Support Of Terrestrial Organic Carbon To Growth And Reproduction Of Daphia Magna And Its Mechanism

Corn straw comes from a typical terrestrial plant and can enter the aquatic ecosystem as terrestrial organic carbon. It consists mainly of lignin, cellulose and hemicelluloses with low concentration of fatty acids, and is considered as low nutritional value for zooplanktons. Therefore, corn straw can’t maintain the zooplankton normal growth and reproduction. However, corn straw can be firstly degraded by bacteria and other microorganisms after the input to freshwater ecosystem. In order to find out how decayed corn straw affect the growth of Daphina magna and the possible mechanisms, following researches were done.1. Decay corn straw in dark and investigate the effect of decayed corn straw on the growth and reproduction of Daphnia magna. The feeding experiments of Daphnia magna showed that using corn straw feeding the Daphnia magna, they were all died in 7th days; while, using the straw decayed liquid to feeding the Daphnia magna, the growth and reproduction status as same as the Daphnia magna eating Scenedesmus. Those two group of Daphnia magna body length respectively reached 3.49 mm and 3.21 mm, survival rates were 86.7 % and 93.3 % respectively, reproduction rate were 71.42 and 72.40 respectively, the intrinsic growth rate were 0.213 and 0.214. So the results showed that corn straw does not directly support the Daphnia normal growth and reproduction, but after the microbial food web decomposition, the straw decayed liquid can maintain the normal growth of the Daphnia magna reproduction.2. Fatty acid analysis was applied to reveal the microbial dynamics, as well as poly-unsaturated fatty acids and sterol accumulation in the corn straw decay system. The results showed that the carbon of cornstalk was assimilated by bacteria and protozoa subsequently. The experiment results showed that the total fatty acid content is 15.68 μg/L in the corn straw and then increased to 206.878 μg/L in the straw decayed liquid, in the before and after straw decomposition, the polyunsaturated fatty acids is 0.391 μg/L and 8.351 μg/L respectively, the sterol content increase to 7.003 μg/L. The results also showed that the total fatty acid content peaked at 3rd day and the polyunsaturated fatty acids and sterols content peaked at 5th day. At 9th day, because of carbon source depletion, the total fatty acid, unsaturated fatty acid and sterol content decreased. The results show the food upgrading by microbial food web of corn straw while decaying.3. In order to further understand which microbial synthesis the polyunsaturated fatty acids and have a key influence on the growth and reproduction of Daphnia magna. Different types of microbes were separated for fatty acids and sterols analysis and their effect on growth and reproduction of Daphnia magna as diet. The results showed that flagellates contributes mostly on synthesis of polyunsaturated fatty acids during the straw decay, and is vital to food quality promotion of corn straw. Feeding experiments showed that Dahnia magna feeding with single types of microbial exhibit higher survival rate and reproductive rate than that with undecayed corn straw, but significantly lower than that with decayed corn straw and scenedesmus. I speculated the possible reason might be the richness of saturated fatty acids, single unsaturated fatty acids and high P: C ratio of bacterial part in decayed corn straw.From the researches above, the paper concluded that heterotrophic flagellates can synthesis of unsaturated fatty acids in vivo, and can upgrade food quality of corn straw during decay. Ultimately, the decayed corn straw can support the growth of Daphnia Magna. My results suggests that how terrestrial organic carbon subsidy the higher trophic level as zooplankton.