1. Introduction

Phosphorous (P) is an essential macronutrient in plants [1, 2] which represents 0.12% of earth’s crust [3, 4]. The sources of P are available in both organic and inorganic forms [2, 5], which are non-renewable and cannot be substituted [4, 6]. Rock phosphate (apatite) is the best P source compared to the other P resources such as soil, clay, plant and animal matter [4], which is commonly used for phosphate fertiliser production also [7]. Phosphorous is reaching to be a plant nutrient that will limit crop production in the next millennium. P is a major growth-limiting mineral, and unlike nitrogen, there is no large atmospheric source that can provide biologically available P. Furthermore, the low bioavailability of phosphate limits the efficacy of P fertiliser. As the option to manage the bioavailability of phosphate for plants, phosphate-solubilising microorganisms (PSM) can be an effective, eco-friendly and sustainable tool. PSM contributes to improving plant growth and yield while reducing the need for chemical fertilisers that can have negative impacts on the environment. Additionally, PSM phosphate solubilisation can help to improve the overall health and fertility of the soil, promoting sustainable agriculture practices [8].

2. Importance of P for plants

P is engaged in biological processes that are shared by all living species [1, 2, 9]. P, in particular, is the second most limiting macronutrient for plant growth, accounting for 0.2% of plant dry weight [10, 11] and serving as a critical element in animal bodies [12, 13, 14]. As a result, P has become a fundamental and necessary component for human well-being [15].

Functions of P in all living organisms included energy metabolism and transmission [1, 9], cell division [2], protein and nucleic acids (DNA and RNA) synthesis [9, 16, 17], being a key component of enzymes, coenzymes, and phospholipids [18] and cell division [2]. In addition, P is essential for plants for photosynthesis [2], nitrogen fixation [14], improving crop quality [14, 16], development of disease resistance [18], root development [3], development of the stems and stalks [15], flower and seed formation [10], conversion of sugar to starch [10], laying down the primordia of propagative parts during the early stages of plant development [16], and proper stress reduction and maturation of plant [10]. A lack of phosphorus can lead to stunted growth, delayed maturity, and lower yield. Therefore, it is essential to ensure that plants receive adequate phosphorus to maintain their health and productivity.

The world population is expected to exceed 9000 million in 2050 [19] and food production requires to be increased globally by 50% as compared to the demand in 2012 [20]. Usability of phosphate has been severely limited due to its low bioavailability. Furthermore, the wastage of phosphate that is unable to be utilised by plants and animals causes additional issues such as eutrophication [10, 21]. The low bioavailability of phosphate is driving its increasing usage, with peak extraction occurring in 2030 [22]. Therefore, the sustainable utilisation of phosphate is needed to increase its bioavailability to avoid the wastage of phosphate [4]. Using microorganisms to increase the bioavailability of phosphate is the emerging eco-friendly technique for sustainable utilisation of phosphate [10, 23].

3. Main problems related to the over-exploitation of natural reserves of P and chemical fertilisers

Global food production needs to be increased due to the rapidly increasing world population [20]. The low bioavailability of phosphate directly affects to not being able to fulfil the required amount of crop production. Due to the low bioavailability, an extra amount of phosphate has to be utilised as fertilisers to provide the optimum requirement of P of plants. Therefore, it is directed to high utilisation, and the peak extraction of phosphate will take place in 2030 [22]. Besides the depletion of natural P resources, over-exploitation of natural reserves of P and excessive use of chemical P fertilisers lead to several environmental and agricultural problems such as soil degradation, groundwater contamination, eutrophication and climate change [10, 21]. Hence, it is highly required for sustainable utilisation of phosphate to minimise wastage [4]. State-of-the-art technique to use microorganisms to solubilise the insoluble forms of phosphates and increase their bioavailability is an effective option [10, 23].

