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Chapter 12. Geomicrobial Interactions with Phosphorus

Chapter 12. Geomicrobial Interactions with Phosphorus

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An important source of free organic phosphorus compounds in the biosphere is the breakdown of

animal and vegetable matter. However, living microbes such as Escherichia coli and organisms from

activated sludge have been found to excrete aerobically assimilated phosphorus as inorganic phosphate when incubated anaerobically (Shapiro, 1967). Organically bound phosphorus is for the most

part not directly available to living organisms because it cannot be taken into the cell in this form.

To be taken up, it must be freed from organic combination through mineralization. This is accomplished through hydrolytic cleavage catalyzed by phosphatases. In the soil, as much as 70–80% of

the microbial population are able to participate in this process (Dommergues and Mangenot, 1970,

p. 266). Active organisms include bacteria such as Bacillus megaterium, B. subtilis, B. malabarensis,

Serratia sp., Proteus spp., Arthrobacterr spp., Streptomyces spp., and fungi such as Aspergillus sp.

Penicillium sp., Rhizopus sp., and Cunninghamella sp. (Dommergues and Mangenot, 1970, p. 266;

Paul and Clark, 1996). These organisms secrete, or liberate upon their death, phosphatases with

greater or lesser substrate specificity (Skujins, 1967). Such activity has also been noted in the marine

environment (Ayyakkannu and Chandramohan, 1971).

Phosphate liberation from phytin generally requires the enzyme phytase:

Phytin + 6H2O → inositol + 6Pi


Phosphate liberation from nucleic acid requires the action of nucleases, which yield nucleotides,

followed by the action of nucleotidases, which yield nucleosides and inorganic phosphate:

Nucleic acid nucleases


→ nucleotides nucleotidase


→ nucleosides ϩ Pi

ϩH O

ϩH O




Phosphate liberation from phosphoproteins, phospholipids, ribitol, and glycerol phosphates requires

phosphomono- and phosphodiesterases. Phosphodiesterases attack phosphodiesters at either the 3′

or 5′ carbon linkage, whereas phosphomonoesterases (phosphatases) attack monoester linkages

(Lehninger, 1975, pp. 184, 323–325), for example,




R O P O R′







+ HO P O R′








Synthesis of organic phosphates (monomeric phosphate esters) is an intracellular process and normally proceeds through reaction between the hydroxyl (OH) of a carbinol group (CHOH), as for

instance in glucose, and ATP in the presence of an appropriate kinase. For example,

Glucose ϩ ATP glucokinase

→ glucose 6-phosphate ϩ ADP

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Geomicrobial Interactions with Phosphorus


Phosphate esters in cells may also arise through phosphorolysis of certain polysaccharide polymers

such as starch or glycogen [(glucose)n]:

(Glucose)n ϩ H3PO4 phosphorylase

→ (glucose)n −1 ϩ glucose 1-phosphate


Glucose 1-phosphate 

→ glucose 6-phosphate




ATP may be generated from ADP by adenylate kinase,



or by substrate level phosphorylation, as in the reaction sequence:

3-Phosphoglyceraldehyde ϩ NADϩ ϩ Pi triosephosphate


→1,3-diphosphoglycerate ϩ NADH ϩ Hϩ




1,3-Diphosphoglycerate ϩ ADP ADP


→ 3-phosphoglycerate ϩ ATP

It may also be generated by oxidative phosphorylation:

transport system

ADP ϩ Pi electron






ADP ϩ Pi photosynthetic



light, ATPase


or by photophosphorylation:

Phosphate polymers are generally produced through reactions such as

(Polynucleotide)n −1 ϩ nucleotide triphosphate polymerase

→ (polynucleotide)n ϩ P ϳ P

In many organisms, inorganic pyrophosphate (P∼P) is enzymatically hydrolyzed,

P ϳ P pyrophosphatase


→ 2Pi


without conservation of the energy released by cleavage of the anhydride bond.

However, in a few bacteria, inorganic pyrophosphate has been reported to be able to serve as an

energy source (Liu et al., 1982; Varma et al., 1983). Although it is easy to understand that this ability can be of great importance for energy conservation from intracellularly formed pyrophosphate

in bacteria, it remains to be clarified how important it may be for extracellularly available pyrophosphate in nature. Liu et al. (1982) found gram-positive and gram-negative motile and nonmotile

bacteria in pyrophosphate enrichments from freshwater anaerobic environments, which grew at the

expense of the pyrophosphate as energy source. Nothing appears to be known about the mechanism

of pyrophosphate uptake in these organisms.

