Phosphorus Cycle K R M Mackey, Stanford University, Stanford, CA, USA a Paytan, University of California Santa Cruz, Santa Cruz, CA, USA
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Phosphorus Cycle K R M Mackey, Stanford University, Stanford, CA, USA A Paytan, University of California Santa Cruz, Santa Cruz, CA, USA ª 2009 Elsevier Inc. All rights reserved. Defining Statement Anthropogenic Alteration of the P Cycle: Eutrophication Introduction in Aquatic Ecosystems Microbially Mediated Processes Conclusion Genetic Regulation of Microbially Mediated Processes Further Reading Glossary mineral formation Processes in which anions react assimilation (transitory immobilization) The process with cations in the environment to form insoluble of incorporating nutrients into cellular biomass. precipitates. authigenic Formed in situ rather than by having been mineralization The conversion of organic matter into transported or deposited in a location through simple inorganic compounds, primarily by microbial secondary processes. decomposition. chelation The reversible binding (complexation) of a nutrient limitation The cessation of photosynthetic ligand to a metal ion. biomass production in response to low availability of a diagenesis The process by which sediment physiologically important nutrient or nutrients. undergoes chemical and physical changes during its organic phosphorus A class of chemical compounds lithification. that comprises only those existing in or derived from eolian transport Advection of material via the living organisms. atmosphere or wind. orthophosphate Form of orthophosphoric acid eutrophication Excessive nutrients in a body of water (H3PO4) in which three protons have been lost (i.e., 3– that cause a dense algal growth, followed by algal PO4 ); also called the phosphate anion. decomposition, anoxia, and impairment of the aquatic primary production The conversion of radiant or community. chemical energy into organic matter. immobilization The process by which labile productivity The rate at which radiant energy is used phosphorus is sequestered and removed from the by producers to form organic substances as food for environmental reservoir of reactive phosphorus for a consumers. period of time. solubilization The conversion of insoluble compounds inorganic phosphorus A class of chemical into soluble compounds. compounds that comprises those not existing in or weathering Processes by which rocks, minerals, and derived immediately from living organisms (e.g., soils get broken down through direct contact with the orthophosphate). atmosphere; can be physical or chemical. Abbreviations PNP para-nitrophenyl phosphate ELF enzyme-labeled fluorescence Pst phosphate-specific transport PhoA alkaline phosphomonoesterase SRP soluble reactive phosphorus Pit phosphate transport system Defining Statement phosphorus in the environment, microbially mediated transformations of phosphorus and their genetic regulation, This article includes a discussion of the global phosphorus and a discussion of microbial responses to anthropogenic cycle, including sources, sinks, and transport pathways of changes to the phosphorus cycle. 322 Environmental Microbiology and Ecology | Phosphorus Cycle 323 Introduction growth of producers, the phosphorus cycle contributes to the regulation of global climate. Phosphorus is an essential nutrient for all living organisms, In aquatic environments, photosynthetic microbes (i.e., and the phosphorus cycle is an important link between phytoplankton) may comprise a substantial portion of the Earth’s living and nonliving entities. The availability of photosynthetic biomass and hence primary production. phosphorus strongly influences primary production, the Marine and freshwater phytoplankton are characterized process by which photosynthetic organisms fix inorganic by extensive biodiversity, and as a group, inhabit an carbon into cellular biomass. Therefore, knowledge of the incredible number of different niches within aquatic phosphorus cycle is critically important for understanding environments. Phytoplankton, like other microbes, have the global carbon budget and hence how biogeochemical strategies that enable them to adapt to changes in the cycles impact and are influenced by global climate. amounts and forms of phosphorus available within their environment. For example, the production of phosphatase enzymes, which hydrolyze organic P compounds to inor- ganic phosphate, helps to mediate the mineralization of organic phosphorus compounds in surface waters and Global Significance of the Phosphorus Cycle allows cells to adapt when phosphate levels are low. Natural assemblages of microbes have a critical role in the Recent estimates suggest that phytoplankton are phosphorus cycle because they forge a link between phos- responsible for as much as half of global carbon fixation, phorus