4. Diversity of phosphate-solubilising microorganisms (PSM)

PSM are diverse and can be found in various environments such as water, and plant tissues while the soil is the key habitat [10, 12]. The population of PSMs in soil ranges between 10⁴ and 10⁶ g⁻¹ of soil, and they are accumulated at the rhizosphere of plants, and those organisms are highly metabolically active [10, 24]. In the microbial profile of soil, phosphate-solubilising bacteria (PSB) were responsible for 1–50%, whereas phosphate-solubilising fungi (PSF) represent 0.1–0.5%. The population density of PSB ranged between 8 × 10⁵ and 5.33 × 10⁹ in the different rhizospheres of vegetable fields, with PSB accounting for 3.98% of the total population of bacteria [25]. Most of PSBs are coccus, bacillus or spirillum in shape while the bacillus is the most abundant and spirillum are rare. However, there is a high diversity of PSM in the soil. Among these microbial species, Bacillus and Pseudomonas are the foremost bacterial genera [4, 12, 24].

The number of PSM is not sufficiently high to compete with other microbial species in the rhizosphere. The profile and population of these PSM vary between ecosystems due to the influence of complex biological factors [25]. Physical-chemical factors of soil such as the soil nutrient status, moisture content, organic matter, soil pH, and soil enzyme activities affect these variations [4, 12, 24]. The diversity of PSM (Table 1) is important for maintaining healthy ecosystems, as they play a critical role in the phosphorus cycle by making this important nutrient more available to plants [25].

Microorganisms	Country	References
Achromobacter xylosoxidans	China	[26]
Acinetobacter calcoaceticus	Iran	[27]
Advenella mimigardefordensis	Spain	[28]
Arthrobacter luteolus	Iran	[27]
Aspergillus awamori	Indonesia	[29]
Aspergillus niger	India, China	[30, 31]
Aspergillus terreus	Indonesia	[29]
Bacillus amyloliquefaciens	Italy	[32]
Bacillus aryabhattai IA20	Pakistan	[33]
Bacillus cereus	Spain, Iran	[27, 28]
Bacillus cereus MZUTZ01	India	[34]
Bacillus firmus	Pakistan	[35]
Bacillus licheniformis	Pakistan	[35]
Bacillus megaterium	China, Brazil, Spain, Thailand	[28, 36, 37, 38, 39]
Bacillus mojavensis	Thailand	[36]
Bacillus pumilus	Mexico	[37]
Bacillus safensis	Pakistan	[35]
Bacillus safensis IALR1035	USA	[38]
Bacillus siamensis	Mexico	[37]
Bacillus subtilis	India, Indonesia, Brazil	[29, 34, 39]
Bacillus subtilis IA6	Pakistan	[33]
Bacillus subtilis IALR1033	USA	[38]
Bacillus thuringensis MZUTZ13	India	[34]
Burkholderia cenocepacia	Indonesia	[29]
Burkholderia cepacia	Indonesia	[29]
Burkholderia cepacia ISOP5	China	[40]
Burkholderia fungorum	Spain	[28]
Burkholderia gladioli	India	[34]
Burkholderia seminalis	Indonesia	[29]
Burkholderia vietnamiensis	Iran	[27]
Cellulosimicrobium cellulans	China	[26]
Enterobacter bugandensis	Morocco	[41]
Enterobacter cloacae C8	China	[42]
Enterobacter hormaechei(LMG 27195)	China	[43]
Funneliformis mosseae	China	[44]
Funneliformis mosseae BEG234	Italy	[32]
Geobacillus stearothermophillus MZUTZ08	India	[34]
Klebsiella variicola	Brazil	[45]
Nocardiopsis alba	Morocco	[46, 47]
Novosphingobium barchaimii(LL02)	China	[43]
Novosphingobium resinovorum	China	[42]
Ochrobactrum haematophilum	China	[26]
Ochrobactrum pseudogrignonense	Brazil	[45]
Ochrobactrum pseudogrignonense(CCUG30717)	China	[43]
Paenibacillus polymyxa IA7	Pakistan	[33]
Pantoea agglomerans	Tunisia, Morocco	[41, 48]
Pantoea agglomerans IALR1325	USA	[38]
Pantoea agglomerans pv. P5	Iran	[49]
Pantoea ananatis	Brazil	[45]
Pantoea roadsii(LMG26273)	China	[43]
Pantoea stewartii subsp. Indologenes	Morocco	[41]
Pantoea vagans IALR611	USA	[38]
Paraburkholderia caffeinilytica(CF1)	China	[43]
Penicillium oxalicum	Pakistan	[35]
Pseudomonas agglomerans	Tunisia	[50]
Pseudomonas azotoformans	India	[51]
Pseudomonas brassicacearum supsp. Neoaurantiaca	Morocco	[41]
Pseudomonas cepaceae	Egypt	[52]
Pseudomonas donghuensis(HYS)	China	[43]
Pseudomonas fluorescens	Tunisia	[50]
Pseudomonas grimontii(CFML97 514)	China	[43]
Pseudomonas lactis	Morocco	[41]
Pseudomonas libanensis	Mexico	[37]
Pseudomonas mallei	Egypt	[52]
Pseudomonas palleroniana	India	[51]
Pseudomonas plecoglossicida C10	China	[42]
Pseudomonas proteolytica	India	[51]
Pseudomonas psychrotolerans IALR632	USA	[38]
Pseudomonas putida pv. P13	Iran	[49]
Ralstonia pickettii C9	China	[42]
Rhizophagus irregularis BEG72	Italy	[32]
Rhodopseudomonas palustris ISP-1	China	[40]
Serratia rubidaea	Morocco	[41]
Staphylococcus pastueri MZUTZ02	India	[34]
Stenotrophomonas maltophilia	Tunisia, China	[26, 50]
Streptomyces alboviridis	Morocco	[47]
Streptomyces fulvissimus	Morocco	[46]
Streptomyces griseorubens	Morocco	[46, 47]
Streptomyces microflavus	Morocco	[46]
Streptomyces pratensis	Morocco	[46]
Streptomyces youssoufiensis	Morocco	[46]
Talaromyces minioluteus	Iran	[27]
Talaromyces pinophilus	Iran, Indonesia	[27, 29]
Talaromyces stipitatus	Iran	[27]
Trichoderma asperellum LZ1	China	[53]