Like pyrophosphate, intracellular inorganic polyphosphate granules formed by some microbes

(Friedberg and Avigad, 1968) are a form of metaphosphate and can represent an energy storage

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compound (van Groenestijn et al., 1987) as well as a phosphate reserve. In the case of the cyanobacterium Anabaena cylindrica, it may also play a role as detoxifying agent by combining with aluminum ions that are taken into the cell (Petterson et al., 1985) (see also Chapter 11).



Inorganic phosphorus may occur in soluble and insoluble forms in nature. The most common inorganic

form is orthophosphate (e.g., H3PO4). As an ionic species, the concentration of phosphate is controlled

by its solubility in the presence of an alkaline earth cation such as Ca2+ or Mg2+ or in the presence

of metal cations such as Fe2+, Fe3+, or Al3+ at appropriate pH values (see Table 12.1). In seawater, for

instance, the soluble phosphate concentration (∼3 × 10−6 M, maximum) is controlled by Ca2+ ions

(4.1 × 102 mg L−1), which form hydroxyapatites with phosphate in a prevailing pH range of ∼7.9–8.1.

Insoluble phosphate occurs most commonly in the form of apatite [Ca5(PO4)3(F, Cl, OH)] in

which the (F, Cl, OH) radical may be represented exclusively by F, Cl, or OH or any combination of

these. In soil, insoluble phosphate may also occur as an aluminum salt (e.g., variscite, AlPO4 ∙ 2H2O)

or as iron salts vivianite (Fe3(PO4)2 ∙ 8H2O) and strengite (FePO4 ∙ 2H2O).

Insoluble forms of inorganic phosphorus (calcium, aluminum, and iron phosphates) may be solubilized through microbial action. The mechanism by which the microbes accomplish this solubilization varies. The first mechanism may be the production of inorganic or organic acids that attack

the insoluble phosphates. A second mechanism may be the production of chelators such as gluconate and 2-ketogluconate (Duff and Webley, 1959; Banik and Dey, 1983; Babu-Khan et al., 1995) (see

also Chapter 10), citrate, oxalate, and lactate. All of these chelators can complex the cationic portion

of the insoluble phosphate salts and thus force the dissociation of the salts. A third mechanism of

phosphate solubilization may be the reduction of iron in ferric phosphate (e.g., strengite) to ferrous

iron by enzymes and metabolic products of nitrate reducers such as Pseudomonas fluorescens and

Alcaligenes sp. in sediment (Jansson, 1987). A fourth mechanism may be the production of hydrogen

sulfide (H2S), which can react with the iron in iron phosphate and precipitate it as iron sulfide,

thereby mobilizing phosphate, as in the reaction

2FePO4 + 3H2S → 2FeS + 2H3PO4 + S0


Table 12.2 lists some of the organisms active in phosphate solubilization.

Solubilization of phosphate minerals has been noted directly in soil (Alexander, 1977; Babenko

et al., 1984; Chatterjee and Nandi, 1964; Dommergues and Mangenot, 1970; Patrick et al., 1973).

Indeed, soil containing significant amounts of immobilized calcium, aluminum, or iron phosphates

has been thought to benefit from inoculation with phosphate-mobilizing bacteria (see discussion by

Dommergues and Mangenot, 1970, p. 262). Important microbial phosphate-solubilizing activity in

soil occurs in rhizospheres (Alexander, 1977), probably because root secretions allow phosphatesolubilizing bacteria to generate sufficient acid or ligands to effect dissolution of calcium and other

TABLE 12.1

Solubility Products of Some Phosphate Compounds


CaHPO4 ⋅ 2H2O




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2.18 × 10

1.53 × 10−112

2.8 × 10−29

1.35 × 10−18


Kardos (1955, p. 185)

Kardos (1955, p. 188)

Kardos (1955, p. 184)

From ∆G

G of formation

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Geomicrobial Interactions with Phosphorus


TABLE 12.2

Some Organisms Active in Phosphate Solubilization


Mechanism of Solubilization

Bacillus megaterium

H2S production, FeS precipitation

Thiobacillus sp.

