Mechanisms Of Improving Phosphorus Release From Bioapatite By Enterobacter Sp.

Phosphorus is an essential nutrient for plant growth.Phosphorus deficiency is common in soil,which limits the primary productivity of ecosystems.Biomineral phosphorus source,which includes animals’bones and teeth,plays an important role in natural and agricultural ecosystems.Biomineral phosphorus source in animals is mainly composed of bioapatite,with organic matters.The mineral accounts for up to 50%of dry biomass and holds 95%of the phosphorus in organisms.The decomposition of animal corpses releases phosphorus in bioapatite at specific time and places for a long period of time,which alleviates local phosphorus limitations.To get rid of the risk of available-P deficiency in agricultural ecosystem and promote the sustainable development of phosphorus resources,it is imperative to study the mechanisms of phosphorus release from bioapatite.The solubility of bioapatite is relatively high compared with hydroxylapatite and fluorapatite（the main types of phosphate ore）and is affected by minerals,environmental conditions,and microbial factors.Its intrinsic crystal structures,chemical substitution,phase transition will influence the solubility production of bioapatite,and thus influence the phosphorus release.Furthermore,phosphorus release of bioapatite in environment is altered by various environmental factors.This research focuses on two environmental factors:environmental carbonation and temperature.Carbonation is a bridge between carbon and phosphorus cycling.Heating by natural fire can also change the surface properties of biomineral phosphorus source and thus control the phosphorus release.To better understand the mechanisms of“phosphorus supply”,it is particularly important to integrate mineral factors with these abiotic environmental factors.Microorganisms drive the soil phosphorus cycling.Microorganisms can facilitate phosphorus release from apatite dramatically through bioweathering.Microorganisms with a strong ability of dissolving phosphate,named phosphate-solubilizing microorganisms（PSM）,are ubiquitous in terrestrial ecosystem.In particular,phosphate solubilizing bacteria（PSB）grow faster and have higher abundance.They are able to colonize in some micropores or on the surface of bioapatite,and hence promote the availability of phosphorus.To figure out the interactions among environmental factors,bioapatite and PSM,this research tested the mechanisms of phosphate solubilizing under the influence of exogenous calcium carbonate,endogenous carbonate as well as the effects of heating on phosphorus release from bioapatite with the presence of PSB.This study applied a type of PSB,Enterobacter sp.,as the research object.Through changing the properties of bioapatite and solution ambience,several factors that affect the ability of solubilizing phosphate were considered.The main results and discussions are as follows:1.Mineralogical characteristics of bioapatiteAnimal bones consist of numerous mineral particles and collagen fibers.The coexistence of them impedes the mineralogical research to the microcrystal structure of bioapatite（BAp）.In this chapter,a standard BAp was applied and its crystal size and morphological change under long-term dissolution were monitored.AFM showed the tensely compacted mineral crystallites and confirmed the extremely low organic matter content（mainly collagenous fibers）in it.In addition,the crystallites in bioapatite were with irregular plate shape and different sizes.Compared with the larger grains（～50nm）,smaller grains（～11×24 nm）are more easily to be detached from plate-like bioapatite prims.Furthermore,various crystal orientations were observed at the edge of the prism,which demonstrated that BAp grown in a random direction when it was formed.The bone BAp was similar to that of normal animals’,which had a low solubility（with P concentration of～3.8 mg/L and Ca concentration of～6.7 mg/L after 150 d dissolution）.The corresponding solubility product is～10^-52.This chapter lays a mineralogical foundation for the following study on phosphorus release.2.The effect of carbonation on phosphate solubilization by microorganismsExogenous carbonate in the environment can influence P availability of BAp in the environment while the degree of carbonate substitution determines the solubility production.This chapter will investigate the influence of the two factors on P release with the presence of PSB.It showed that carbonate could neutralize acid efficiently.The addition of carbonate hampered the P release and transformed tricalcium phosphate into hydroxylapatite.Moreover,the adsorption of PSB on carbonates obstructed bioweathering of phosphate.However,precipitation of Ca-P complex on the surface of calcium carbonate was inhibited by Ca²⁺consumption when gypsum was formed.Meanwhile,the improvement of carbonate content in BAp cannot consistently enhance the solubility of BAp.Endogenous carbonate’s hydrolysis can lead to an alkaline environment and alter the solubilizing efficiency of PSB.P release from BAp reached a zenith when carbonate content was 7-10%due to an ideal balance between decrease in crystallinity（positive effect）and hydrolysis of carbonate（negative effect）.3.The influence of heating on P release from bioapatiteIn this chapter,P release from charred bioapatite that heated at different temperature（100-300 ^oC）with the assistance of PSB was compared.With the absence of PSB,P release in pure water fluctuated with elevating temperature and culminated at 200 ^oC（8.66 mg/L）.Similarly,the kinetics showed the highest solubility and dissolution rate of CB heated at～250 ^oC.After the addition of PSB,P release from each CB was enhanced.Among all treatments,P release from CB heated at 100 ^oC was the greatest enhanced by PSB,which rise from 3.51 to 77.37 mg/L.ATR-IR and XPS pattern confirmed the loss of organic matter（mainly consisted of collagen）in bones.However,there was no significant changes in composition of BAp.Meanwhile,it was inferred that collagen cannot provide extra P for this PSB strain but promote the adsorption of PSB on the surface of bones.Thus,P release was enhanced.In conclusion,the solubility product of standard bioapatite is extremely low,and PSM improve the release of phosphorus via acid secretion.The presence of carbonate in the environment and heating control over the efficiency of phosphate solubilizing with the assistance of PSB through buffering acid and denaturing organic matter,respectively.This paper dedicates to investigating the effects of the two environmental factors on the decomposition of bioapatite by PSB,which contribute to the study of the role of bioapatite to the phosphorus cycle in ecosystem.