Table 1.

Major phosphate solubilizing microorganisms.

5. Ability of microorganisms to solubilise phosphates and key mechanisms of microbial P Solubilisation

Phosphorous is available in the soil as a number of organic and inorganic compounds due to its high reactivity, and those are unavailable for plants [54]. PSMs solubilise these insoluble forms of phosphates into soluble by secreting complex compounds such as organic acid anions, protons, exopolysaccharides, siderophores, hydroxyl ions and extracellular enzymes [55]. The phosphate-solubilising mechanism of PSM has two major aspects as inorganic phosphate solubilisation and organic phosphate solubilisation based on the substrate of P [3].

5.1 Inorganic phosphate solubilisation by PSM

Microbial mobilisation of inorganic phosphates is involved with the production and secretion of organic acids, inorganic acids, siderophore and exopolysaccharide and proton extrusion by the PSM [3].

5.1.1 Organic acid production

The key mechanism of phosphate solubilisation is the secretion of the organic acids that result from the carbon metabolism of PSM, which is closely related to the concentration of soluble phosphate [3, 4, 24]. Low-molecular-weight organic acids are synthetised during glucose oxidation through direct periplasmic oxidation and intracellular phosphorylation. The synthesis of organic acids from glucose by the cells of PSMs in phosphate deficient conditions is higher than in phosphate sufficient conditions, which correlates with the effect of soluble phosphate on organic acid production [56].

The release of these organic acids into the environment is accompanied by a decrease in pH and chelate of the cations (such as Al³⁺, Fe³⁺, and Ca²⁺) bound to phosphate ions to release the phosphate [3, 8]. These organic acids compete with the phosphate binding sites of the medium and allow phosphates to be available in free [15] as HPO₄⁻² and HPO₄⁻³ [10]. However, there is no correlation between pH and the amount of solubilised phosphates [56]. Gluconic acid is the frequent secretion among all organic acids released by PSM (Table 2) [3, 10, 54]. PSF may be even more important than PSB since they typically produce and excrete more acids [56].

