Energy and chemical elements flow through ecosystems and control the biological and geological processes that shape them. Biogeochemical cycling refers to the flux of energy and chemical elements through living (biotic) and nonliving (abiotic) components of the biosphere. Many important biogeochemical cycles, such as carbon, nitrogen, phosphorus, water, and iron, exist on Earth.
Each cycle is complex and differs in substantive ways from the other cycles, though they do share some unifying characteristics. All chemical elements have abiotic reservoirs, meaning they are found in large amounts in non-living materials. For example, nitrogen (N2) makes up 78% of the atmosphere and phosphorus is bound to other elements in rock. Both the atmosphere and rocks are abiotic reservoirs. There is a net movement, or flux, of each element between biotic and abiotic components of ecosystems. For example, during photosynthesis the carbon, water, and oxygen cycles interact in algae and plants. Carbon from the atmosphere and water from the soil are used to synthesize the sugars plants use as food, and oxygen is released into the atmosphere. In all of the biogeochemical cycles, the elements are recycled. When an animal eats a plant that fixed atmospheric carbon dioxide during photosynthesis, the animal uses the energy in the plant in the process of cellular respiration and releases carbon dioxide back into the atmosphere.
Microorganisms (microbes)—bacteria, archaea, and some fungi—play significant roles in biogeochemical cycles. In the cycles of some elements—for example, nitrogen and iron—they are able to change redox states of the elements through oxidation and reduction reactions by adding or removing hydrogen or oxygen molecules. Redox changes alter the overall charge on molecules, which changes what other molecules will or will not react with them. In this way, microbes change the availability of nutrients in the soil or water. Some microbes are also autotrophic, and, collectively, they perform about half of the photosynthesis on the planet. This is calculated by measuring the amount of oxygen produced and carbon dioxide used by autotrophic species. Microbes also perform most of the decomposition processes that recycle molecules in dead animal, plant, and microbial materials. Through these processes, microbes exert far greater influence on the biosphere than all other organisms.
The three major biogeochemical cycles are the carbon cycle, the hydrologic (water) cycle, and the nitrogen cycle. Several minor cycles play some importance in maintaining life, including the iron, phosphorus, and sulfur cycles.
Major Biogeochemical Cycles
| Biogeochemical Cycle | Function | Microbial Roles |
|---|---|---|
| Carbon cycle | Producing and recycling organic carbon for organisms to use | Algae, bacteria—produce organic carbon via photosynthesis Fungi, bacteria, archaea—recycle carbon from decomposing organic matter in the soil |
| Hydrologic cycle | Recycling water over the planet | Bacteria—serve as particles for rain or snow to form around |
| Nitrogen cycle | Producing and recycling organic nitrogen for organisms to use | Bacteria—produce organic nitrogen from inorganic nitrogen Fungi, bacteria—recycle organic nitrogen from decomposing organic matter in the soil |
Each of the three major biogeochemical cycles has a specific function and involves specific actions by the microbes that are involved.