Differential sensitivity of three cyanobacterial and five green algal species to organotins and pyrethroids pesticides

Differential sensitivity of three cyanobacterial and five green algal species to organotins and pyrethroids pesticides

Science of the Total Environment 341 (2005) 109 – 117 www.elsevier.com/locate/scitotenv Differential sensitivity of three cyanobacterial and five gre...

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Science of the Total Environment 341 (2005) 109 – 117 www.elsevier.com/locate/scitotenv

Differential sensitivity of three cyanobacterial and five green algal species to organotins and pyrethroids pesticides Jianyi Ma* Department of Plant Protection, Henan Institute Science and Technology, Xinxiang, 453003, People’s Republic of China School of Life Sciences, Zhejiang Forestry College, Lin-An 311300, People’s Republic of China Received 15 January 2004; received in revised form 10 June 2004; accepted 9 September 2004

Abstract In this work, five organotins and pyrethroids pesticides were tested to examine their effects on the three cyanobacteria Anabaena flos-aquae, Microcystis flos-aquae, Mirocystis aeruginosa and on the five green algae Selenastrum capricornutun, Scenedesmus quadricauda, Scenedesmus obliqnus, Chlorella vulgaris, Chlorella pyrenoidosa through 96 h acute toxicity tests. The results indicated that: (1) the decreasing order of the average acute toxicity to cyanobacteria and green algae of five dissimilar organotins and pyrethroids pesticides was: fentin hydroxideNcyhexatinNazocyclotinNfenbutatin oxideNbetacyfluthrin. (2) Wide variations occurred in response to the tested pesticides among the eight individual species of cyanobacteria and green algae. The sensitivity of various species of algae exposed to fenbutatin-oxide varied over one order of magnitude, exposed to cyhexatin/fentin-hydroxide/beta-cyfluthrin varied over two orders of magnitude and exposed to azocyclotin varied three orders of magnitude. (3) In contrast with the sensitivity of cyanobacteria and green algae, cyanobacteria were much less sensitive to beta-cyfluthrin than green algae. The pollutants may result in a shift of green algal and cyanobacterial group structure, especially in a shift from dominance by green algae to dominance by cyanobacteria, and may sustain cyanobcterial blooms during the special period. Thus, the decreasing order of the aquatic ecological risk was: beta-cyfluthrinNfentin hydroxideNcyhexatinNazocyclotinNfenbutatin oxide. There was a strong variance between toxicity and ecological risk, i.e. blow toxicityQ does not automatically imply blow ecological riskQ. The toxicity of pyrethroids pesticides was lower than that of organotins pesticides, whereas the aquatic ecological risk of pyrethroids pesticides was higher than that of organotins pesticides. D 2004 Elsevier B.V. All rights reserved. Keywords: Organotins pesticides; Pyrethroids pesticides; Toxicity; Sensitivity; Cyanobacteria; Green algae

1. Introduction

* Tel.: +86 571 63695853; fax: +86 571 85826279. E-mail address: [email protected] 0048-9697/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.09.028

The organotins are a group of acaricides that double as fungicides. Of particular interest is cyhexatin, one of the most selective acaricides known, introduced in 1967. Fenbutatin-oxide has been used

