Microbial ecology is a study of the relationship between microorganisms and their environment. Microbial ecology has branched out and includes on how microbial populations interact in various environments with plants and animals. Among the concerns of microbial ecologist are pollution and toxic chemicals in the environment.
The chemical elements nitrogen, carbon, oxygen, sulfur, and phosphorus are essential for life, through large amounts. Nitrogen, oxygen, and carbon (as carbon dioxide) exist as gases in the atmosphere. Sulfur and phosphorous are elements stored on earth’s crust. For the most part, microorganism converts these elements into forms that can be used by plants and animals.
A certain soil bacteria and cyanobacteria in water combine gaseous nitrogen with other elements (nitrogenfixation) to form compounds that other organisms can use. When other helpful bacterium help to decompose dead plants and animals, nitrogen-containing chemicals are released into the soil; still other microbes then convert the soil nitrogen back to nitrogen gas.
The cyanobacteria is a type of helpful bacterium that are essentially aerobic. These helpful bacterium carry out oxygen-producing photosynthesis. However, some of these helpful bacterium are capable of using reduced sulfur compounds for anoxygenic photosynthesis. Few numbers of cyanobacteria are capable of fixing nitrogen. Specialized cells called heterocysts contain enzymes that fix nitrogen gas (N2) into ammonium (NH4) necessary for use by the growing cell. Cyanobacteria are helpful bacterium that are motile and move about by gliding.
The cyanobacteria are helpful bacteria that are morphologically varied. This helpful bacterium have unicellular, forms that divide by simple fission colonial forms that divide by multiple fission and filamentous forms that reproduce by fragmentation of the filaments. The filamentous forms usually exhibit some differentiation, of cells, which are often bound together within an envelope or sheath if examined through the microscope.
The cyanobacteria, especially those that fix nitrogen, are helpful bacteria that are extremely important to the environment. They occupy environmental niches similar to those occupied by the eucaryotic algae, but the ability of cyanobacteria to fix nitrogen makes them even more adaptable.
Nitrogen Fixation
During the initial phase of the nitrogen cycle, nitrogen gas is converted into ammonia in a process called nitrogen fixation. Only a few species of bacteria arid cyanobacteria, as seen through microscope, are capable of enacting this process. The nitrogenase enzyme responsible for nitrogen fixation is anaerobic, so it probably evolved early in the history of the planet, before the atmosphere contained molecular oxygen and before nitrogen-containing compounds were available from decaying organic matter. Nitrogen fixation is brought about by two types of organisms, nonsymbiotic and symbiotic.
Nonsymbiotic (free-living) nitrogen-fixing bacteria are found in particularly high concentrations in the rhizosphere as examine with microscope, the region where the soil and roots make contact-especially in grasslands. Among the nonsymbiotic bacteria that can fix nitrogen are aerobic species such as Azotobacter. These aerobic organisms apparently shield the nitrogenase enzyme from oxygen by, among other things, having a very high rate of oxygen utilization that minimizes the diffusion of oxygen into the cell where the enzyme is located.
There are many species of aerobic, photosynthesizing cyanobacteria that fix nitrogen. Because their energy supply is independent of carbohydrates in soil or water, they are especially useful suppliers of nitrogen to the environment. Cyanobacteria are hempful bacteria that usually carry their nitrogenase enzymes in specialized structures called heterocysts that provide anaerobic conditions for fixation.
Most of the nonsymbiotic nitrogen-fixing organisms are capable of fixing large amounts of nitrogen under laboratory conditions. However, in the soil there is usually a shortage of usable carbohydrates to supply the energy needed for the reduction of nitrogen to ammonia, which is then incorporated into protein. Nevertheless, these nitrogen-fixing bacteria, as seen through a microscope, make important contributions to the nitrogen economy of such areas as grasslands, forests, and the arctic tundra.
Symbiotic nitrogen-fixing bacteria serve an even more important role in plant growth for crop production. Members of the genera Rhizobium and Bradyrhizobiuni infect the roots of leguminous plants, such as soybeans, beans, peas, peanuts, alfalfa, and clover. (These agriculturally important plants are only a few of the thousands of known leguminous species, many of which are bushy plants or small trees found in poor soils in many parts of the world.) Rhizobia are specially adapted to particular leguminous plant species.
Closely examining these helpful bacteria with a microscope, the bacteria attach to the root of the host legume, usually at a root hair. In response to the-bacterial infection, an indentation forms in the root hair and an infection thread synthesized by the, plant passes down the root hair into the root itself. The helpful bacteria follow this infection thread and enter the cells in the toot. Inside these cells, the bacteria alter their morphology into larger forms called bacteroids that eventually pack the plant cell. The root cells are stimulated by this infection to form a tumor like nodule of bacteroid packed cells, as seen through a microscope. Nitrogen is then fixed by a symbiotic process of the plant and the bacteria. The plant furnishes anaerobic conditions and growth nutrients for the helpful bacteria, and the bacteria fix nitrogen to be incorporated into plant protein. Millions of tons of nitrogen are fixed in this way each year.
There are similar examples of symbiotic nitrogen fixation in nonleguminous plants, such as alder trees. These trees are among the first to appear in forests after fires or glaciation. The alder tree is symbiotically infected with an actinomycete (Frankia) and forms nitrogen-fixing root nodules. About 50 kg of nitrogen can be fixed each year by the growth of one acre of alder trees; the trees thus make a valuable addition’ to the forest economy.
Another important contribution, to the nitrogen economy of forests is made by lichens, which are a symbiosis between a fungus and an alga of cyanobacterium, as seen through a microscope. When one symbiont is a nitrogen-fixing cyanobacterium, the product is fixed nitrogen that eventually enriches the forest soil. Free-living cyanobacteria can fix significant amounts of nitrogen in desert soils after rains and on the surface of arctic tundra soils. Rice paddies can accumulate heavy growths of such nitrogen-fixing organisms. The cyanobacteria also form a symbiosis with a small floating fern, Azolla, which grows thickly in rice paddy waters. So much nitrogen is fixed by these organisms that other nitrogenous fertilizers are often unnecessary for rice cultivation.