Organic acid	PSM		References
Acetic acid	Bacillus amyloliquefaciens Bacillus atrophaeus Bacillus licheniformis Chryseomonas Luteola Enterobacter aerogenes Enterobacter asburiae	Enterobacter taylorae Kluyvera cryocrescens Penibacillus macerans Pseudomonas aerogenes Vibrio proteolyticus Xanthobacter agilis	[57]
Caproic acid	Actinomadura oligospora Bacillus subtilis var.2	Citrobacter sp.	[57]
Citric acid	Arrhrobacter Aspergillus flavus Aspergillus niger FS 1 Enterobacter Eupenicillium ludwigii FS 27	Penicillium canescens Penicillium canescens FS 23 Penicillium islandicum FS 30 Penicillium rugulosum Penicillium trivialis	[12]
	Arthrobacter sp. Aspergillus flavus Aspergillus foetidus Aspergillus japonicas Aspergillus niger Bacillus firmus B-7650	Bascillus sp. Chaetomium nigricolor Enterobacter agglomerans Penicillium fluorescens Penicillium canescens Penicillium rugulosum	[57]
	Aspergillus Azospirillum sp. Bacillus sp.	Penicillium sp. Proteus sp. Pseudomonas	[10, 58, 59, 60]
Formic Acid	Pseudomonas trivialis		[12]
Formic Acid	Actinomadura oligospora Bacillus pumilus var.2 Bacillus subtilis var.2	Citrobacter sp. Pseudomonas trivialis	[57]
Fumaric acid	Azospirillum sp. Bacillus sp.	Proteus sp. Pseudomonas	[10, 59, 60]
Gluconic acid	Arrhrobacter Aspergillus flavus Aspergillus niger FS 1 Enterobacter Enterobacter intermedium Enterobacter sps Fs 11	Eupenicillium ludwigii FS 27 Penicillium canescens FS23 Penicillium islandicum Penicillium rugulosum Pseudomonas fluorescens	[12]
	Actinomadura oligospora Aspergillus flavus Aspergillus foetidus Aspergillus japonicas Aspergillus niger Bacillus pumilus var.2	Bacillus subtilis var.2 Citrobacter sp. Penicillium canescen Penicillium radicum Penicillium rugulosum Pseudomonas fluorescens	[57]
	Aspergillus Azospirillum sp. Bacillus sp. Erwinia herbicola	Penicillium sp. Proteus sp. Pseudomonas	[10, 58, 59, 60]
Glycolic acid	Aspergillus niger	Penicillium sp.	[12]
Glycolic acid	Aspergillus sp.	Penicillium sp.	[10, 58]
Heptonic acid	Actinomadura oligospora Bacillus pumilus var.2	Bacillus subtilis var.2 Citrobacter sp.	[57]
Indole acetic acid	Psedomonas nitroreducens		[12]
Isobutyric acid	Bacillus amyloliquefaciens Bacillus atrophaeus Bacillus licheniformis Chryseomonas luteola Enterobacter aerogenes Enterobacter asburiae	Enterobacter taylorae Kluyvera cryocrescens Penibacillus macerans Pseudomonas aerogenes Vibrio proteolyticus Xanthobacter agilis	[57]
Isocaproic acid	Actinomadura oligospora Bacillus pumilus var.2	Bacillus subtilis var.2 Citrobacter sp.	[57]
Isovaleric acid	Actinomadura oligospora Bacillus atrophaeus Bacillus licheniformis Bacillus pumilus var.2 Bacillus subtilis var.2 Bacillus amyloliquefaciens Chryseomonas luteola Citrobacter sp.	Enterobacter asburiae Enterobacter taylorae Enterobacter aerogenes Kluyvera cryocrescens Penibacillus macerans Pseudomonas aerogenes Vibrio proteolyticus Xanthobacter agilis	[57]
