Table 1 shows the pretreatment results of the eight experiments.
When the FNA concentration reached 1.09 mg HNO2–N/L
(Exp. 3, 4, and 6), the colour of the algal liquor turned to yellowish-
green from dark green (natural colour). With a further increase
in FNA concentration to 26.98 mg HNO2–N/L (Exp. 7 and 8), the
colour turned to brown, indicating denaturation of chlorophyll pigments
in the algal cells. This result is consistent with the microscope
observations (Fig. S1). Under the fluorescent microscope,
chlorophyll fluoresces red. The cells retained autofluorescence in
Exp. 6 but very little red autofluorescence was observed in Exp. 7
and Exp. 8 (Fig. S1g). Severe treatment conditions in Exp. 7 and
Exp. 8 even caused some cells to disintegrate into fragments
(Fig. S1f, yellow circle).