Meet the microbes that feed phosphorus to plants

Plants need phosphorus to make biological molecules like DNA, ATP, and the phospholipid bilayers that form cell membranes. Most phosphorus on Earth is found in its most oxidised form, phosphate (PO₄^3-). Plant roots can only absorb soluble phosphate ions, but 80% of the phosphate in soil is insoluble and therefore unavailable for plant growth.

Enter phosphate-solubilising micro-organisms.

What are phosphate-solubilising micro-organisms?

Phosphate solubilisation is the process by which micro-organisms convert insoluble phosphorus sources, like rocks or the biomass of dead organisms, into bioavailable phosphate ions (Figure 1). Examples of phosphate-solubilising bacteria come from the genera Bacillus, Pseudomonas, Rhizobium, Escherichia, Streptomyces, and Micromonospora, as well as some cyanobacteria. Phosphate-solubilising fungi include Aspergillus, Penicillium, Mucor, Rhizopus, Rhizophagus, and Glomus. The latter two fungal genera are arbuscular mycorrhizal (AM) fungi - more on them later. 

The chemistry underpinning phosphate solubilisation is complex but can broadly be split into inorganic and organic processes (Figure 1). Some of these inorganic and organic processes are described in the rest of this article.

Solubilising inorganic phosphate

Inorganic insoluble phosphate is solubilised by microbial acids. When phosphate-containing rocks like apatite are broken down by weathering, the resulting smaller rock particles enter the soil. Micro-organisms secrete organic acids – usually gluconic acid but occasionally lactic, citric, oxalic, or other acids – to solubilise these rock particles. Acids work on inorganic phosphate in two ways. Firstly, they dissolve weathered rock pieces due to their low pH. Secondly, negatively charged acid anions (lactate, citrate, etc) displace the phosphate captured by aluminium, iron, magnesium, and calcium minerals in the rock. Organic acids are just some of the chemicals secreted by microbes to solubilise inorganic phosphate.

Solubilising organic phosphorus

On the other hand, microbial enzymes solubilise organic phosphorus during the decomposition of organic matter. The two types of phosphate-solubilising enzymes are phosphatases, which solubilise 90% of organic phosphorus, and phytases, which solubilise the remaining 10%. Both types of enzyme break the ester bonds linking a PO₄^3- group to the rest of a biological molecule. By expressing genes encoding phytases and phosphatases, soil micro-organisms make phosphorus available for plants.

Arbuscular mycorrhizae (AM)

AM fungi provide plants with phosphorus in a symbiotic relationship. These fungi consist of hyphae, which are long, thin strands of cells that extend a plant’s root network and access phosphorus where roots cannot (Figure 2). AM fungi have a three-pronged approach to improving a plant’s phosphorus uptake: firstly, they absorb phosphate from the soil and give it to the plant in exchange for carbon. Secondly, they solubilise phosphate by secreting acids and phosphatases. Finally, AM fungi recruit phosphate-solubilising bacteria to the root system by feeding them sugars and amino acids.

Conclusion

Phosphate-solubilising bacteria and fungi provide plants with phosphorus, an essential element for making nucleic acids and ATP. Most phosphate is inaccessible to plants, locked up in rocks and biomass. By secreting organic acids and enzymes, soil micro-organisms convert this inaccessible phosphate into a form that plant roots can absorb and incorporate into their own biomass. When that plant dies, the organic phosphorus is solubilised again for another plant to use, so phosphorus never runs out. Therefore, phosphate-solubilising microbes are a small part of the invisible world that keeps our planet green. 

Written by Simran Patel

REFERENCES

Silva LI da, Pereira MC, Carvalho AMX de, et al. Phosphorus-Solubilizing Microorganisms: A Key to Sustainable Agriculture. Agriculture 2023; 13: 462.

Pang F, Li Q, Solanki MK, et al. Soil Phosphorus Transformation and Plant Uptake Driven by Phosphate-solubilizing Microorganisms. Front Microbiol; 15. Epub ahead of print 27 March 2024. DOI: 10.3389/fmicb.2024.1383813.

Schipanski ME, Bennett EM. Chapter 9 - The Phosphorus Cycle. In: Weathers KC, Strayer DL, Likens GE (eds) Fundamentals of Ecosystem Science (Second Edition). Academic Press, pp. 189–213.

Tian J, Ge F, Zhang D, et al. Roles of Phosphate Solubilizing Microorganisms from Managing Soil Phosphorus Deficiency to Mediating Biogeochemical P Cycle. Biology 2021; 10: 158.