Ref ID : 4525
Shaoli Zhang, Honggang Ma, Henglong Xu, and Weibo Song; [On experimental ecology of the marine ciliate Paranophrys magna IV: effects of competition of population growth]. Acta Ecologica Sinica 21(2):2039-2044, 2001
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The scuticociliate Paranophrys magna Borror, 1972 is commonly found in a variety of marine waters, especially in mariculture biotope. The effects of different initial population density, temperature, salinity and pH on the population growth have been studied using experimental methods in previous reports. As the fourth part of this series, the present paper deals with the effects of the competitor (a hypotrichous ciliate) and food on the population growth of Paranophrys magna. The experiment was divided into four treatments: 1) the medium without any ciliate as a control; 2) the medium inoculated with Paranophrys magna as Paranophrys-treatment; 3) with Euplotes vannus as Euplotes-treatment and 4) the medium inoculated with both Paranophrys magna and Euplotes vannus (as mixing-treatment). Each treatment was carried out as three independent replicates. All work was performed in room temperature with initial density of 30 ind./ml of ciliates. During experiments, the density of cells was recorded periodically with counting slide, while the density of bacteria was assessed with the globulimeter. According to the formula In(Nt)=In(N)+rt, the increasing rate of populations was calculated with regression analysis based the method of least square. In control treatment, the population growth turned into exponential growth phase in a few hours due to high temperature and concentrations for organic materials. During equilibrium phase the density of bacteria remained 1.02x10E8 ind./ml. The medium turned turbid as the density of bacteria was amplified. In Paranophrys-treatment, the natural rate of population growth of ciliates was significantly lower than that of bacteria due to shortage of food from 0 to 1 day. As the density of bacteria increased, the maximum density of ciliates reached 2.97x10E4 ind./ml and the natural rate of population growth remained 3.22+/-0.046/day after one day. With the increasing of density of ciliates, the growth of bacteria was inhibited by ciliates and the density decreased of 6.42x10E7 ind./ml. After the growth of ciliates came into equilibrium phase, the density of bacteria leveled off steadily at a lower value as 2/3 of that in control treatment. In Euplotes-treatment, the interrelation of growth between ciliates and bacteria was similar to that in Paranophrys-treatment, however, the density of Euplotes vannus decreased to a lower value as 1/10 of that of Paranophrys magna, the maximum value reached 2328 ind./ml and the natural rate of population growth remained 1.57+/-0.002/day. In the same way as in Paranophrys-treatment, the growth of bacteria was also inhibited by ciliates. In mixing-treatment, the growth of the two species of ciliates developed into the exponential growth phase after 24 hours. During this phase the increasing of density of Paranophrys magna lasted for 1 day, then the maximum density reached 900 ind./ml and the natural rate of population growth remained 1.79+/-0.021/day, while the growth of Euplotes vannus lasted for 1.5 about days, then the maximum density reached 1775 ind./ml and the natural rate of population growth increased to 2.10+/-0.046/day. It was obvious that the density of Euplotes vannus remained higher than that of Paranophrys magna in this treatment. It was revealed that there was a difference between mixing culture and non-mixing culture on the population growth of the two ciliates. The maximum density of the former was only about 3% of that in the latter (870 vs. 29679 ind./ml) and the natural rate of population growth decreased to about 1/2 during exponential growth phase, while the population growth of Euplotes vannus was inhibited at a lower degree. The natural rates of population growth of the two species ciliates were different among all the treatments (Paranophrys with single culture > Euplotes with mixing culture > Paranophrys with mixing culture > Euplotes with single culture). Compared with control, the population growth of bacteria were inhibited in different degree in other three treatments (mixing treatment > Paranophrys-treatment > Euplotes-treatment). The results also indicate that the population growth of both ciliated protozoa, Paranophrys magna and Euplotes vannus, were inhibited by the competition and the their densities dropped to a steady and lower level when two species were cultured together. The maximum population density and the natural rate of increase during the exponential phase in Paranophrys magna was only 870 ind./ml, 1.79 ind./ml when mixed with Euplotes vannus while the values were 29670 ind./ml, 3.22 ind./ml when culture alone. The results support the viewpoint that there is a negative correlativity between size and increasing rate of population in ciliate protozoa (Fenchel, 1968). The growth of bacteria was inhibited strongly when ciliate were cultured either individually or mixed. The density of bacteria dropped sharply at beginning of exponential growth and decreased finally to a lower level during experiments. As effective predators, ciliates used might be very helpful for maintaining the quality of aquiculture waters.