Impacts of Identified Bacterium Ensifer adhaerens on Microcystis aeruginosa and Subsequent Microcystin Release

The release of microcystin and dissolved organic matter (DOM) during algae-lysing process draws much attention. In this study, Ensifer adhaerens (bacterium r23) was screened for algicidal activity against Microcystis aeruginosa (MA). The effects of dosage (2.04 × 10⁶, 3.90 × 10⁶, and 7.15 × 10⁶ CFU ml⁻¹) and dosing modes (single, double, and quadruple treatments totaling 7.15 × 10⁶ CFU ml⁻¹) on the release of microcystin-LR (MC-LR) and DOM were investigated. Besides cell density, intra/extra-cellular, and total MC-LR were measured during the lysing treatment. The DOM components were analyzed by parallel factor analysis (PARAFAC). Results show that the lowest dosage (2.04 × 10⁶ CFU ml⁻¹) not only stimulated MA growth during the initial 4 days but also triggered the production of more toxins, resulting in higher total MC-LR than the controls on 2–16 days. The higher dosages suppressed MA growth and MC-LR production simultaneously, and the total MC-LR content was substantially lower than those of the controls during the whole experimental period. The total MC-LR for the double and quadruple treatments were lower than the controls after 30-day treatment, but still higher than the single dosage. Both intracellular and extracellular DOM (IDOM, EDOM) of MA had five components, namely tyrosine-like (C1), tryptophan-like (C2 and C3), fulvic acid-like (C4), and humic acid-like (C5) substances. C1 and C3 in the EDOM resulted from MA lysis and increased along with the treatment. C2 in the EDOM came from the nutrient broth and could be consumed by r23. C4 and C5 in the EDOM mainly resulted from the growth of r23.