Itaconic acid	Bacillus atrophaeus Bacillus licheniformis Bacillus amyloliquefaciens Chryseomonas luteola Enterobacter asburiae Enterobacter taylorae	Enterobacter aerogenes Kluyvera cryocrescens Penibacillus macerans Pseudomonas aerogenes Vibrio proteolyticus Xanthobacter agilis	[57]
Lactic acid	Aspergillus niger Penicillium sp.	Pseudomonas trivialis	[12]
Lactic acid	Arthrobacter sp. Aspergillus niger Bacillus atrophaeus Bacillus licheniformis Bacillus amyloliquefaciens Bacillus firmus B-7650 Bacillus megaterium Chryseomonas luteola Enterobacter asburiae Enterobacter taylorae	Enterobacter aerogenes Escherichia freundii Kluyvera cryocrescens Pseudomonas trivialis Penibacillus macerans Penicillium sp. Pseudomonas aerogenes Bacillus subtilus Vibrio proteolyticus Xanthobacter agilis	[57]
Malic acid	Arrhrobacter Enterobacter sps Fs 11	Pseudomonas fluorescens	[12]
	Bacillus megaterium Bacillus subtilus	Pseudomonas fluorescens	[57]
	Aspergillus Bacillus sp. Penicillium sp.		[10, 58, 59]
Malonic acid	Actinomadura oligospora Bacillus pumilus var.2	Bacillus subtilis var.2 Citrobacter sp.	[57]
Oxalacetic acid	Actinomadura oligospora Bacillus pumilus var.2	Bacillus subtilis var.2 Citrobacter sp.	[57]
Oxalic acid	A. niger FS 1 Aspergillus flavus Eupenicillium ludwigii FS 27	Penicillium canescens FS23 Penicillium canescens, Penicillium islandicum	[12]
	Actinomadura oligospora Aspergillus flavus Aspergillus foetidus Aspergillus japonicas Aspergillus niger Bacillus pumilus var.2	Bacillus subtilis var.2 Chaetomium nigricolor Citrobacter sp. Enterobacter agglomerans Penicillium canescens	[57]
	Aspergillus sp.	Penicillium sp.	[10, 58]
Propionic acid	Actinomadura oligospora Bacillus pumilus var.2	Bacillus subtilis var.2 Citrobacter sp.	[57]
Succinic acid	Aspergillus flavus Aspergillus niger	Penicillium canescens	[12]
	Actinomadura oligospora Aspergillus flavus Aspergillus foetidus Aspergillus japonicas Aspergillus niger	Bacillus pumilus var.2 Bacillus subtilis var.2 Chaetomium nigricolor Citrobacter sp. Penicillium canescens	[57]
	Aspergillus Azospirillum sp. Bacillus sp.	Penicillium sp. Proteus sp. Pseudomonas	[10, 58, 59, 60]
Tartaric acid	Arrhrobacter Enterobacter	Pseudomonas trivialis	[12]
	Aspergillus japonicas Aspergillus foetidus	Pseudomonas fluorescens	[57]
	Bacillus sp.		[10, 59]
Valeric acid	Actinomadura oligospora Bacillus pumilus var.2	Bacillus subtilis var.2 Citrobacter sp.	[57]
2-Keto gluconic acid	Enterobacter intermedium		[12]
	Aspergillus sp. Chaetomiumnigricolor	Enterobacter intermedium Penicillium sp.	[57]
	Erwinia herbicola	Pseudomonas	[10, 59, 60]

