Straw biochar modulates manganese/cadmium enrichment in paddy algae under simulated warming: Combined experimental...

Periphytic algae play essential roles in retaining heavy metals at the paddy soil-water interface. Climate warming imposes growing stress on paddy ecosystems, and straw biochar is widely used as an agricultural soil amendment. This study employed a combined experimental and modeling approach to investigate how straw biochar modulated Mn(II) and Cd(II) enrichment by the cyanobacterium Nostoc sp. under simulated warming (25-37 °C). Two-way ANOVA confirmed temperature, biochar dosage and their interaction significantly affected metal adsorption (p < 0.001), with biochar switching from mild metal stress alleviation at 25 °C to a synergistic stress amplifier under warming ≥ 34 °C. This shift caused algal biomass to drop by over 60% and metal enrichment capacity to decrease by 44.2-48.4%. Physicochemical characterization indicated that combined stress induced excessive extracellular polymeric substances (EPS) secretion, altered EPS composition, and impaired Mn(II) bio-oxidation. Under warming, straw biochar further caused oxidative damage and redirected algal gene expression from metal metabolism toward cellular defense and matrix production. Mn and Cd exhibited divergent adsorption kinetics, reflecting distinct uptake mechanisms. A random forest model (R2 = 0.77) identified temperature and biochar dose as key negative predictors of metal enrichment. These findings highlight that biochar's effects on algal metal retention depend strongly on temperature, guiding rational biochar application to reduce heavy metal risks in warming paddy soils.