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extensively against mites on deciduous fruits, citrus, greenhouse crops, and ornamentals. The organotin pesticides inhibit oxidative phosphorylation at the site of dinitrophenol uncoupling, preventing the formation of the high-energy phosphate molecule adenosine triphosphate (ATP). These trialkyl tins also inhibit photophosphorylation in chloroplasts, the chlorophyll-bearing subcellular units, and could therefore serve as algicides. Over the past two decades many synthetic pyrethroids have become available. These are very stable in sunlight and are generally effective against most agricultural insect pests when used at the very low rates. Their modes of action are apparently worked by keeping open the sodium channels in neuronal membranes (Ware, 2001). However, its usage may enter freshwater ecologicals by spray drift, leaching, run-off, or accidental spills and present potential risks for aquatic flora (Van den Brink and Ter Braak, 1999; Wang and Freemark, 1995). Alterations of the species composition of an aquatic community as a result of toxic stress may affect the structure and the functioning of the whole ecological (Campanella et al., 2000; Wong, 2000; Verdisson et al., 2001). Algae and cyanobacteria (blue-green algae) are known to be comparatively sensitive to many chemicals (Real et al., 2003). Their ecological position at the base of most aquatic food webs and the essential roles in the nutrient and phosphorus cycling are critical to all ecologicals (Ka¨llqvist and Svenson, 2003; Sabater and Carrasco, 2001). Some information on the toxicological aspects of pesticides on green algae has been reported. However, little is known on the toxicological aspects of pesticides on cyanobacteria (Abou-Waly et al., 1991; Sabater and Carrasco, 2001). Cyanobacteria can produce algal toxins, which has important implications for humans and aquatic organisms (An and Kampbell, 2003). Tests on a certain species of algae are of limited applicability in assessing the effects of environmental contaminants on algal communities, which are composed of an array of species with different sensitivities (Sanchez and Tarazona, 2002). A few works have been published about the comparative sensitivity of pesticides toward various green algae (Ma et al., 2004a,b). Yet, there were few reports concerning the differential response of various cyanobacteria and green algae. In the present work, five organophosphates and pyrethroids pesticides were tested to

examine their effects on three cyanobacteria Anabaena flos-aquae, Microcystis flos-aquae, Mirocystis aeruginosa and five green algae Selenastrum capricornutun, Scenedesmus quadricauda, Scenedesmus obliqnus, Chlorella vulgaris, Chlorella pyrenoidosa, and the differential sensitivity of cyanobacteria and green algae are compared.

2. Materials and methods 2.1. Chemicals All of tested pesticides were purchased from People’s Republic of China. Their CAS, REG. NO. and their respective formulations were shown in Table 1. The tested pesticides were dissolved by 99.5% acetone. The concentration of the acetone in the medium was kept minimizing and was less than 0.05%. The US Environmental Protection Agency recommends the allowable maximal limits of 0.05% solvent for acute tests and 0.01% for chronic tests, this level was not significant with regard to toxicity (Jay, 1996). 2.2. Test organisms The toxicity tests were carried out with the freshwater cyanobacteria A. flos-aquae, M. flosaquae, M. aeruginosa, and green algae S. capricornutun, S. quadricauda, S. obliqnus, C. vulgaris, C. pyrenoidosa, which obtained from the institute of hydrobiology, the Chinese academy of science. 2.3. Nutrient media The medium for cyanobacteria and green alga growth inhibiting test were HGZ and HB-4 medium, respectively. The culture medium was sterilized at 121 8C, 1.05 kg cm 2 for 30 min (Kong et al., 1999), which was described in detail in the works of Ma et al. (2003a,b). 2.4. Test methods Cyanobacterial or green algal cells were propagated photoautotrophically in a 250 mL Erlenmeyer flask containing 100 mL liquid HGZ or HB-4 medium and

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Table 1 Selected insecticides, their CAS name, REG. NO. and formulation Pesticides

CAS

REG. NO.

Formulationa

Azocyclotin Cyhexatin Fenbutatin-oxide Fentin hydroxide Beta-cyfluthrin

1-(tricyclohexylstannyl)-1H-1,2,4-triazole tricyclohexylhydroxystannane hexakis(2-methyl-2-phenylpropyl)distannoxane triphenylstannylium cyano(4-fluoro-3-phenoxyphenyl)methyl 3-(2,2dichloroethenyl)-2,2-dimethylcyclopropanecarboxylate

41083-11-8 13121-70-5 13356-08-6 76-87-9 68359-37-5

95%TC 93%TC 91%TC 96%TC 90%TC

a

TC stands for technical concentration.

