Rethinking early Earth phosphorus geochemistry

Abstract

Phosphorus is a key biologic element, and a prebiotic pathway leading to its incorporation into biomolecules has been difficult to ascertain. Most potentially prebiotic phosphorylation reactions have relied on orthophosphate as the source of phosphorus. It is suggested here that the geochemistry of phosphorus on the early Earth was instead controlled by reduced oxidation state phosphorus compounds such as phosphite (HPO 3 2؊ ), which are more soluble and reactive than orthophosphates. This reduced oxidation state phosphorus originated from extraterrestrial material that fell during the heavy bombardment period or was produced during impacts, and persisted in the mildly reducing atmosphere. This alternate view of early Earth phosphorus geochemistry provides an unexplored route to the formation of pertinent prebiotic phosphorus compounds, suggests a facile reaction pathway to condensed phosphates, and is consistent with the biochemical usage of reduced oxidation state phosphorus compounds in life today. Possible studies are suggested that may detect reduced oxidation state phosphorus compounds in ancient Archean rocks.meteorites ͉ origins of life ͉ phosphonates ͉ prebiotic ͉ redox chemistry P hosphorus (P) is a key biologic element and is the limiting reagent in many ecosystems. Phosphorus is ubiquitous in biochemistry because phosphorylated biomolecules play major roles in replication and information (as RNA and DNA), metabolism (as ATP, NADPH, and other coenzymes), and structure (as phospholipids). Several key properties of P as phosphate make it advantageous to biologic systems, including thermodynamic instability coupled with kinetic stability, charge and coordination state, and a constant oxidation state under typical redox conditions (1). These features are especially critical to the formation of large informational polymers, and hence highly relevant to the origin and development of early life. Phosphorus Geochemistry and CosmochemistryThe major forms of P in life are summarized in Fig. 1. Inorganic P compounds used by life include orthophosphate, pyrophosphate and other condensed phosphates, phosphite, hypophosphite, and phosphine. These inorganic forms are either used by organisms as sources of P for the synthesis of organic-P biomolecules or are possible metabolic by-products of P metabolism (PH 3 ). Additionally, orthophosphate acts as a buffer in many cells, keeping pH constant at Ϸ7. Organic forms of P include compounds with C-O-P linkages and compounds with C-P linkages, and both are formed from inorganic P by using enzymes.Phosphorus is a lithophile element at the redox conditions on the surface of the Earth, and hence orthophosphate is the dominant form of inorganic P on the surface of the Earth today. The concentration of orthophosphate is water-buffered by the solubility of orthophosphate minerals like apatite, Ca 5 (PO 4 ) 3 (OH,F,Cl), on the present-day Earth, and of whitlockite, Ca 9 MgH(PO 4 ) 7 , and brushite, CaHPO 4 ⅐2H 2 O, before the rise of life-mediated apatite deposition (2). Because ...

Evidence for reactive reduced phosphorus species in the early Archean ocean

“…In contrast to calcium phosphate minerals, schreibersite is more reactive and, hence, may have provided soluble P species like phosphite necessary for the formation of organophosphates during the origin of life (9,13).…”

Inorganic Polyphosphate Modulates TRPM8 Channels

“…As already noted, hypophosphite and phosphite may have been abundant in the primordial ocean [252][253][254][256][257][258] and could participate in the primeval photosynthesis. Moreover, phosphate ions, ultimate products of the (photo)oxidation, owing to their affinity to metal sulfides [279], would have remained adsorbed at the ZnS surface, providing a specific phosphate-rich reaction milieu.…”

“…Since the concentration of phosphate in sea water is low (the phosphates of Ca and Mg are poorly soluble in water), it has been argued that more reduced compounds such as hypophosphite (phosphinate, PO 2 2-) and/or phosphite (phosphonate, PO 3 3-), which have better solubility in sea water, could have been abundant in the primeval, more reduced ocean [252][253][254][255][256][257][258]. This suggestion is supported by findings of diverse systems of hypophosphite and phosphite oxidation in prokaryotes (see [259] for a review).…”

Primitive Genetic Polymers

“…Indeed, phosphate is a limiting reagent in much of contemporary life. Although recent hypotheses have been proposed for the availability of reduced forms of phosphorus on the earth which are more soluble (De Graaf and Schwartz 2000;Bryant and Kee 2006;Schwartz 2006;Pasek 2008), the low aqueous solubility of the contemporary (V) oxidation state of phosphorus, in the presence of divalent cations, and the positive DH associated with phosphoester formation present problems for prebiotic phosphate chemistry.…”