Table 2.

Major examples of the production of organic acids by PSM.

5.1.2 Inorganic acid production

Inorganic acid-producing bacteria also involve with phosphate solubilisation through acidification. Bacteria, engaged in nitrification and sulphur-oxidation, which have the ability to produce inorganic acids, are involved with phosphate solubilisation by secreting nitric, carbonic, sulphuric [3] and hydrochloric acids [4, 24]. Nitrifying bacteria such as Nitrosovibrio, Nitrosomonas, Nitrobacter, Nitrosospira frequently secrete nitric acid [61] and sulphur oxidising bacteria such as Thiobacillus thiooxidans [62] produce sulphuric acid directly involve to solubilisation of phosphate which is required to be further studied. Acidification of the media by secreting inorganic acids and H⁺ substitution reactions release the phosphates by converting insoluble phosphate to its soluble form [4, 24]. However, the efficacy of phosphate solubilisation by inorganic acids is lower than the efficiency of organic acids [3, 8].

5.1.3 Proton extrusion

Another alternative mechanism to solubilise phosphate is extrusion of proton, which minimise the requirement to synthesis acids [3]. Excretion of H⁺ through H₂CO₃ production, NH₄⁺ assimilation and liberation of organic acid anions facilitate the solubilisation of phosphates by acidifying the media [3, 8, 10]. The release of H⁺ to the extracellular surface of PSM through the exchange of cation or the ATPase activity with H+ translocation, lead to the solubilisation of P [56]. Pseudomonas fluorescens [63], Bacillus sp., Azospirillum sp. [64] significantly exhibit this mechanism of H⁺ extrusion in the process of phosphate solubilisation.

5.1.4 Exopolysaccharide production

Exopolysaccharides are high molecular weight compounds that indirectly affect the solubilisation of P in soil [65]. Microorganisms secrete exopolysaccharides under stress conditions; exhibit the potential to promote phosphate solubilisation [3, 66]. Exopolysaccharides have a strong affinity with the metal ions in the soil, which have formed complexes with phosphates and release those phosphates. There is a positive correlation between the rate of phosphate solubilisation and the concentration of exopolysaccharides [3]. Different exopolysaccharides have different binding affinities with various metals, and there are also different binding strengths between the metals themselves. Arthrobacter sp. ArHy-505, Azotobacter sp. AzHy-510 and Enterobacter sp. EnHy-401 exhibits higher tricalcium phosphate solubilisation capacity by producing exopolysaccharides [65].

5.1.5 Siderophore production

Siderophore production is a usual ability of microorganisms, while it is used by PSM as another alternative method to solubilise the phosphate [3, 10, 15]. Siderophores are low-molecular-weight secondary metabolites synthesised by PSM that have a strong affinity for inorganic iron and act as metal chelators. Siderophores contain three functional groups hydroxamates, catecholates, and carboxylates, which catalyse the reduction of Fe³⁺ to Fe²⁺. PSM use siderophores to obtain the Fe as the requirement of cellular functions, and during the dissociation of its bond, P is released and available for plants. Streptomyces sp. successfully increase the bioavailability of P through the production of sideraphores [65].

5.2 Organic phosphate solubilisation mechanisms

5.2.1 Enzyme production

The content of organic phosphorus in the soil can reach 30–50% of the total amount, which is represented by compounds such as phosphonates, phytic acid, polyphosphonates, sugar phosphates [3, 56], phosphomonoesters, phosphodiesters, phosphotriesters [8], phospholipids and nucleic acids [3, 8]. Generally, these high molecular weight organic compounds are resistant to chemical hydrolysis and need to be converted into soluble ionic phosphate or low-molecular-weight organic phosphates for plant uptake [56].

PSM produce and secrete the enzymes to solubilise the organic phosphates [8]. There are several groups of enzymes are secreted by PSM as [3, 8, 54, 55, 56],

Non-Specific Acid Phosphatases (NSAPs)
Phytases
Phosphonatases
Carbon–Phosphorus Lyases

NSAPs are a class of enzymes secreted extracellularly or bound to the lipoprotein membranes of PSM. NSAP are generally known as phosphomonoesterases, which dephosphorylate a wide variety of phosphoesters, solubilising around 90% of organic phosphate in soils. The proportion of phosphatases and abundance of P in the soil has a direct relationship, which influences the availability of phosphates to plants. According to the authors, the activity of NSAPs is more concentrated in the rhizosphere than in other parts of soil and phosphatase activity, P uptake by plants and nodule weight has a positive correlation [56].

Phytase enzymes are phosphatases produced by PSM, which have the capability to hydrolyse phytic acid by acting on the phosphomonoester bonds present in the compound, originating two subgroups, myo-inositol hexaphosphate or phytate. In addition to P, this process makes biologically available other nutrients such as zinc and iron [65].

Phosphatases (phosphonate hydrolases) are enzymes, which promote the breaking of C-P bonds of phosphonates by catalysing this reaction from a group carbonyl electron scavenger that allows heterologous cleavage between nutrients. Phosphonatases act on several substrates, including phosphoenolpyruvate, phosphonoacetate, and phosphoenol-acetaldehyde and make biologically available P for plants [56, 65].