kept on a rotator shaker (100 rpm) at 24 8C, illuminated with cool-white fluorescent lights at a continuous light intensity of 450 Amol m 2 s 1, respectively (Verdisson et al., 2001; Ma, 2002). HGZ (20 mL) or HB-4 medium containing cyanobacterial or green algal cells (initial concentration OD680nm=0.008) were distributed to sterile 50 mL Erlenmeyer flasks, respectively. A wide range of concentrations was examined in a previous test in order to find the adequate range of toxicity for each pesticide. Then, adequate concentrations grads were tested according to the results of the previous test (Moreno-Garrido et al., 2000). The medium were then treated with various pesticidal concentrations, and incubated for 96 h on an orbital shaker (100 rpm) at a temperature of 24 8C and a continuous light intensity of 450 Amol m 2 s 1 (Yan and Pan, 2002; Ma et al., 2001). Biomass was correlated with absorbance over time for 96 h on a Shimadzu UV-2401PC spectrophotometer. The most suitable wavelength for monitoring culture growth was 680 nm. Good linear relationships between dry weight concentration (DWC) or chlorophyll-a (ChlaA) content of algal cultures and OD680nm are highly correlated, thus, growth of cyanobacterial and green algal biomass were calculated indirectly using spectrophotometric data. Three replicates were made for each pesticidal concentrations and control. Appropriate control systems containing no pesticide were included in each experiment. Control and treated cultures grew under the same conditions. In each experiment, percent inhibition values, relative to growth in control systems, were calculated using spectrophotometric data (Ma and Liang, 2001). 2.5. Statistical evaluation EC50 values were calculated using linear regression analysis of transformed pesticidal concentration as natural logarithm data versus percent inhibition

(Ma and Liang, 2001). No observed effect concentration (NOEC) is the test concentration immediately below the lowest significant concentration (Saker and Neilan, 2001). A significant concentration is interpreted to mean a concentration exhibiting a statistically significant reduction in fecundity (at pb0.05) when compared with the control. Weighted analysis of variance (ANOVA) was used, followed by a one-sided Dunnett’s test using a 5% significance level to obtain lowest observed effect concentration (LOEC). NOEC was taken to be the test concentration immediately below LOEC (Saker and Neilan, 2001). Chronic value (CV) was the geometric mean of the NOEC and LOEC. Statistical analysis of the data was by linear regression analysis using SPSS version 11.0.

3. Results 3.1. Acute toxicity of five pesticides to the cyanobacteria and green algae Acute toxicity of five pesticides to cyanobacteria A. flos-aquae, M. flos-aquae, M. aeruginosa, and green algae S. capricornutun, S. quadricauda, S. obliqnus, C. vulgaris, C. pyrenoidosa were shown in Table 2. The 96 h EC50, LOEC, NOEC values of azocyclotin to cyanobacteria and green algae varied around 0.01–0.05 and 0.06–15.6 mg L 1, 0.001–0.05 and 0.02–0.5 mg L 1, 0.005–0.02 and 0.001–2 mg L 1, respectively. The 96 h EC50, LOEC, NOEC of cyhexatin to cyanobacteria and green algae varied around 0.01–0.04 and 0.03–0.14 mg L 1, 0.0005– 0.05 and 0.005–0.05 mg L 1, 0.0002–0.02 and 0.002–0.02 mg L 1, respectively. The average toxicity of cyhexatin to cyanobacteria and green algae was higher than that of azocyclotin. The 96 h EC50,

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Table 2 The effect of various pesticides to cyanobacteria and green algae Pesticides

Regression equationa

Coefficient correlation

Significance level

EC50 (mg L 1)

LOEC (mg L 1)

NOEC (mg L 1)

Azocy-clotin

(1) (2) (3) (4) (5) (6) (7) (8) (1) (2) (3) (4) (5) (6) (7) (8) (1) (2) (3) (4) (5) (6) (7) (8) (1) (2) (3) (4) (5) (6) (7) (8) (1) (2) (3) (4) (5) (6) (7) (8)

