Biogeochemical Cycles: Phosphorus, Carbon, Sulfur, Nitrogen

Phosphorus Cycle

Phosphorus reaches the soil with the help of phosphate-solubilizing bacteria, which make phosphates from urine and other organic sources available to plants. Phosphates are carried by surface waters to the ocean basins. Part of these phosphates is lost in the deep ocean, while another portion is taken up by algae and seaweed. From there, phosphates move through aquatic food webs into animals, where they form skeletons, shells, or other remains; those remains are returned to structural components by marine decomposers. Birds that feed on fish recover some phosphates, and their feces accumulate in guano deposits that are exploited industrially for their agricultural importance.

Carbon Cycle

There are major reservoirs of carbon including carbonate rocks, fossil fuels, organic accumulations, and the atmosphere. Plants, algae, and cyanobacteria are the primary CO₂ fixers: through photosynthesis they incorporate atmospheric CO₂ into organic matter. Heterotrophic organisms feed on the remains of animals and plants, forming accumulations of organic matter. Bacteria and fungi decompose this organic material and return CO₂ to the environment by respiration or fermentation (including anaerobic processes).

• Methanogenic archaea produce methane (CH₄) under anaerobic conditions.
• Methanotrophic bacteria oxidize methane, returning CO₂ to the atmosphere.

Sulfur Cycle

Sulfur in the atmosphere can occur in forms such as hydrogen sulfide (H₂S) and sulfur oxides. Minerals such as pyrite and gypsum store sulfur in the lithosphere and biosphere. Soils accumulate sulfate ions (SO₄2−), which photosynthetic organisms use to synthesize sulfur-containing compounds such as the amino acids cysteine and methionine. These sulfur compounds pass through the food chain to consumers, and when organic matter decomposes some sulfur returns to the soil as hydrogen sulfide or other reduced sulfur species.

Like the nitrogen cycle, sulfur oxides released to the atmosphere can react with water vapor to form sulfate ions, contributing to acid rain and returning sulfate to terrestrial and aquatic systems.

Nitrogen Cycle

Nitrogen precipitates into soil and water primarily as nitrate (NO₃−) and can be absorbed by producers to form biomolecules. When organisms die, their remains are mineralized to produce ammonia (NH₃ or NH₄⁺).

Ammonia is transformed by nitrification and can then be used by plants. Nitrification proceeds in two main stages:

• Ammonia-oxidizing bacteria (and archaea) oxidize ammonia to nitrite (NO₂−).
• Nitrite-oxidizing bacteria convert nitrite to nitrate (NO₃−).

Completing the cycle, denitrifying bacteria reduce nitrate back to molecular nitrogen (N₂), which returns to the atmosphere.