Characterization of Microalgae-Derived Alginate-like Expolymers (ALEs) and Their Crop-Promoting Effects and Resourcefulness

This study selected the typical eutrophication associated algae species Chlorella pyrenoidosa and Microcystis aeruginosa, from which alginate-like expolymers (ALEs) were extracted. Their composition, structural characteristics, and potential as biofertilizers were systematically analyzed. Results indicate that both C. pyrenoidosa-ALE (Cp-ALE) and M. aeruginosa-ALE (Ma-ALE) primarily comprise proteins and polysaccharides as functional components. Cp-ALE exhibited higher extraction yields (35.34 ± 4.32 mg·g−1 VSS, volatile suspended solids) and richer growth-promoting constituents such as tryptophan, while Ma-ALE demonstrated higher aromaticity in its structure. Pot experiments further demonstrated that both ALEs exhibited a “low-concentration promotion, high-concentration inhibition” effect on ryegrass growth: at the optimal concentration (1:10,000), Cp-ALE and Ma-ALE increased ryegrass dry weight by 61.2% and 59.8%, respectively, with no significant difference compared to the algal whole-cell fertilizer (CF). This study has established a simple, environmentally friendly pathway for resource utilization of microalgal waste. Extracting ALEs effectively preserves plant-promoting components within microalgae, providing not only a sustainable solution for high-value utilization of eutrophication associated algae, but also a viable pathway for green agriculture and circular economic development.