0.9510 0.9670 0.9859 0.9946 0.9924 0.9831 0.8967 0.9381 0.9427 0.9519 0.9813 0.9871 0.9894 0.9619 0.9612 0.9576 0.9891 0.9955 0.9592 0.9840 0.9418 0.9691 0.9284 0.9829 0.9432 0.9428 0.9737 0.9792 0.9764 0.9954 0.9920 0.9747 0.9980 0.9742 0.9716 0.9961 0.9637 0.9566 0.9818 0.9879

0.0490 0.0330 0.0140 0.0000 0.0000 0.0169 0.0030 0.0020 0.0163 0.0126 0.0180 0.0000 0.0000 0.0022 0.0000 0.0420 0.0109 0.0045 0.0100 0.0160 0.0000 0.0065 0.0010 0.0170 0.0568 0.0572 0.0000 0.0036 0.0000 0.0000 0.0000 0.0250 0.0020 0.0050 0.0010 0.0000 0.0005 0.0030 0.0000 0.0000

0.0485 0.0438 0.0180 0.0666 2.0014 0.2119 16.542 0.0608 0.0406 0.0388 0.0106 0.0676 0.0315 0.0875 0.1408 0.0577 0.1888 0.1684 0.1661 7.4337 0.5545 1.5092 3.0944 1.7380 0.0194 0.0187 0.0183 0.1323 0.0380 0.0841 0.0033 0.0360 62.3364 87.0454 34.6256 3.5152 2.3663 97.0396 4.4095 885.2353

0.05 0.05 0.001 0.01 0.5 0.2 5 0.002 0.05 0.05 0.0005 0.01 0.005 0.05 0.05 0.01 0.2 0.2 0.05 0.5 0.05 0.2 1 0.2 0.02 0.02 0.005 0.02 0.0002 0.01 0.0002 0.01 25 10 20 0.5 0.5 10 0.5 20

0.02 0.02 0.0005 0.005 0.2 0.1 2 0.001 0.02 0.02 0.0002 0.005 0.002 0.02 0.02 0.005 0.1 0.1 0.02 0.2 0.02 0.1 0.5 0.1 0.01 0.01 0.002 0.01 0.0001 0.005 0.0001 0.005 10 5 10 0.2 0.2 5 0.2 10

Cyhexatin

Fenbutatin oxide

Fentin hydroxide

Beta-cyfluthrin

Y=7.6998+0.4275X Y=7.9001+0.4367X Y=6.8253+0.3547X Y=3.4469+0.1783X Y=3.2175+0.2017X Y=4.3796+0.2525X Y=2.1052+0.1458X Y=3.7168+0.1936X Y=7.3773+0.4041X Y=7.3863+0.4035X Y=3.8969+0.1850X Y=3.6921+0.1934X Y=3.7472+0.1880X Y=3.2892+0.1716X Y=3.3412+0.1801X Y=8.5119+0.4807X Y=8.6526+0.5266X Y=8.7876+0.5314X Y=5.1068+0.2951X Y=3.6492+0.2667X Y=3.4502+0.2048X Y=2.9625+0.1837X Y=3.1818+0.2114X Y=6.7733+0.4730X Y=13.0289+0.7056X Y=12.3425+0.6654X Y=4.5609+0.2279X Y=3.7593+0.2058X Y=3.1773+0.1567X Y=3.4351+0.1802X Y=3.9757+0.1780X Y=7.4094+0.4031X Y=3.5695+0.3170X Y=3.8049+0.3535X Y=3.5936+0.3012X Y=2.7647+0.1803X Y=2.4093+0.1474X Y=1.8223+0.1431X Y=2.3177+0.1474X Y=1.2585+0.1079X

a Y and X stand for percent inhibition and natural logarithm of concentration, respectively. (1) A. flos-aquae (2) M. aeruginosa (3) M. flosaquae (4) S. capricornutun (5) S. quadricauda (6) S. obliqnus (7) C. vulgaris (8) C. pyrenoidosa.

