Calcium‒phosphorus coprecipitation induced by <i>Hydrilla verticillata</i> photosynthesis: response to elevated phosphorus concentration and regulation by light intensity, inorganic carbon and calcium levels

To evaluate whether photosynthesis-induced calcium‒phosphorus (Ca‒P) coprecipitation in Hydrilla verticillata can be harnessed for eutrophic freshwater phytoremediation, we conducted controlled light incubator experiments to determine how key environmental factors regulate this process. We manipulated four drivers—light intensity (66 vs. 110 μmol m⁻² s⁻¹), inorganic carbon (HCO₃−: 0 vs. 50 mg L⁻¹ = 0.82 mM), calcium (CaCl₂: 0 vs. 50 mg L⁻¹) and phosphorus (K₂HPO₄: 0 vs. 1 mg L⁻¹)—and quantified the responses in dry weight total phosphorus (DWTP), total ash phosphorus (TAP) and its fractions: water-soluble H₂O-P, organic NaOH-P and calcium-bound HCl-P. The key results revealed that a higher light intensity (110 vs. 66 μmol m⁻²​​​​​​ s⁻¹) exerted a stronger influence on H₂O-P and HCl-P fractions (by percentage) than on NaOH-P or DWTP in the light‒phosphorus interaction experiments. This suggests that enhanced photosynthesis under high light boosted P adsorption and promoted coprecipitation with supersaturated CaCO₃. For the inorganic carbon × phosphorus interactions, TAP and H₂O-P peaked under high inorganic carbon and high P conditions. Inorganic carbon contributed more to H₂O-P (45.13%) than did P (35.33%), indicating that inorganic carbon-enhanced photosynthesis primarily drove P adsorption. In the calcium‒phosphorus experiments, the maximum HCl-P and TAP occurred under high Ca (50 mg L⁻¹) and high P conditions. Calcium contributed more to HCl-P than did P, identifying Ca as the primary driver of Ca-P coprecipitation in H. verticillata. In summary, photosynthesis-induced Ca-P coprecipitation in H. verticillata is a synergistic, multifactor process—highlighting Ca as a key regulator of remediation efficiency. Ecologically, H. verticillata is particularly effective at reducing P bioavailability via Ca-P coprecipitation in calcium-replete waters, while in calcium-limited systems, P removal relies more on adsorptive/absorptive pathways. These findings highlight the need to integrate nutrient availability and plant physiological processes for optimizing phytoremediation of eutrophic freshwater.