The oceanic phosphorus cycle.

Phosphorus (P) is an essential element to all life, being a structural and functional component of all organisms.1-4 P provides the phosphate-ester backbone of DNA and RNA, and it is crucial in the transmission of chemical energy through the ATP molecule (Figure 1). P is also a structural constituent in many cell components such as phosphoproteins, and phospholipids in cell membranes, teeth, and bones (Figure 1). In some organisms it can also be present as intracellular polyphosphate storage granules (Figure 1). Phosphorus availability can impact primary production rates in the ocean as well as species distribution and ecosystem structure.5-8 In some marine and estuarine environments, P availability is considered the proximal macronutrient that limits primary production.8,9 Specifically, in recent years it has been recognized that phosphorus limitation in the ocean may be more prevalent than previously thought. For example, it is generally accepted that orthophosphate (PO4) is the limiting nutrient in the eastern Mediterranean Sea.10-12 In addition, phosphate is suspected to play an important role in limiting production in the Sargasso Sea13 and may also be limiting in bodies of water receiving large freshwater inputs or fertilizer runoff from agriculture, as in the Chesapeake Bay.14,15 Furthermore, research in the Pacific Ocean gyres indicates that biological P uptake rates far surpass the combined input from atmospheric and deep water sources, suggesting that P is efficiently recycled within oligotrophic euphotic zones.16 It has also been suggested that transitions over the last two decades from nitrogen (N) limitation to P limitation have taken place in the North Pacific subtropical gyre and that this may be responsible for the observed succession of prokaryotic picophytoplankton such as Prochlorococcus and Synechococcus in oligotrophic waters.6 In other marine environments, P may only be limiting a subset of organisms within the ecosystem.17-19 Phosphorus, in the form of orthophosphate, plays a key role in photosynthesis (i.e., primary productivity). The chemical equation representing average ocean photosynthesis can be written as