Sterilized flasks were filled with 200 mL of new BG-11 medium and 600 mL of phytoplankton assemblages, then the concentrated N. minor exudates were added to make the concentration 0, 0.8%
1.0% and 1.2% mL/L (v/v) respectively. All experiments were con- ducted in triplicate, and the cultural conditions were the same as those mentioned earlier. During the experiments, a certain amount of microalgae suspension was collected from each flask every other day to measure the contents of chlorophyll a using the colorimet- ric method [16] and TTC reduction, respectively. TTC, a water- soluble compound, can be changed into triphenyl formazan (TPF, a red-colored compound) by reduction through the catalysis of de- hydrogenase [17]. Three milliliter of 0.6% TTC was added into 10 mL of microalgae suspension with 10 mL of 0.5 M Na2HPO4–KH2PO4 (pH 7.4) buffer solution, extracted by ethyl acetate, followed by OD485 measurement. A standard curve was made, and the TPF of treat- ment groups were calculated based on the curve [6,18].