Nitrifying bacteria

H2SO4 production from sulfur

NH3 oxidation to HNO3

Pseudomonads, Arthrobacter, and

Erwinia-like bacterium

Chelate production; glucose

converted to gluconate or




Aspergillus niger, A. flavus,

Sclerotium rolfsii, Fusarium

oxysporum, Cylindrosporium sp.,

and Penicillium sp.

Organic acid production (e.g., citric



Sperber (1958a); Swaby and

Sperber (1958)

Lipman and McLean (1916)

Dommergues and Mangenot

(1970, p. 263)

Duff et al. (1963); Sperber

(1958b); Dommergues and

Mangenot (1970, p. 262);

Babu-Khan et al. (1995)

Dommergues and Mangenot

(1970, p. 262)

Agnihotri (1970)

insoluble phosphates. Phosphate-deficient soil may be beneficially fertilized with insoluble inorganic phosphate rather than soluble phosphate salts because the former will be solubilized slowly

and thus will be better conserved than soluble phosphate salts, which can be readily leached.

Soluble phosphate in soil may consist not only of orthophosphate but also of pyrophosphate (metaphosphate). The latter is readily hydrolyzed by pyrophosphatase, especially in flooded soil (Racz

and Savant, 1972).

Recent experiments with Bacillus megaterium found that phosphate mobilization when the

organism is in direct contact with apatite was 3–10 times slower than when it is not in direct contact

with the mineral (Hutchens et al., 2006). The authors suggest that when in direct contact, the organism may block reactive sites at the mineral surface.


Microorganisms can cause fixation or immobilization of phosphate, either by promoting the formation of inorganic precipitates or by assimilating the phosphate into organic cell constituents or intracellular polyphosphate granules. The latter two processes have been called transitory phosphate

immobilization by Dommergues and Mangenot (1970) because of the ready solubilization of phosphate through mineralization upon death of the cell. In soil and freshwater environments, transitory

phosphate immobilization is often more important, although fixation of phosphate by Ca2+, Al3+,

and Fe3+ is recognized. In a few marine environments (coastal waters or shallow seas) where phosphorite deposits occur, the precipitation mechanism may be more important (McConnell, 1965).


Phosphorite (Figure 12.1) in nature may form authigenically or diagenetically. In authigenesis, the

phosphorite forms as a result of a reaction of soluble phosphate with calcium ions forming corresponding insoluble calcium phosphate compounds. In diagenesis, phosphate may replace carbonate in

calcareous concretions. The role of microbes in these processes may be one or more of the following:

(1) making reactive phosphate available, (2) making reactive calcium available, or (3) generating or

maintaining the pH and redox conditions that favor phosphate precipitation.

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FIGURE 12.1 Micronodules of phosphorite (phosphatic pellets) from the Peru shelf. The average diameter

of such pellets is 0.25 mm. According to Burnett (personal communication), such pellets are more representative of what is found in the geologic record than the larger phosphorite nodules. (Courtesy of Burnett WC,

Institute for International Cooperative Environmental Research, Florida State University, Tallahassee FL.) Authigenic Formations

Models of authigenic phosphorite genesis assume the occurrence of mineralization of organic phosphorus in biologically productive waters, such as at ocean margins, that is, at shallow depths on

continental slopes, shelf areas, or plateaus. Here detrital accumulations may be mineralized at the

sediment–water interface and in interstitial pore waters, liberating phosphate, some of which may

then interact chemically with calcium in seawater to form phosphorite grains. These grains may

subsequently be redistributed within the sediment units (Riggs, 1984; Mullins and Rasch, 1985).