LOEC, NOEC values of fentin hydroxide to cyanobacteria and green algae varied around 0.018–0.020 and 0.003–0.133 mg L 1, 0.005–0.02 and 0.0002– 0.02 mg L 1, 0.002–0.01 and 0.001–0.01 mg L 1, respectively. The average toxicity of fentin hydroxide was the highest among all of tested five pesticides. The 96 h EC50, LOEC, NOEC values of fenbutatin oxide to cyanobacteria and green algae varied around

0.16–0.19 and 0.5–0.75 mg L 1, 0.05–0.02 and 0.05– 1 mg L 1, 0.02–0.1 and 0.02–0.5 mg L 1, respectively. The average toxicity of fenbutatin oxide was the lower than that of fentin hydroxide. For betacyfluthrin, its 96 h EC50, LOEC, NOEC values varied around 34–88 and 2–856 mg L 1, 10–25 and 0.5– 20 mg L 1, 5–10 and 0.2–10 mg L 1, respectively, its average toxicity to cyanobacteria and green algae was

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the lowest among all of tested five pesticides. Thus, the decreasing order of the toxicity to cyanobacteria and green algae of five dissimilar pesticides was: fentin hydroxideNcyhexatinNazocyclotinNfenbutatin oxideNbeta-cyfluthrin. 3.2. The sensitivity of the eight algae to the five pesticides Wide variations were found occurring in response to the tested pesticides among individual species of eight algae. For azocyclotin, according to magnitude of EC50, the decreasing order of the sensitivity to cyanobacteria and green algae was: M. flos-aquaeNM. aeruginosa NA. flos-aquae NC. pyrenoidosa NS. capricornutunNS. obliqnusNS. quadricaudaNC. vulgaris, the sensitivity of various species of algaebetween M. flos-aquae and S. obliqnus, between S. quadricauda and A. flos-aquae/M. aeruginosa/M. flos-aquae/S. capricornutun/C. pyrenoidosa, when exposed to azocyclotin varied one order of magnitude; between M. flos-aquae and A. flos-aquae/C. vulgaris, between C. vulgaris and A. flos-aquae/M. aeruginosa/S. capricornutun/C. pyrenoidosa, varied two orders of magnitude. According to magnitude of CV, the decreasing order of the sensitivity was: M. flosaquaeNC. pyrenoidosaNS. capricornutunNM. aeruginosa NA. flos-aquae NS. obliqnus NS. quadricaudaNC. vulgaris, the sensitivity between S. quadricauda/C. pyrenoidosa and A. flos-aquae/M. aeruginosa, between M. los-aquae and S. capricornutun/A. los-aquae/M. eruginosa, when exposed to azocyclotin varied one order of magnitude; between

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M. los-aquae/C. pyrenoidosa and S. quadricauda/S. obliqnus/M. aeruginosa, between C. vulgaris and A. flos-aquae/M. aeruginosa, varied two orders of magnitude; between C. vulgaris and M. flos-aquae/ C. pyrenoidosa varied three orders of magnitude (see Fig. 1). The sensitivity of five green algae is much higher than that of three cyanobacteria. As for cyhexatin, in accordance with EC50, the decreasing order of sensitivity was: M. flos-aquaeNS. quadricauda NA. los-aquae /M. aeruginosa NC. pyrenoidosaNS. capricornutunNS. obliqnusNC. vulgaris, the sensitivity of various species of algae exposed to cyhexatin varied small. According to the CV, the decreasing order of sensitivity was: M. flos-aquaeNS. quadricaudaNS. capricornutun/C. pyrenoidosaNA. flos-aquae/M. aeruginosa/S. obliqnus/C. vulgaris, the sensitivity between M. flos-aquae/S. capricornutun/C. pyrenoidosa, between S. quadricauda and A. flos-aquae/M. aeruginosa/S. obliqnus/C. vulgaris, varied one order of magnitude; between M. flosaquae and A. flos-aquae/M. aeruginosa/S. obliqnus/ C. vulgaris, varied two orders of magnitude (see Fig. 2). For fenbutatin-oxide, in conformity to EC50, the decreasing order of sensitivity was: M. flos-aquaeNM. aeruginosa NA. flos-aquae NS. quadricauda NS. obliqnusNC. pyrenoidosaNC. vulgarisNS. capricornutun, the sensitivity of various species of algae— between M. aeruginosa/M. flos-aquae and S. capricornutun/C. vulgaris/C. pyrenoidosa, between A. flos-aquae and S. capricornutun/C. vulgaris, between S. quadricauda and S. capricornutun, varied one order of magnitude. According to CV, the decreasing

