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Finally, P23 at 0.15 mg/mL significantly promoted the growth of L. minor even in real secondary sewage effluent (Figure 8, Table 4). After 7 days' culture, the frond number and biomass production were increased about 1.7-fold by P23 inoculation relative to the control. These results demonstrate that P23 can enhance duckweed biomass production even in real-world secondary sewage effluent.

Table 4

Effects of P23 on the growth of L. minor in secondary effluent

Treatment	Increase in number of fronds per flask during 7 days^a (and ratio of P23 to control)	Biomass production (increase in dry weight; mg) of plants per flask during 7 days^a (and ratio of P23 to control)
P23	266 ± 15 (1.7)*	32.6 ± 2.1 (1.7)*
Control	159 ± 15	19.0 ± 1.3

Treatment	Increase in number of fronds per flask during 7 days^a (and ratio of P23 to control)	Biomass production (increase in dry weight; mg) of plants per flask during 7 days^a (and ratio of P23 to control)
P23	266 ± 15 (1.7)*	32.6 ± 2.1 (1.7)*
Control	159 ± 15	19.0 ± 1.3

Values are means ± SD. Values in parentheses are the ratio of the P23 value to the control (no P23) value.

^aFinal value minus initial value.

*Significant difference (P < 0.05) between values with and without P23 within a treatment.

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Figure 8

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Effects of P23 (0.15 mg dry weight/mL) on the growth of L. minor in secondary effluent for 7 days. (a) Changes in the number of fronds growing without (●) or with P23 cells (○). (b) Biomass production (final minus initial dry weight) of L. minor during 7 days. Values are mean ± SD (n = 3). *Significant difference (P < 0.05) between treatments.

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