Effects Of Pesticides And Chloride Fertilizers On The Growth And Photosynthesis Of The Edible Cyanobacterium Ge-Xian-Mi

Ge-Xian-Mi (Nostoc) is an edible nitrogen-fixing cyanobacterium. It occupies a prestigious position in Chinese social behavior and has been used as a food delicacy or ingredient of Chinese medicine since the Eastern Jin Dynasty. In recent years, the decline of Ge-Xian-Mi resource and the research advancement of its large-scale cultivation have raised concerns from many scholars. In the present paper, its taxonomy, geographical distribution and habitats, development, ecophysiology, nutritive components and resource utilization were reviewed, and the effects of pesticides and chloride fertilizers on the growth and photosynthesis of Ge-Xian-Mi were studied.In order to gain insight into the relationship between the reduction of Ge-Xian-Mi productivity and the abusive use of pesticides in the field, three representative pesticides, such as butachlor, bensulfuron-methyl and dimethoate were examined for their effects on the growth, photosynthesis and photoinhibition of Ge-Xian-Mi. Severe inhibition of growth was found in the presence of 4-6 fold field concentration of butachlor and very high concentrations of bensulfuron-methyl (> 200μM) and dimethoate (> 3000μM). Slight stimulation of photosynthesis was observed over a limited range of low concentrations of these three pesticides. We found that PSII and PSI were respectively the inhibitory sites of 150μM butachlor and 150μM bensulfuron-methyl. However, the inhibitory site of 2000μM dimethoate seems to be situated at the terminal of photosynthetic electron transport chain or dark reaction. The colonies exposed to 150μM butachlor were more sensitive to high light than the control and those exposed to bensulfuron-methyl, dimethoate or low butachlor concentration. Dim light-induced rapid recovery of photoinhibited colonies was observed on the control, or 10μM butachlor, bensulfuron-methyl, dimethoate-treated cells. However, the maximal PSII photochemical efficiency of photoinhibited colonies treated with 150μM butachlor was maintained at a relatively stable value in low light. Our findings suggested that the abusive utilization of butachlor might be an important limiting factor for the productivity of Ge-Xian-Mi in the field.Effects of NaCl, KCl and NH₄Cl on the growth and photosynthesis of Ge-Xian-Mi were also examined in order to probe into whether they are responsible for the decline of this resource. Unlike 500 mM NaCl or KCl resulted from short-term dryness or salt stress, no recovery of F_v/F_m was observed in Ge-Xian-Mi in fresh BG₁₁ medium within 72 hours following the 24 h exposure to 500 mM NH₄Cl. Severe inhibition of growth and F_v/F_m were observed in the presence of field concentration of NH₄Cl (≥1 mM) and very high concentrations of NaCl and KCl within the 10 d exposure. When treated with 1 mM NH₄Cl, the inorganic carbon-saturated photosynthetic rate (V_max) and K_0.5(DIC) of Ge-Xian-Mi were respectively reduced to 42% and 30% of the control on the 4th day; its light-saturated photosynthetic rate (P_m) was severely suppressed, but dark respiratory rate was significantly increased by 213% on the 5th day. Therefore, our findings suggested that the utilization of ammonium chloride in the rice field might result in the reduction of Ge-Xian-Mi resource.