15
P
Phosphorus
30.974
Essential: all life
Phosphorus
Major functions in cells: (1)
-Nucleic acids (DNA, RNA)
-NTPs
-Phospholipids
-Metabolism
-Regulation
Environmental and health impacts:
-Limitation affects oceanic primary production, agriculture.
-Major reservoir in nature is in the form of phosphates
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(1) Phosphorus: Required For Life
Phosphorus is required in all cells and is considered one of the macronutrients that are essential for life.. Phosphate forms the linkage unit of nucleic acids and is therefore essential for the storage, transmission, and expression of genetic information. Nucleoside triphosphates (NTPs) serve as precursors for the synthesis of DNA and RNA, and they function as the universal energy currency in the cell. Phosphorus is also used in the synthesis of the phospholipids of the membrane lipid bilayer. Phosphorus-containing polymers are also abundant in the cell walls of Gram-positive bacteria (in teichoic acids).
(2) Membrane Phospholipid Remodeling
Bacteria
Many regions of the open ocean are P deficient, yet that does not appear to limit productivity because the inhabiting Prochlorococcus species have adapted to the low P content of that niche and reduced their P quota by replacing membrane anionic phospholipids by anionic sulfolipids, specifically sulfoquinovosyl diglyceride (SQDG) (van Mooy et al., 2006). This P-sparing adaptation is important for the success of prokaryotic Prochlorococcus and cyanobacterial species as well as eukaryotic phytoplankton in P-deplete aquatic environments.
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The replacement of phospholipids is a common acclimation response in bacteria faced with P deficiency. It has also been documented in :
-Rhizobia, where SQDG, ornithine-containing lipids, and diacylglyceryl trimethylhomoserine are used as substitutes (Geiger et al., 1999).
-Pseudomonas, where acidic glycolipids are used as substitutes (Minnikin et al., 1974).
-Gram-positive Marinococcus species, where sulfolipid is used as a substitute (Sprott et al., 2006).
-Anoxygenic photosynthetic bacteria, where glycolipids and betaine lipids are used as substitutes (Benning et al., 1995).
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Eukaryotes
Phospholipid remodeling is also seen in eukaryotic organisms. In Chlamydomonas, phosphatidylglycerol is substituted with sulfolipids. However, because of specific binding sites for phosphatidylglycerol in PSII, it cannot be completely replaced in the thylakoid membrane (Yu et al., 2002). P sparing via membrane lipid remodeling is conserved throughout the plant lineage and has also been documented in:
-Moss (Wang et al., 2008)
-Arabidopsis (Yu et al., 2002)
-Perennial rye grass (Byrne et al., 2011)
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(3) Cell Wall Remodeling in B. subtilis
In media containing sufficient phosphate, B. subtilis cell walls contain an abundant anionic polymer known as teichoic acid, composed of glycerol and phosphate. Cells grown in P-limited medium repress the expression of the wall teichoic acid biosynthetic pathway and activate the expression of an alternative anionic polymer, teichuronic acid (Qi and Hulett, 1998). In teichuronic acid, the carboxylates provide the negative charge that the phosphates provide in the teichoic acids.
(4) Degradation or Synthesis Reduction of Ribosomes
Ribosomes represent a major fraction of cell mass in rapidly growing bacteria, so ribosomes can provide a source of macronutrients in a limited environment. In general, starvation for P often elicits a reduction in synthesis of ribosomes, which is a general response to reduced growth rates. However, in at least some systems it seems that existing ribosomes, particularly the individual subunits, may be degraded to recycle P.
(5) Degradation of Plastid DNA
Nucleic acids represent the other major reservoir of P other than phospholipids. In C. reinhardtii, where the plastid genome is polyploid with up to 80 copies per cell, there is evidence for copy number reduction, which could release P for recycling to other processes (Yehudai-Resheff et al., 2007).
(6) Degradation of Phospholipids
Degradation of pre-existing phospholipids is a P-recycling mechanism. In Rhizobium meliloti, P recycling is part of the phosphate-deficiency program mediated by the response regulator PhoB. A specific intracellular phospholipase C is induced so that the phospholipids (whose function can be covered by non-P-containing molecules) can be used as a pool of mobilizable P, which is recycled for molecules in which P is essential, like nucleotides (Zavaleta-Pastor et al., 2010).