Fig. 1. Differential sensitivities of eight algae to azocyclotin.

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Fig. 2. Differential sensitivities of eight algae to cyhexatin.

order of the sensitivity was: M. flos-aquae/S. quadricaudaNA. flos-aquae/M. aeruginosa/S. obliqnus/C. pyrenoidosaNS. capricornutunNC. vulgaris (see Fig. 3). As to fentin-hydroxide, from EC50, the decreasing order of sensitivity was: C. vulgarisNM. flos-aquaeNM. aeruginosaNA. flos-aquaeNC. pyrenoidosaNS. quadricaudaNS. obliqnusNS. capricornutun, the sensitivity of various algal species—between C. vulgaris and S. capricornutun/S. quadricauda/S. obliqnus/C. pyrenoidosa, varied one order of magnitude. According to CV, the decreasing order of the sensitivity was: S. quadricauda/C. vulgarisNM. flos-aquaeNS. obliqnus/C. pyrenoidosaNA. flos-aquae/M. aeruginosa/S. capricornutun, the sensitivity between S. quadricauda/S. obliqnus and M. flos-aquae/S. obliqnus/C. pyrenoidosa, varied one order of magnitude; between S. quadricauda/S. obliqnus and A. flos-aquae/M. aeruginosa/S. capricornutun, varied two orders of magni-

tude (see Fig. 4). The sensitivity of five green algae is much lower than that of three cyanobacteria. With respect to beta-cyfluthrin, from EC50, the decreasing order of the sensitivity was: S. quadricaudaNS. capricornutunNC. vulgarisNM. flosaquaeNA. flos-aquae/M. aeruginosa/S. obliqnusNC. pyrenoidosa, the sensitivity of various species of algae—between S. capricornutun/S. quadricauda/C. vulgaris and A. flos-aquae/M. aeruginosa/M. flosaquae/S. obliqnus, varied one order of magnitude; between C. pyrenoidosa and S. capricornutun/S. quadricauda/C. vulgaris, varied two orders of magnitude. According to CV, the decreasing order of the sensitivity was: S. capricornutun/S. quadricauda/C. vulgarisNS. obliqnus/M. aeruginosaNC. pyrenoidosa/M. flos-aquaeNA. flos-aquae. The sensitivity between S. capricornutun/S. quadricauda/S. obliqnus and A. flos-aquae/M. aeruginosa/M. flosaquae/S. obliqnus/C. pyrenoidosa, varied one order

Fig. 3. Differential sensitivities of eight algae to fenbutatin-oxide.

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Fig. 4. Differential sensitivities of eight algae to fentin-hydroxide.

of magnitude (see Fig. 5). The sensitivity of green algae is much higher than that of cyanobacteria.

