Effects Of Heavy Metals And Herbicides On Induced Formation And Photosynthesis

As the mian pollutions in water environment, heavy metals and pesticides have been reported to impair the growth, physiological activity and enzyme activity of algae. But, there are no reports about the effect of these pollutions on inducible defenses of algae. Inducible defenses, which is the result of the interspecific interaction, play an important role in maintaining the stability of community and trophic structure. There are many reports for inducible defenses, many biotic and abiotic all influence the inducible defenses in Scenedesmus. Analysis about the strategy between pollutions and grazers in Scenedesmus is useful to the development of inducible defenses. So we explored the effects of heavy metals and herbicides on grazer-induced colony formation in Scenedesmus.The experimental materials were filtered water from Daphnia magna and Scenedesmus obliquus. The treatment is the group with filtrate from D. magna, while the control is the group without filtrate from D. magna. Light intensity, light cycle and incubation temperature are consistent throughout the experiments. And each experiment lasted for 9 days. We studied some heavy metals and pesticides, and adding D. magna water to S. obliquus, samples were collected then counted using a blood-cell counting chamber under a microscope. We calculated the growth rate, the numbers of cells per particle, the mean proportion of cells, the photosynthetic efficiency and enzyme activity in different particles, detailed results are as follows:1. Effect of heavy metals on grazer-induced colony formation and photosynthesis in S. obliquusWe inoculated the alga S. obliquus into media with or without Daphnia filtrate at different concentrations of Cd (0,0.05,0.08,0.10,0.32 mg L"1) and free Cu2+(0.03 ×10-3,0.89×10-3,2.91×10-3,10.3×10-3,23.3×10-3μmol L-1).Results showed:in the presence of Daphnia filtrate, Cd-free populations of S. obliquus were comprised of 46.4% eight-celled colonies on day 3. As the Cd concentrations increased from 0.10 to 0.32 mg L-1, the algal growth and photosynthesis were decreased with the result of reduced proportions of colonial populations. In the presence of Daphnia filtrate, S. obliquus cultured in healthy conditions (0.03×10-3 μmol L-1 free Cu2+) was comprised of 60.4% eight-celled colonies on day 3. At Cu2+ concentrations higher than 2.91×10-3 μmol L-1, algal growth and photosynthetic efficiency decreased along with reduction in the proportions of colonial populations. Exposure to< 0.08 mg L-1 free Cd and< 0.89×10-3μmol L-1 free Cu2+ had no significant effects on algal growth and photosynthesis; however, the ability of S. obliquus to form colonies in response to Daphnia filtrate was impaired. These results suggested the high sensitivity of grazer-induced morphological defenses in Scenedesmus to Cd and Cu toxicity.2. Effect of herbicides on grazer-induced colony formation and photosynthesis in S. obliquusWe inoculated the alga S. obliquus into media with or without Daphnia filtrate at different concentrations of three herbicides(0,0.002,0.02,0.2,2 mg L-1). Results showed:different sensibilities have been found in S. obliquus on three herbicides, and the toxicity of atrazine is higher than glyphosate and 2,4-D on S. obliquus. At the concentration(atrazine)≥ 0.2 mg L-1, the algal growth decreased significantly. But, glyphosate and 2,4-D had no significant effects on algal growth. Besides, the photosynthesis is also affected by these herbicides. In the presence of Daphnia filtrate, the ability of S. obliquus to form colonies was impaired by atrazine(≥0.2 mg L-1), glyphosate(≥2 mg L-1) and 2,4-D(≥2 mg L-1), however, Exposure to glyphosate(≥2 mg L-1) and 2,4-D(≥2 mg L-1) had no significant effects on algal growth. These results suggest high sensitivity of grazer-induced morphological defense to herbicide toxicity in Scenedesmus.Results showed that the growth rate, photosynthesis and grazer-induced colony formation are affected by the heavy metals and herbicides in S. obliquus. For algae, the normal growth is the premise of other activities, and may seek the best balance in resource allocation in order to maintain the population survival and continuation when they face the double pressures of heavy metals/herbicides and kairomones. The reasons of the decrease in grazer-induced colony formation:maybe the action and degradation rate of kairomones were affected, or the interaction between S. obliquus and kairomones has been disrupted by heavy metals and herbicides.