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Frontiers in Microbiology - VI. Microbes' Inluence on Earth

The impacts of microbes on earth's environment have been and continue to be substantial. The sheer number of microbes on Earth is staggering. It has been estimated that we share the planet with some 5×1031 microbes, which weigh more than 50 quadrillion metric tons. Put another way, microbes constitute about 90% of the earth’s biomass, excluding cellulose, and more than 60 percent of the biomass when cellulose is included (ASM, 2004).

These diverse microbes are responsible for cycling the elements essential for life, such as oxygen, nitrogen, carbon, sulfur, and hydrogen. By making these elements available in soils, microbes increase its fertility and allow for plant growth. The plants, in turn, are the producers of our ecosystem, providing the energy needed by animals, including humans. Microbes also cycle the gases in our atmosphere. We tend to think of green plants as being responsible for providing us with oxygen through photosynthesis. Actually, more photosynthesis is carried out by microbes than by plants. Ultimately, all photosynthesis derives from microbes. The cyanobacteria (formerly called blue-green algae) can live aerobically or anaerobically. They are responsible for the initial rise of atmospheric oxygen around 2.3 billion years ago (Kastling & Siefert, 2002). Eukaryotoic algae and land plants acquired their photosynthetic capabilities from cyanobacteria through endosymbiosis. This conclusion is based on sequence analysis of ribosomal RNA and chloroplast DNA.

Microbes are also major recyclers of other atmospheric gases that contribute to the earth’s “greenhouse effect.” Theses gases, including water vapor, carbon dioxide, nitrous oxide, methane, and ozone, allow light energy from the sun to reach the earth. This energy is absorbed by the earth’s surface and re-radiated as heat. The greenhouse gases then absorb much of this heat and re-radiate it in all directions. This means that less heat escapes into outer space and the earth stays warmer than it would otherwise. If the greenhouse gases were not present, the average temperature of the earth would fall by some 60°F. However, it is possible to have too much of a good thing. Today, excessive quantities of greenhouse gases are increasing the average temperature of the earth. This problem is called global warming and threatens the delicate balance of life on this planet.

Ecosystems are so complex that researchers often resort to using modeling techniques that take into account the many interrelationships found within the ecosystem. Sometimes models produce unexpected results. For example, a recent study (Knight, 2004) found that the production of acid rain can actually work to lessen global warming. A significant portion of the human-induced global warming comes from agriculture. For example, microbes that use hydrogen and acetate found in peat produce methane. Living alongside the methane-producing microbes are others that metabolize sulfur. The sulfur-eating microbes can outcompete the methane-producing ones, thereby reducing the amount of methane released into the atmosphere. This study demonstrates the importance of accounting for sulfur pollution when estimating methane emissions. It does not, however, suggest a strategy for controlling methane emissions. After all, acid rain upsets the chemical balance of rivers and lakes, killing fish and damaging trees.

Nitrogen is an element essential to support the lives of all life. It is a required component of proteins, nucleic acids and other cellular components. Although the atmosphere is nearly 80 percent nitrogen, this gas is unavailable for use by most organisms. The strong bonds between the two nitrogen atoms render the N2 molecule almost inert. In order for the nitrogen to be useful for growth, it must first be combined or “fixed” in the form of ammonium or nitrate ions.

Some species of bacteria can convert nitrogen gas to ammonia through the process of nitrogen fixation. Other species can transform ammonia to nitrate. Still other species decompose organic matter, releasing fixed nitrogen in the process. Of special interest are the species of nitrogen-fixing bacteria that form symbiotic relationships with plants of the legume family. Legumes such as peas and beans have root nodules that contain Rhizobium bacteria. These microbes supply the plants with fixed nitrogen, reducing the need for farmers to use commercial fertilizers.

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