Dissolution of fish debris (bones) has also been considered an important source of phosphate in

authigenic phosphorite genesis (Suess, 1981). Upwelling probably plays an important role in many

cases of authigenic phosphorite formation on western continental margins at latitudes in both the

northern and southern hemispheres, where prevailing winds (e.g., trade winds) cause upwelling (see,

e.g., discussion by Burnett et al., 1982; Jahnke et al., 1983; Riggs, 1984). Nathan et al. (1993) cite

evidence that in the southern Bengula upwelling system (Cape Peninsula, western coast of South

Africa) during nonupwelling periods in winter, phosphate-sequestering bacteria of the oxidative genera Pseudomonas and Acinetobacterr become dominant in the water column. Fermentative Vibrios

and Enterobacteriaceae are dominant during upwelling in summer. It has been suggested that

Pseudomonas and Acinetobacter,

r which sequester phosphate as polyphosphate under aerobic conditions and hydrolyze the polyphosphate under anaerobic conditions to obtain energy of maintenance

and to sequester volatile fatty acids for polyhydroxybutyrate formation, contribute to authigenic

phosphorite formation. Locally elevated, excreted orthophosphate becomes available for precipitation as phosphorite by reacting with seawater calcium. In the northern Bengula upwelling system

off the coast of Namibia, where upwelling occurs year-round, Nathan et al. (1993) found that phosphate-sequestering cocci occurred in the water column. They suggested that these organisms such

as Pseudomonas and Acinetobacterr may release sequestered phosphate when they reach waters with

low oxygen concentration below 10 m water depth and thereby contribute to phosphorite formation.

Authigenic phosphorite at some eastern continental margins, where upwelling, if it occurs at all,

is a weak and intermittent process, may have been formed more directly as a result of intracellular bacterial phosphate accumulation, which became transformed into carbonate fluorapatite upon

death of the cells and accumulated in sediments in areas where the sedimentation rate was very

low (O’Brien and Veeh, 1980; O’Brien et al., 1981). Ruttenberg and Berner (1993) concluded that

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Geomicrobial Interactions with Phosphorus


carbonate fluorapatite accumulations in Long Island Sound and Mississippi Delta sediments are the

result of mineralization of organic phosphorus. These accumulations increased as organic phosphorus concentrations decreased with depth. Thus, important phosphorus sinks occur in sediments of

continental margins outside upwelling regions.

Youssef (1965) proposed that phosphorite could be formed in a marine setting through mineralization of phytoplankton remains that have settled into a depression on the seafloor, leading

to localized accumulation of dissolved phosphate. According to him, this phosphate could then

precipitate as a result of reaction with calcium in seawater. Piper and Codespoti (1975) proposed

that carbonate fluorapatite [Ca10(PO4,CO3)6F2–3] precipitation in the marine environment may be

dependent on bacterial denitrification in the oxygen minimum layer of the ocean as it intersects with

the ocean floor. A loss of nitrogen due to denitrification means lowered biological activity and can

lead to excess accumulation of phosphate in this zone. The lower pH (7.4–7.9) in the deeper waters

compared to the surface waters keeps phosphate dissolved and allows for its transport by upwelling

to regions where phosphate precipitation is favored (pH >8) (Figure 12.2). This model takes into

account the conditions of marine apatite formation described by Gulbrandsen (1969) and helps to

explain the occurrence of probable contemporary formation of phosphorite in regions of upwelling

pH 8.0 − 8.5




+ 6(Pi +




= Ca10(PO4,CO3)F2−3





O2 minimum layer



pH 7.4 − 7.9






FIGURE 12.2 Schematic presentation of phosphorite formation in the marine environment. Note that the

rising Pi upslope is due to upwelling. (Based on Piper DZ, Codespoti LA, Science, 179, 564–565, 1975.)

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such as the continental margin off Peru (Veeh et al., 1973; see also, however, Suess, 1981) and on the

continental shelves of southwestern Africa (Baturin, 1973; Baturin et al., 1969). To explain the more

extensive ancient phosphorite deposits, a periodic warming of the ocean can be invoked, which

would reduce oxygen solubility and favor more intense denitrification in deeper waters, resulting in

temporarily lessened biological activity and a consequent increase in dissolved phosphate concentration that would lead to phosphate precipitation (Piper and Codespoti, 1975). Mullins and Rasch

(1985) proposed an oxygen-depleted sedimentary environment for biogenic apatite formation along

the continental margin of central California during the Miocene. They view oolitic phosphorite

as having resulted from organic matter mineralization by sulfate reducers in sediments in which

dolomite was concurrently precipitated. The phosphate, according to their model, then tended to

precipitate interstitially as phosphorite, in part around bacterial nuclei. O'Brien et al., 1981 had previously reported the discovery of fossilized bacteria in a phosphorite deposit on the East Australian

margin. Diagenetic Formation

Models of phosphorite formation through diagenesis generally assume an exchange of phosphate

for carbonate in accretions that have the form of calcite and aragonite. The role of bacteria in

this process is to mobilize phosphate by mineralizing detrital organic matter. This has been demonstrated in marine and freshwater environments under laboratory conditions (Lucas and Prévot,

