.
Bacteria are also involved in the inorganic cycling of nitrogen compounds. Nitrifying bacteria are responsible for the biological oxidation of ammonia. These bacteria are chemolithotrophs, obtaining chemical energy from the oxidation process. This energy is used to elaborate organic compounds from carbon dioxide. Nitrifying bacteria such as those of the genus Nitrosomonas produce nitrite ions from the oxidation of ammonia. Bacteria of the genus Nitrobacter and a few other genera can oxidize nitrites to nitrates.
Nitrates may be used by some bacteria instead of oxygen for a type of respiration referred to as dissimilatory nitrate reduction. During this process, nitrate is reduced to nitrite and thence to ammonia. This may then be assimilated into organic compounds as described above. Not all bacteria follow this pathway, however. Bacteria of the genus Pseudomonas, micrococci and Thiobacillus species can reduce nitrates to liberate nitrogen gas into the environment.
To complete the inorganic nitrogen cycle, nitrogen gas must be fixed in a form that can be used by living organisms. Nitrogen is converted into ammonia in the process of nitrogen fixation. Bacteria are the only life-forms capable of the biological fixation of nitrogen. They are of vital importance if life is to continue on this planet.