reservoirs in the living and nonliving environment. thereby contributing significantly to the regulation of Microbes facilitate the weathering, mineralization, and solu- Earth’s climate. Phytoplankton productivity is strongly bilization of nonbioavailable phosphorus sources, making influenced by nutrient availability, and which nutrient orthophosphate available to microbial and plant commu- ultimately limits production depends on (1) the relative nities and hence to higher trophic levels within the food web. abundance of nutrients and (2) the nutritional require- However, microbes also contribute to immobilization of ments of the phytoplankton. With some exceptions, phosphorus, a process that diminishes bioavailable phos- production in most lakes is limited by the availability of phorus by converting soluble reactive forms into insoluble phosphorus, whereas limitation of primary production in the ocean has traditionally been attributed to nitrogen, forms. The mechanisms of microbial involvement in these another nutrient required by cells in large quantities. processes vary from passive (e.g., resulting from microbial However, phosphorus differs from nitrogen in that the metabolic byproducts) to highly regulated active contribu- major source of phosphorus to aquatic environments is tions (e.g., the regulation of gene expression in response to the weathering of minerals on land that are subsequently environmental cues). introduced into water bodies by fluvial and aeolian Microbially mediated processes also link the phos- sources. In contrast, microbially mediated nitrogen phorus cycle to the carbon cycle and hence to global fixation, in which bioavailable forms of nitrogen are gen- climate. During photosynthesis, photoautotrophs incor- erated from nitrogen gas in the atmosphere, is a major porate phosphorus and carbon at predictable ratios, with pathway by which phytoplankton gain access to nitrogen approximately 106 carbon atoms assimilated for every (in addition to continental weathering). Because there is one phosphorus atom for marine photoautotrophs and no phosphorus input process analogous to nitrogen fixa- terrestrial vegetation. The phosphorus cycle therefore tion, marine productivity over geological timescales is plays an important role in regulating primary productiv- considered to be a function of the supply rate of phos- ity, the process in which radiant energy is used by phorus from continental weathering and the rate at which primary producers to form organic substances as food phosphorus is recycled in the ocean. Accordingly, the for consumers, as in photosynthesis. phosphorus cycle influences phytoplankton ecology, pro- Because phosphorus is required for the synthesis of ductivity, and carbon cycling in both marine and numerous biological compounds, its availability in the freshwater ecosystems. environment can limit the productivity of producers when other nutrients are available in excess. This fact carries important implications for the global carbon bud- Characterizing and Measuring Environmental get because the rate of incorporation (fixation) of carbon Phosphorus Pools dioxide into photosynthetic biomass can be directly con- Occurrence of elemental phosphorus in the environment trolled by the availability of phosphorus; if phosphorus is is rare, given that it reacts readily with oxygen and com- not available, carbon dioxide fixation is halted. The inter- busts when exposed to oxygen; thus phosphorus is section of the phosphorus and carbon cycles is of typically found bound to oxygen in nature. Phosphorus particular significance for global climate, which is affected has the chemical ability to transfer a 3s or p-orbital by atmospheric carbon dioxide levels. Hence, through the electron to a d-orbital, permitting a relatively large 324 Environmental Microbiology and Ecology | Phosphorus Cycle number of potential configurations of electrons around dissolved because it is associated with water molecules the nucleus of the atom. This renders the structures of homogeneously in solution rather than being held within phosphorous-containing molecules quite variable and a salt crystal.) By contrast, colloidal forms include relatively reactive. These properties are likely responsi- any tiny particles that are distributed evenly throughout ble for the ubiquity and versatility of phosphorus- the solution but that are not dissolved in solution. Soluble containing compounds in biological systems. phosphorus is commonly reported because colloidal Because phosphorus exists in many different physical phosphorus particles are very small, and differentiating and chemical states in the environment, specific definitions between colloidal and dissolved phosphorus is methodo- are needed to clarify different parts of the phosphorus pool. logically difficult. Chemical names reflect the chemical composition of