1984; Hirschler et al., 1990a,b). Adams and Burkhart (1967) propose that diagenesis of calcite to

form apatite explains the origin of some deposits in the North Atlantic. Phosphorite deposits off

Baja California and in a core from the eastern Pacific Ocean seem to have formed as a result of

partial diagenesis (d’Anglejan, 1967, 1968).


Sizable phosphorite deposits are associated with only six brief geological intervals: the Cambrian,

Ordovician, Devonian-Mississippian, Permian, Cretaceous, and Cenozoic eras. Because in many

instances these phosphorite deposits are associated with black shales and contain uranium in

reduced form (Altschuler et al., 1958), they are presumed to have accumulated under reducing conditions. Nowadays, apatite appears to be forming in the sediments at the Mexican continental margin (Jahnke et al., 1983) and in the deposits off the coast of Peru (Burnett et al., 1982; Suess, 1981;

Veeh et al., 1973).



Microbes can also play a role in the authigenic or diagenetic formation of other phosphate minerals

such as vivianite, strengite, and variscite. In these instances, the bacteria may contribute orthophosphate to the mineral formation by degrading organic phosphate in detrital matter and they may

contribute iron or aluminum by mobilizing these metals from other minerals. Authigenic formation of such phosphate minerals is probably most common in soil. A case of diagenetic formation

of vivianite from siderite (FeCO3) in the North Atlantic coastal plain has been proposed by Adams

and Burkhart (1967). Microbial control of pH and Eh can influence the stability of these phosphate

minerals (Patrick et al., 1973; Williams and Patrick, 1971).

Citrobacterr sp. has been reported to form metal phosphate precipitates, for example, cadmium

phosphate (CdHPO4) and uranium phosphate (UO2HPO4), which encrust the cells (Macaskie et al.,

1987, 1992). The precipitates form as a result of the action of a cell-bound, metal-resistant phosphatase on organophosphates such as glycerol-2-phosphate, liberating orthophosphate (HPO42–) that

reacts in the immediate surroundings of the cells with metal cations to form corresponding metal

phosphates. The metal phosphates form deposits on the cell surface.

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Geomicrobial Interactions with Phosphorus


Some microbes, such as certain strains of Arthrobacter sp., Chromohalobacter marismortui,

Flavobacterium sp., Listeria sp., and Pseudomonas sp., can cause struvite (MgNH4PO4 ∙ 6H2O) to

form, at least under laboratory conditions (Rivadeneyra et al., 1983, 1992b, 2006). A major, but not

necessarily exclusive, microbial contribution to this process appears to be ammonium formation

(Rivadeneyra et al., 1992a). Struvite is formed when orthophosphate is added at pH 8 to seawater

solutions in which the NH

H+4 concentration is 0.01 M (Handschuh and Orgel, 1973). The presence

of calcium ion in sufficient quantity can suppress struvite formation and promote apatite formation

instead (Rivadeneyra et al., 1983). Although struvite formation is probably of little significance in

nature today, it may have been significant in the primitive world of Precambrian times if NH4+ was

present in concentrations as high as 10−2 M (Handschuh and Orgel, 1973).


Phosphorus may also undergo redox reactions; some or all of which may be catalyzed by microbes.

Of biogenic interest are the +5, +3, +1, and −3 oxidation states, as in orthophosphate (H3PO4),

orthophosphite (H3PO3), hypophosphite (H3PO2), and phosphine (PH3), respectively. Reduction of

phosphate to phosphine by soil bacteria has been reported (Rudakov, 1927; Tsubota, 1959; Devai

et al., 1988). Mannitol appeared to be a suitable electron donor in the reaction described by Rudakov