Carbon–phosphorus lyases are a complex of membrane enzymes released by PSM that also allow the making available of P, cleaving the C–P bonds of phosphonates, producing hydrocarbons and inorganic Phosphate. This complex is the leading mechanism for the utilisation of phosphonates by PSM. The enzymes and proteins of C–P lyases are complex and specific to their substrates. Escherichia coli and Enterobacter cloacae K7 are commonly involved with solubilisation of phosphates by using C–P lyases [65].

6. Capability of microorganisms to formulate phosphate-related nanoparticles

Nanotechnology is a state-of-the-art technique of using particles between 1 to 100 nm, which originated as both organic and inorganic forms [67, 68, 69]. Nano minerals are currently used as plant fertilisers due to their Nano size, high surface area, and higher solubility in different solvents and penetrative capacity than conventional mineral fertilisers. Reduction of the particle size is directed to an increment of the surface area of particle and the number of particles per unit in fertiliser, which facilitate higher nutrient utilising efficacy. Nanoparticles enter into plants through nano and microscale openings, which are most commonly available in roots and leaves [70, 71]. Generally, root tips, rhizodermis, lateral root junctions and wounding of roots are the entering pathways to nanoparticles. Enhanced bioavailability of nano fertilisers than conventional fertilisers cause to prevent loss of nutrients from leaching, denitrification, volatilisation, and fixation in the soil to confirm the sustainable utilisation of minerals [70].

Microorganisms including bacteria, fungi, actinobacteria and viruses have the ability to synthesise phosphorus-related nanoparticles with well-defined chemical composition, morphology and size either intra-or extracellularly [71, 72, 73]. But no in-depth study or work has been done so far [74]. Enzymes [75, 76] such as phytase, phosphatases [75], extracellular polymeric substances (EPS) [76] and proteins [75] are the key potential materials to synthesise phosphorus-related nanoparticles which are produced by microorganisms. In addition, proteins act as capping agents to stabilise the produced nanoparticles [75]. This is a new arising area in nano formulations [74]. Therefore, limited research has been conducted on synthesising phosphorus-related nanoparticles by using microorganisms, as mentioned in Table 3.

Microbe	Type of Nano P	Size/Shape	Application	References
Aspergillus flavus TFR-1	Phosphorous	17–64 nm	Plant fertiliser	[77]
Aspergillus fumigatus TFR-8	Phosphorous	37-81 nm	Plant fertiliser	[77]
Aspergillus oryzae TFR-9	Phosphorous	23–88 nm	Plant fertiliser	[77]
Aspergillus terreus CZR-1	Phosphorous	11–74 nm	Plant fertiliser	[77]
Aspergillus tubingensis TFR-3	Phosphorous	5–49 nm	Plant fertiliser	[77]
Bacillus megaterium JCT13	Phosphorous	20–91 nm	To maintenance of soil fertility	[73, 75]
Emericella nidulans TFR-14	Phosphorous	55–92 nm	Plant fertiliser	[77]
Rhizoctonia bataticola TFR-6	Phosphorous	12–36 nm	Plant fertiliser	[77]
Yeast	Zinc Phosphate	10–80 nm in width and 80–200 nm in length, Butterfly like shape	Antirust pigment and electronic luminophore	[78]

Table 3.

Microbial synthesis of phosphorus-related nanoparticles.

7. Potential of PSM to develop as a commercial level biofertiliser

The distinctive and unique ability of PSMs to solubilise organic and inorganic phosphates, leads to their developed as commercial level bio fertilisers [10, 12]. Biofertilisers are microbiologically active, eco-friendly, low-cost products applied to soil expecting soil health and growth promotion of plants [10, 15]. Biofertilisers can be replaced by 50% instead of chemical fertilisers without any reduction of the yield [15]. These phosphate bio fertilisers are not crop-specific and able to be used for any type of plant, expecting growth promotion, high yield and crop quality through optimum phosphate absorption [10]. Azotobacter inoculation and Bacillus inoculants have success stories with yield increments of sugarcane and wheat. Application of the combination of Bacillus megaterium and Azotobacter chroococcum as biofertiliser remark the yield increment by 10–20%. Bacillus circulans, Bacillus megaterium, Bacillus subtilis, and Pseudomonas striata reach the commercial level in the phosphate biofertiliser industry [10].

