Bacillus megaterium in phosphorus solubilization

Bacillus megaterium is a well‑documented phosphate‑solubilizing bacterium (PSB) that mobilizes inorganic and organic P and can also enhance availability of several micronutrients under a wide range of soil conditions.

Bacillus megaterium_petri_colony_solubilizing phosphorous_XL

Mechanisms of phosphorus solubilization

• Organic acid production: B. megaterium secretes low‑molecular‑weight organic acids (e.g. gluconic, citric, lactic acids), which chelate cations (Ca, Fe, Al) and lower pH, converting insoluble phosphates such as tricalcium phosphate, rock phosphate and bone‑derived phosphates into H2_22PO4−_4^-4− and HPO42−_4^{2-}42− forms.

• Acidification of the microenvironment: During solubilization, culture pH commonly drops from around 7.0 to 4.5–5.0, and the extent of P or Zn release is closely correlated with this pH decrease.

• Enzymatic mineralization: Many strains produce phosphatases and phytases that hydrolyze organic P esters and phytate, contributing to mineralization of organic phosphorus pools.

• Chelation and complexation: Production of siderophores and other chelators mobilizes P bound to Fe and Al oxides and can simultaneously mobilize micronutrient cations.

• Strain‑specific traits: Transcriptomic work with strain P68 in potato shows that inoculation modulates plant P‑uptake pathways and root development genes, indicating both direct (solubilization) and indirect (plant signaling, root architecture) effects.

Environmental conditions for activity

• Temperature: P‑solubilizing B. megaterium isolates generally grow and remain active from about 30 °C up to at least 40–44 °C, with good solubilization performance reported within this range.medcraveonline+2

• pH:

  • P availability in soil is usually optimal at pH 6–7.5, but P‑solubilizing B. megaterium strains can grow and function up to pH 9, suggesting suitability even in moderately alkaline soils.

  • In vitro assays show strong solubilization at initial pH around 7 followed by self‑induced acidification to ~4.5–5.5.

• Salinity: Isolates from non‑saline soils tolerated up to 6% NaCl in minimal media, but P solubilization efficiency declined as NaCl increased from 4 to 6%, indicating moderate halotolerance but some sensitivity at higher salinity.

• Carbon source and P source: Availability of utilizable carbon (e.g. glucose) enhances organic‑acid production, while solubilization efficiency varies with the P source (tricalcium phosphate, rock phosphate, bone meal, ash phosphates), usually higher for more reactive materials like poultry/fish bone phosphates than for some rock phosphates.

Quantitative efficiency for P and micronutrients

Phosphorus solubilization

• In vitro, a cow‑dung‑derived B. megaterium isolate (CDK25) solubilized up to about 281.6 μg P ml−1^{-1}−1 from insoluble phosphate after 8 days, with a clear increase of soluble P over time.

• Another soil isolate, B. megaterium mj1212, showed marked solubilization of tricalcium phosphate in liquid cultures and significantly increased P availability and growth in mustard, although absolute μg ml−1^{-1}−1 values vary with medium and assay.

• Column and batch assays using poultry and fish bones or phosphorite reported significantly higher P release (expressed as P2_22O5_55) in B. megaterium‑treated systems than in uninoculated controls, with P release increasing with the dose of P‑bearing material.

• Parameter‑optimization studies with B. megaterium as a PSB inoculant show that inoculation can increase plant‑available soil P and plant P uptake in pot and field trials, typically in the range of 10–30% over non‑inoculated controls, depending on soil and crop.

Micronutrient solubilization (Zn, Fe, others)

• Zinc:

  • B. megaterium isolates can solubilize insoluble Zn sources (ZnO, ZnCO3_33) with solubilization indices above 4.0 and zones up to 5.0 cm in plate assays.

  • Quantitatively, the best strain reached ≈20.3 ppm soluble Zn from ZnO and ≈12.0 ppm from ZnCO3_33 after 15 days, again associated with medium acidification to pH 4.5–5.0.

• Iron and other metals:

  • Siderophore production by B. megaterium contributes to Fe mobilization from sparingly soluble Fe compounds and also supports Zn solubilization via metal chelation.

  • Co‑inoculation studies with zinc‑solubilizing Bacillus strains (including B. megaterium) in crops report increases in plant N, P, K, Fe and Zn contents up to roughly 26–30% for NPK and around 19–27% for Fe and Zn relative to controls.

Compatibility with soil conditions

• Soil pH and texture:

  • Ability to function up to pH 9 suggests good performance in neutral to moderately alkaline mineral soils, including calcareous conditions where Ca‑phosphates predominate.

  • In situ work with columns packed with different P sources (bones, phosphorite, ash) shows that B. megaterium remains effective across varying substrates, indicating adaptability to diverse soil textures and P mineralogy.

• Salinity and temperature stress:

  • Growth and P solubilization under 4–6% NaCl suggest potential for moderately saline soils, though efficiency decreases as salinity rises.

  • Tolerance of temperatures up to about 44 °C supports use in warm climates and in surface soils exposed to high temperatures.

• Moisture and aeration: Most reports are from well‑aerated, moist conditions; as an aerobic spore‑forming bacterium, B. megaterium can survive transient drought via spore formation and resume activity when moisture returns.

Compatibility with other microorganisms

• Co‑inoculation with other PGPR:

  • Mixed inocula with other Bacillus strains (e.g. Zn‑solubilizing Bacillus spp.) improve plant growth and nutrient uptake more than single strains, indicating functional compatibility and complementary mechanisms (P + Zn solubilization, phytohormone production, etc.).

  • Studies developing PSB consortia often include B. megaterium together with other genera (e.g. Pseudomonas, Azotobacter), with positive or additive effects on P solubilization and plant performance.

• Rhizosphere interactions:

  • Transcriptome analysis in potato shows that B. megaterium P68 influences plant genes related to root development and P transport, suggesting beneficial plant–microbe signaling without evidence of antagonism toward native beneficial microbiota.

  • As a Bacillus, it can produce antibiotics and lipopeptides, but in the context of PSB formulations it is typically reported to be compatible with common rhizosphere bacteria and mycorrhizal fungi used in biofertilizer programs.

Conclusions

In synthesis, B. megaterium acts primarily via organic‑acid‑mediated solubilization and enzymatic mineralization, operates effectively over a relatively broad range of pH, temperature and moderate salinity, quantitatively mobilizes both P and key micronutrients like Zn and Fe, and is generally compatible with diverse soils and co‑inoculated beneficial microorganisms, making it a robust candidate for P‑solubilizing biofertilizer formulations.