(1927) and glucose in the experiments described by Tsubota (1959). Phosphite and hypophosphite

were claimed to be intermediates in the reduction process (Rudakov, 1927; Tsubota, 1959). Devai

et al. (1988) detected phosphine evolution in anaerobic sewage treatment in Imhoff tanks in Hungary

and confirmed the observation in anaerobic laboratory experiments. Gassmann and Schorn (1993)

detected phosphine in surface sediments in Hamburg Harbor. The phosphine was most readily

detected in porous sediments in which the porosity was due to gas bubbles. Iron phosphide (Fe3P2)

is reported to have been formed when a cell-free preparation of Desulfovibrio was incubated in the

presence of steel in a yeast extract broth under hydrogen gas (Iverson, 1968). Inositol hexaphosphate, a product of plants and present in yeast extract, may be a substrate for phosphine formation

(Iverson, 1998). Hydrogenase from Desulfovibrio may have been responsible for the formation of

phosphine from the inositol phosphate in the yeast extract, using the hydrogen in the system as the

reductant in Iverson’s (1968) experiment. The phosphine could then have reacted with ferrous iron

from the steel corrosion to form Fe3P2 (Iverson, 1968).

Questions have been raised about the ability of microbes to reduce phosphate. Liebert (1927)

showed that on the basis of thermodynamic calculations using heats of formation, the reduction of

phosphate to phosphite by mannitol is an energy-consuming process and could therefore not serve

a respiratory function. He calculated a heat of reaction value (∆H) of +20 kcal on the basis of the

following equation:

C6H14O6 ϩ 13Na 2HPO 4 → 13Na 2HPO3 ϩ 6CO2 ϩ 7H 2O

316 kcal

5390 kcal

4460 kcal

566 kcal

478 kcal


He also calculated a ∆H

H of +438 kcal for the reduction of phosphate to hypophosphite and a ∆H

of +1147 kcal for the reduction of phosphate to phosphine by mannitol. These conclusions can also

be reached when the free energy of reaction (∆G) is considered instead of heats of reaction (∆H).

Woolfolk and Whiteley (1962) reported that phosphate was not reduced by hydrogen in the presence

of an extract of Veillonella alcalescens (formerly Micrococcul lactilyticus),

s although this extract

could catalyze the reduction of some other oxyanions with hydrogen. Skinner (1968) also questioned

the ability of bacteria to reduce phosphate. He could not find such organisms in soils he tested.

Burford and Bremner (1972), while unable to demonstrate phosphine evolution from water-logged

soils, were not able to rule out microbial phosphine genesis because they found that soil constituents

can adsorb phosphine. Thus, unless bacteria form phosphine in excess of the adsorption capacity of

a soil, phosphine detection in the gas phase may not be possible.

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Interestingly, Barrenscheen and Beckh-Widmanstetter (1923) reported the production of hydrogen phosphide (phosphine, PH3) from organically bound phosphate during putrefaction of beef

blood. Much more recently, Metcalf and Wanner (1991) presented evidence supporting the existence of a C–P lyase in Escherichia coli that catalyzes the reductive cleavage of compounds such as

methyl phosphonate to phosphite and methane:


HO P CH3 + 2(H)


C−P lyase

HO P H + CH4






This enzyme activity was previously studied in Agrobacterium radiobacterr (Wackett et al., 1987),

although it was described by these authors as a hydrolytic reaction. Phosphonolipids are known to

exist in organisms from bacteria to mammals (Hilderbrand and Henderson, 1989, cited by Metcalf

and Wanner, 1991). Thus, biochemical mechanisms for synthesizing orthophosphonates exist, and it

is therefore highly likely that an organophosphate such as methyl- or ethylphosphonate that requires

a C–P lyase to release the phosphorus as orthophosphite (Metcalf and Wanner, 1991) is an intermediate in the conversion of orthophosphate to orthophosphite and that C–P lyase activity represents

the reductive step in this transformation. This needs further investigation.


