Chapter 21: Nutrient Cycles

The carbon cycle involves the movement of carbon dioxide among major reservoirs including the atmosphere, oceans, terrestrial biomass, and sediments, with microbial processes such as photosynthetic carbon fixation, respiration, and decomposition regulating the balance between organic carbon production and carbon dioxide release. Under anoxic conditions, microbial consortia degrade organic matter through syntrophic interactions that produce methane and carbon dioxide, with methanogenic archaea carrying out methanogenesis in environments such as wetlands, sediments, and animal digestive systems. The nitrogen cycle encompasses a series of microbial redox reactions that interconvert nitrogen compounds, including nitrogen fixation that converts atmospheric nitrogen gas into ammonia, ammonification during organic nitrogen decomposition, nitrification in which ammonia is oxidized to nitrate, and denitrification that returns nitrogen to the atmosphere as gaseous nitrogen compounds. Additional nitrogen transformations include dissimilatory nitrate reduction to ammonia and anaerobic ammonium oxidation, both of which influence nitrogen availability in natural and engineered environments. The sulfur cycle involves the interconversion of sulfide, elemental sulfur, and sulfate through microbial sulfate reduction and sulfide oxidation, processes that occur widely in aquatic sediments and sulfur-rich environments. The chapter also explores additional nutrient cycles including iron, manganese, phosphorus, calcium, and silicon, emphasizing microbial processes such as metal reduction and oxidation, extracellular electron transfer to insoluble mineral oxides, and the role of marine organisms like diatoms and coccolithophores in cycling silicon and calcium through biomineralization. Finally, the chapter addresses the profound influence of human activity on nutrient cycling, including increased atmospheric carbon dioxide from fossil fuel combustion, widespread nitrogen fertilization through the Haber–Bosch process, ocean acidification, eutrophication, and the release of greenhouse gases such as methane and nitrous oxide. Microbial transformations of mercury, including methylmercury formation and microbial detoxification pathways, further illustrate how microorganisms influence environmental chemistry and ecosystem health. Together, these interconnected nutrient cycles demonstrate how microbial metabolism regulates global biogeochemical processes that maintain ecological stability and support the biosphere.