Biofilm inoculants and nano-bio inoculants are ultra-modern techniques in biofertiliser formulation technology. Biofilm inoculants are the combination of two microorganisms, while one microorganism colonises over the other microorganism. The second microorganism act as a biotic surface for the first microorganism to develop a metabolically enhanced biofilm rather than a single culture [3]. Even though these relationships affect the high phosphate solubilisation and growth promotion of plants, the abundance in the soil is very low [3, 79]. Therefore, applying these biofilms as biofertilisers through artificial formulation is advantageous in agriculture [3]. Pleurotus ostreatus, Xanthoparmelia mexicana and Penicillium spp. are the most prominent PSMs, which have the potential to be used in biofilms for substantial impact [80].

8. Contribution of PSM to achieving sustainable development goals (SDGs)

Collection of 17 interlinked objectives focusing on prosperity for people and the planet called United Nations Sustainable Development Goals (SDGs) able to be enforced with PSMs. Microbial phosphate solubilisation makes way for sustainable development by promoting plant growth, food security, industrial growth, environmental sustainability and water security [81, 82].

Continuous fertilisation to supply the optimum requirement of phosphate for crops due to the low bioavailability is not the appropriate way, and it is a waste of the resource [83]. Responsible consumption of the resource (SDG 12) able to be confirmed by the involvement of PSMs in increasing the bioavailability of phosphate. It reduces the accumulation of excess phosphate with associated other chemicals as fertilisers to soil and contributes to building up sustainable terrestrial ecosystems with reverse land degradation to halt biodiversity loss (SDG 15) [84]. Moreover, the sequestration of phosphate in the soil leads to eutrophication, which causes disruptions of valuable aquatic resources and pollution the water [85]. Therefore, reducing the excess utilisation of phosphate fertilisers by developing PSM as biofertilisers will ensure the sustainable conservation of aquatic resources (SDG 14) and sustainable management of available clean water (SDG 06) [82, 86].

The contribution of PSM to sustainable agriculture through plant growth promotion ensures sufficient food production empowering food security, which promotes public nutrition and ends hunger (SDG 02) [87, 88]. Development of the sustainable agricultural industry (SDG 09) confirm sustainable economic growth (SDG 08) with moving forward to the reduction of poverty (SDG 01) [89]. Society, which reaches economic development by overcoming poverty and hunger, ensures a better living environment promoting a sustainable peaceful society by accessing justice (SDG 16) [90]. The direct and indirect contributions of PSMs to achieve the SDGs affect significantly strengthen the framework of the 2030 agenda of the United Nations [82].

9. Conclusion

The application of PSMs as a tool in managing soil and plant nutrients through Solubilising phosphate is an efficient and eco-friendly method. Bacterial and fungal species are majorly involved in the phosphate solubilisation in soil, remarking Bacillus, Pseudomonas, Penicillium and Aspergillus species as the most frequent organisms. Even the PSMs are using a number of mechanisms to solubilise the phosphate, organic acid production is the leading mechanism, and gluconic acid is the foremost organic acid involved in the Phosphate immobilisation process. As a cutting-edge technology, PSMs can be used for the formulation of phosphate-related nanoparticles to increase bioavailability. Current arising biofertiliser technology using this potential of PSMs at the commercial level. Biotechnological applications are able to be used to develop the ability of PSMs to obtain maximum output. Furthermore, PSMs are directly and indirectly affecting to achievement SDGs to serve as a “shared blueprint for peace and prosperity for people and the planet, now and into the future”.

Acknowledgments

The paper was supported by the research grant of “Accelerating Higher Education and Development” (AHEAD) under development-oriented research (DOR) of STEM grant number AHEAD/RA3/DOR/WUSL/LAS No.57 funded by the World Bank.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Caroline Zabiegaj-Zwick

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