Reduced forms of phosphorus can be aerobically and anaerobically oxidized by bacteria. Thus,

Bacillus caldolyticus, a moderate thermophile, can oxidize hypophosphite to phosphate aerobically (Heinen and Lauwers, 1974). The active enzyme system consists of an (NH4)2SO4-precipitable

protein fraction, nicotinamide adenine dinucleotide (NAD), and respiratory chain components. The

enzyme system does not oxidize phosphite. Adams and Conrad (1953) first reported the aerobic

oxidation of phosphite by bacteria and fungi from soil. All phosphite that was oxidized by these

strains was assimilated. None of the oxidized phosphite, that is, phosphate, was released into the

medium before the organisms died. Phosphate added to the medium inhibited phosphite oxidation. Active organisms included the bacteria Pseudomonas fluorescens, P. lachrymans, Aerobacter

(now known at Enterobacter)

r aerogenes, Erwinia amylovora; fungi Alternaria, Aspergillus niger,


Chaetomium, Penicillium notatum; and some actinomycetes. In later studies, Casida (1960) found

that a culture of P. fluorescens strain 195 formed orthophosphate aerobically from orthophosphite

in excess of its needs and released phosphate into the medium. The culture was heterotrophic, and

its phosphite-oxidizing activity was inducible and stimulated by yeast extract. The enzyme system

involved in phosphite oxidation was an orthophosphite-NAD oxidoreductase, which was inactive

on arsenite, hypophosphite, nitrite, selenite, and tellurite, and was inhibited by sulfite (Malacinski

and Konetzka, 1966, 1967).

Oxidation of reduced phosphorus compounds can also occur anaerobically. A soil bacillus has

been isolated that is capable of anaerobic oxidation of hypophosphite and phosphite to phosphate

(Foster et al., 1978). In a mixture of phosphite and hypophosphite, phosphite was oxidized first.

Phosphate inhibited the oxidation of either phosphite or hypophosphite. The organism did not

release phosphate into the medium.

Because phosphite and hypophosphite have not been reported in detectable quantities in natural

environments, it has been suggested that microbial ability to utilize these compounds, especially

anaerobically, may be a vestigial property that originated at a time when the Earth had a reducing

atmosphere surrounding it that favored the occurrence of phosphite (Foster et al., 1978).

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Geomicrobial Interactions with Phosphorus





Intracellular Porg, Pi



au cret

tol ion

ys s,


Extracellular Porg




Extracellular Pi










The phosphorus cycle.


In many ecosystems, phosphorus availability may determine the extent of microbial growth and

activity. The element follows cycles in which it finds itself alternately outside and inside the living

cells, in organic and inorganic form, free or fixed, dissolved or precipitated. Microbes play a major

role in these changes of state, as outlined in Figure 12.3 and as discussed in this chapter.


Phosphorus is a very important element for all forms of life. It is used in cell structure as well as cell

function. It plays a role in transducing biochemically useful energy. When free in the environment,

it occurs primarily as organic phosphate esters and as inorganic phosphates. Some of the latter, such

as calcium, aluminum, and iron phosphates, are insoluble at neutral or alkaline pH. To be nutritionally available, organic phosphates have to be enzymatically hydrolyzed to liberate orthophosphate.

Microbes play a central role in this process. Microbes may also free orthophosphate from insoluble

inorganic phosphates, by producing organic or mineral acids or chelators, or in the case of iron

phosphates by producing H2S. Under some conditions, microbes may promote the formation of

insoluble inorganic phosphates such as those of calcium, aluminum, and iron. They have been

implicated in phosphorite formation in the marine environment.

Microbes have been implicated in the reduction of pentavalent phosphorus to lower valence

states. The experimental evidence for this is somewhat equivocal, however. It is likely that organic

phosphorus compounds are intermediates in these reductions. Microbes have also been implicated

in the aerobic and anaerobic oxidation of reduced forms of phosphorus to phosphate. The experimental evidence in this case is strong. It includes demonstration of enzymatic involvement. The

geomicrobial significance of these redox reactions in nature is not clearly understood.


Adams JK, Burkhart B. 1967. Diagenetic phosphates from the northern Atlantic coastal plain. Abstr Annu

Geophys Soc Am Assoc Soc Joint Meeting. New Orleans, LA. November 20–22, 1967. Program, p. 2.

Adams F, Conrad JP. 1953. Transition of phosphite to phosphate in soil. Soil Sci 75:361–371.

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Agnihotri VP. 1970. Solubilization of insoluble phosphates by some soil fungi isolated from nursery seedbeds.

Can J Microbioll 16:877–880.

Alexander M. 1977. Introduction to Soil Microbiology. 2nd ed. New York: Wiley.

Altschuler S, Clarke RS, Jr, Young EJ. 1958. Geochemistry of Uranium in Apatite and Phosphorite. U.S. Geol

Surv Prof Pap 314D.

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