4. Discussion A few works, which was related to the toxicity of pollutants on algae, has mostly compared the various sensitivities of green algae. Our previous works has found that the sensitivity of various species of green algae (S. capricornutun, S. quadricauda, S. obliqnus, C. vulgaris and C. pyrenoidosa) exposed to ametryn, chlorimuron-ethyl, chlorotoluron, cinmethylin, mefenacet, paraquat, pendimethalin, pretilachlor and quinclorac, varied by over three orders of magnitude (Ma et al., 2002a,b, 2003a,b, 2004a,b). It may give rise to risk in the aquatic ecosystem owing to the shift of green algal community structure. However, little is known about the sensitivity of cyanobacteria and

green algae to pollutants, especially to the pesticides which were widely used in agriculture. Most research showed that the formation of algal bloom is attributed to the overabundance of green algal or cyanobacterial growth, and the gradual shift of their community structure in the aquatic ecosystem (Jorgensen, 1976). The shift usually indicates that from dominance by green algae to dominance by cyanobacteria, or a gradual shift within the cyanobacterial population from dominance by the species to dominance by another species. The reasons are physical (e.g. light factor and temperature factor), nutritional (e.g. nitrogen and phosphorus in water) and biological as well (e.g. food chain and food web) (Pei and Ma, 2002). However, there were few reports as to whether there exist other factors, such as pollutants, to which the green algae and cyanobacteria have greater differential sensitivity. This is possible. The pollutants may enter the aquatic ecological and result in a shift of

Fig. 5. Differential sensitivities of eight algae to beta-cyfluthrin.

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green algal and cyanobacterial group structure, especially in a shift from dominance by green algae to dominance by cyanobacteria. It may also be important for sustaining cyanobcterial blooms during the special period. Cyanobacteria can also produce a variety of toxins including hepatotoxins, e.g. microcystins, and endotoxins such as lipopolysaccharides (Best et al., 2002). Therefore, the research on comparing the differential sensitivity of cyanobacteria and green algae is of important scientific significance and realistic value. If green algae and cyanobacteria have greater differential sensitivity, especially when the sensitivity of cyanobacteria is strongly lower than that of green algae, the contamination may result in a shift of green algal and cyanobacterial group structure, i.e. from dominance by green algae to dominance by cyanobacteria, then sustain cyanobcterial blooms during the special period, thus, the contamination would present higher ecological risk. The result indicates that the decreasing order of the average toxicity to eight algae of five dissimilar pesticides was: fentin hydroxideNcyhexatinNazocyclotinNfenbutatin oxideNbeta-cyfluthrin. However, according to my assumption, the decreasing order of the ecological risk was: beta-cyfluthrinNfentin hydroxideNcyhexatinN azocyclotinNfenbutatin oxide. However, according to sensitivity magnitude of four organotins pesticides, the decreasing order of the ecological risk was the same taxis. The toxicity of pyrethroids pesticides was lower than that of organotins pesticides, but the aquatic ecological risk of pyrethroids pesticides was higher than that of organotins pesticides. There was a strong disaccord between toxicity and ecological risk, i.e. blow toxicityQ does not always imply blow ecological riskQ. However, the aquatic ecological system is complicated. And the supposition we proposed as to whether there exists a strong positive relativity between the effect on the respective cultivation of certain alga in laboratory and the effect on the actual mixed growth of multiple algae in field. It remains to be further studied and proved by more experimental data. Single-species toxicity test has historically been the source of biological data for risk evaluation. However, it has been discussed as to whether information from these tests alone is suitable to predict effects at the ecological level (Cairns et al., 1996; Sanchez and

Tarazona, 2002). Furthermore, multiple-species toxicity tests such as microcosm and mesocosm tests enable the observation of the indirect effects of chemicals caused by interactions among species. However, conducting mesocosm tests to assess the impact of chemicals on ecology involves skilled labor and high cost (Cairns et al., 1996; Naito et al., 2003). Therefore, I think that we should select a lot of special genus organisms, through single-species toxicity test respectively, instead of multiple-species toxicity tests such as microcosm and mesocosm tests, in risk evaluation.

Acknowledgments The project was supported by the Zhejiang Provincial Natural Science Foundation of China and the Educational Committee of Zhejiang Province, China.

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