Marine algae 106 16 1
Soil bacteria 31 5 1 Grassland soil
Native 191 6 1
Cultivated, fertilized 119 9 1
Weakly weathered soil 80 5 1
the phosphate anhydride bond (Fig. 15.4d), plays an important role in storing and transferring biochemically useful energy. A free energy change (AG0) of -7.3 kcal mol-1 (-30.6kJ mol-1) of adenosine 5'-triphosphate (ATP) is associated with the hydrolysis of its terminal anhydride bond, yielding adenosine 5'-diphosphate and a phosphate group (see Chap. 9). Unlike many other anhydrides, phosphate anhydrides such as ATP are unusually resistant to hydrolysis in the aqueous environment (Westheimer, 1987). At neutral pH and physiological temperature, hydrolysis proceeds at an optimal rate only in the presence of appropriate enzymes (e.g., ATPase). The relative resistance of phosphate anhydride bonds to hydrolysis is attributable to the negative charges on the phosphates at neutral pH and is the probably reason ATP was selected in the evolution of life as a universal transfer agent of chemical energy in biological systems (Westheimer, 1987).
There is good evidence that P is the dominant element controlling C and N immobilization in biological systems. In a classic paper, Redfield (1958) hypothesized that P controls the C, N, and S cycles of marine systems. He noted that the oceanic C:N:P ratio paralleled that of the plankton and believed the following general relationship occurs:
106CO2 + 16NO- + 122H2O + 18H+-solar energy,trace elements :
C106H263O110N16P1 + 138O2.
The C:N:P ratios for a number of terrestrial situations are shown in Table 15.3. The aquatic algae and soil bacteria have similar C:N ratios of approximately 6:1. The algae have lower C:P ratios than the bacteria, but fall within the range of C:P found in soil organic matter.
MICROBIAL TRANSFORMATIONS OF PHOSPHORUS Mineralization
Organically bound P is not directly available to organisms because it cannot be absorbed into cells in this form. For cellular uptake to occur, P must first be released from the organic molecule through mineralization. The final stage in the conversion of organically bound P to inorganic phosphate occurs through the action of phosphatase enzymes. The phosphatase group of enzymes includes phytase enzymes that catalyze the release of phosphate from phytin and nuclease enzymes that liberate phosphate from nucleic acids. These enzymes are produced by up to 70-80% of the microbial population, including bacteria such as Bacillus megaterium, B. subtilis, Serratia spp., Proteus spp., Arthrobacter spp., and Streptomyces spp. and fungi such as Aspergillus spp., Penicillium spp., Rhizopus spp., and Cunninghamella spp. Once P is mineralized, it can be taken up by plants, immobilized by the microbial biomass, precipitated in inorganic complexes, or sorbed to mineral surfaces.
Soil microorganisms can cause fixation or immobilization of P, either by promoting the formation of inorganic precipitates or by assimilation into organic cell constituents or intracellular polyphosphate granules. In soils and freshwater sediments, cellular immobilization is important, though fixation of P by Ca2+, Al3+, or Fe3+ has been observed. In some marine sediments, where phosphorite minerals occur, the precipitation mechanism is more important. Microorganisms are indirectly involved in phosphorite precipitation by making reactive phosphate available, by making reactive calcium available, or by creating or maintaining the environmental conditions that favor phosphate precipitation.
The extent of immobilization of P is affected by the C:P ratio of the organic materials being decomposed and the amount of available P in solution. If insufficient P is available in the substrate for assimilation of the substrate C, inorganic P from the soil solution will be used and net immobilization occurs. Generally, a C:P < 200 will result in net mineralization, a C:P > 300 results in net immobilization, and C:P ratios between 200 and 300 result in little net change in soluble P concentrations.
A number of soil bacteria and fungi have been shown to be capable of oxidizing reduced phosphorus compounds (e.g., phosphite, hypophosphite) either aerobically (Adams and Conrad, 1953) or anaerobically (Foster et al., 1978). The biochemical pathway for such a microbially mediated reaction has been characterized molecu-larly and genetically, providing some evidence for a previously underappreciated microbial redox cycle for P. The relatively high solubility in water of phosphites, hypophosphites, and phosphonates suggests they may have been important precursors of biochemical P compounds, but the fact that there have been only trace quantities of phosphite and hypophosphite detected in the current environment suggests that the existence of microbial pathways of P oxidation might represent an ancient evolutionary property (Foster et al., 1978; Schink and Friedrich, 2000).
There is increasing appreciation in the literature for the presence of reduced forms of P such as phosphine (PH3), phosphites, and organic phosphonates, which provide a small gaseous link to the P cycle (Glindemann et al., 2005). Microbially mediated reduction of phosphate remains a controversial topic in the literature (see Morton and Edwards, 2005; Roels and Verstraete, 2001). The controversy is fueled by thermodynamic calculations that show that the reduction of phosphate is energetically unfavorable. However, this does not imply that reduced P compounds cannot be formed biogenically because other linked mechanisms exist in nature by which energetically unfavorable reactions, such as N2 fixation, can occur.
The low solubility of P in soils makes it one of the major nutrients limiting plant growth. Frequent applications of soluble forms of P are needed, more than really necessary, because only a fraction is used by plants while the rest rapidly forms insoluble complexes. Traditional P fertilizer production is based on chemical processing of insoluble mineral phosphate ore, which is expensive and environmentally undesirable. In areas where commercially produced P fertilizer is too costly, the microbial solubilization of phosphate rock is seen as a viable alternative (see Whitelaw, 2000). In India, for example, there is an estimated 40 million tons of P-containing rock deposits that could provide a cheap source of P fertilizer. One strategy currently used is to mix rock phosphates with various plant residues in a composting mixture. In some cases, the compost is enriched with known P-solubilizing bacteria. The development of agricultural inoculants has been difficult and knowledge of the genetics of phosphate solubilization is still sparse (Rodriguez et al., 2006).
Phosphate-solubilizing microorganisms are suspected to convert the insoluble rock phosphates into soluble forms through the processes of acidification, chela-tion, and exchange reactions, but uncertainty remains concerning the detailed mechanisms, which may be organism-dependent. Carbonic acid and HCO- derived from respiratory CO2 are of prime importance in the weathering of soil minerals, but there is poor correlation between CO2 levels and dissolution of apatite. Illmer and Schinner (1995) showed that Aspergillus niger produced citrate, oxalate, and gluconate and suggested that organic acid production may be an important mechanism for solubilizing aluminum phosphates, but not the only effective mechanism. They found that other organisms, such as Penicillium aurantiongriseum and Pseudomonas sp. (P/18/89), were effective at solubilizing aluminum or calcium phosphates without producing organic acids. Proton release associated with respiration or ammonium assimilation was proposed as the mechanism responsible. Organic acids produced in the rhizosphere by plant roots and associated microorganisms may act as chelating agents. These organic chelates form complexes with Ca, Fe, or Al, thereby releasing the phosphates to solution. It is currently impossible to select any given mechanism from the alternatives, and some researchers have questioned whether sufficient acidity or chelating agents can be generated microbially to appreciably affect P solubility.
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