Terrestrial cross-boundary inputs of leaf litter serve as a critical foundation for secondary productivity in freshwater ecosystems. The regulatory mechanisms of tannins in leaf litter on degradation rates under submerged conditions remain unclear. This study employed leaf litter from low-tannin plants Osmanthus fragrans (A) and Canna glauca (B) as decomposition substrates, with the high-tannin species Myriophyllum verticillatum (C) incorporated to adjust tannin levels. A 140-day hydroponic degradation experiment was conducted under controlled temperature and dark conditions, which included four mixed litter treatments with a gradient of tannin additions (AB as the control, 0 g; ABC1: 0.5 g; ABC2: 2.5 g; ABC3: 4.5 g) along with two single-species treatments (A and B). The following results were found: (1) Low tannin levels (ABC1) promoted degradation rates of A and B (increased by 1.33–12.70%), whereas high tannin (ABC3) inhibited decomposition (decreased by 6.21–6.82%). (2) Tannin–protein complexes reduce nitrogen bioavailability and inhibit nitrification, thereby disrupting the nitrogen cycle in aquatic systems. In ABC3, total nitrogen content in A and B litter increased by 17.69–26.46% compared to AB, with concurrent 59.29% elevation in water NH₄⁺-N concentration. (3) High tannin induced dominance of oligotrophic stress-resistant bacterial communities (e.g., Treponema) through nutrient limitation and toxicity stress; however, their low metabolic efficiency reduced overall decomposition efficiency. Research reveals that the ecological benefits of plant secondary metabolites outweigh their nutritional quality attributes.

Heavy metal wastewater often contains multiple metal ions, and competition among them reduces the adsorption efficiency of microalgae. Enhancing this efficiency is crucial for improving heavy metal removal. This study optimized lysine addition to facilitating the formation of a ternary complex between microalgae, lysine, and heavy metals, thereby enhancing adsorption in both single- and mixed-metal systems. In a single-metal system at 64 mg/L, lysine improved the removal rates of copper, zinc, and cadmium by 13.96%, 41.21%, and 33.26%, respectively. In binary systems (Cu-Zn, Cu-Cd, and Cu-Pb) at 32 mg/L, lysine increased copper adsorption by 11.81%, 15.71%, and 25.25%, while improving zinc, cadmium, and lead adsorption by 15.41%, 12.51%, and 3.93%, respectively. Competitive adsorption analysis revealed that lead most strongly inhibited copper adsorption, while copper significantly suppressed zinc adsorption. Mechanistic investigations using 3D-EEM, FTIR, and XPS demonstrated that humic substances in the extracellular polymeric substances (EPSs) of microalgae play a key role in lysine binding. This interaction increases the number of carboxyl functional groups on the cell surface, thereby enhancing the microalgae’s capacity to adsorb heavy metals.