The Cycle of Life
A balanced natural ecosystem can remain healthy and biologically vibrant year after year because it is essentially a closed system. A corn field, for example, is not a closed system because nutrients in the corn are exported to distant locations and cannot return to the soil in the form of organic wastes. An undisturbed rainforest, on the other hand, retains most of its fertility from year to year because nutrients are not removed in the form of crops or timber. But these conserved nutrients are not always in a convenient form. Tree roots, for example, cannot absorb minerals locked up in the branches of a shrub that recently died. This is where bacteria become involved.
A World of Bacteria
Bacteria are an extremely diverse group of organisms. Almost every environment on Earth provides a suitable home for certain bacterial species, and different types of bacteria are tolerant to acidic substances, high temperatures, lack of oxygen, nutrient deficiencies and various other stressful conditions. The numerous species of bacteria that help to recycle nutrients are known as decomposers. These microscopic, single-celled creatures sustain life on Earth by decomposing dead organisms so that their nutrients are returned to the ecosystem in a form that can be utilized by future generations.
The Cleanup Crew
Decomposer bacteria contribute to nutrient recycling in a wide variety of ways. In garden soil, for example, bacteria help to transform fresh plant and animal residue into humus, which is a stable organic substance that is crucial to long-term soil fertility. Forest bacteria help to decompose certain portions of woody residue, and in so doing they maintain appropriate levels of carbon in the atmosphere and ensure that the remnants of dead organisms do not perpetually accumulate on the forest floor. Decomposers are also directly consumed by various higher organisms, which benefit from the recycled nutrients stored in bacterial cells.
Certain types of bacteria are not decomposers in the typical sense but nonetheless contribute to important nutrient cycles. Some of the most widely known and appreciated species in this category are the numerous bacteria that can convert atmospheric nitrogen into plant-available forms of soil nitrogen. Many of these bacteria are in the Rhizobium genus, but various other genera include species that can "fix" nitrogen from the atmosphere and store it in the soil. Nitrogen, an essential plant nutrient that is often in limited supply, can escape into the atmosphere through processes known as denitrification and volatilization. Nitrogen-fixing bacteria recycle this lost nitrogen back into the soil.