Inter-specific Competition Of Two Species Of Marine Microalgae And The Effects Of Marine Bacteria On Their Population Growth: What's The Possible Mechanism?

The microorganism from marine environment can decompose and inhibit the growth and reproduction of microalgae. It is a basic theory of prevention and control of Red tide by environmental microorganism. The red tide microalgae—Amphidinium carterae Hulburt and the microalgae of bait—Platymonas helgolandica var.tsingtaoensis were chosen as the target organisms. This study carried out the population growth and competition mechanism of these two microalgaes under axenic culture of two microalgaes which was set up in laboratory condition. Simultaneously, the effect of five bacteria which were separated from the marine environment on the growth of two marine microalgaes was studied with a view to provide a foundamental research data to the control of red tide by microorganism.The results show that:1. Axenic microalgal system set-up and the changes of microalgal growth with and without sterilizationBasing on the prelinimary experiment of antibiotic sensitivity and the inhibition cycle, three antibioticses: ineomycin, streptomycin sulfate, Kanarese sulfate at a concentration of 10μg/mL were used for bacterial elimination in A. carterae and P.helgolandica cultures. The bacterial numbers significantly reduced after antibiotic treatment as compared to that without sterilization by acridine orange staining and epifluorescence microscopy observation. Hence, the axenic microalgal culture could be obtained according to the method above described.The microalgal growth changed obviously with and without sterilization almost no significance was observed in the relative growth rate in either axenic or nonaxenic cultured in the exponential phase. The axenic microalgae entered the stationary phase two days later than the nonaxenic one, and the greatest cell density increased by 24% as compared to the latter. P.helgolandica growing in the nonaxenic culture was quicker than that in the axenic one in the exponential phase; however, the microalgal cell density of algal cells was no less than in the axenic one. In the stationary phase, the growth status changed obviously in either axenic P. helgolandica or A. carterae cultures as compared to that in the nonaxenic one, and the microalgae maintained a good condition of suspension while almost nine tenths sanked in the respective nonaxenic cultures.2. The change of growth of two microalgaes in co-culture system and the competition between themCompared with the mono-culture, both carrying capacity (K) and intrinsic rate of increase (r) of the two microalgaes reduced obviously in the co-culture and the inhibitory degree to A.carterae was stronger than that to P. Helgolandica. The interaction of two sterilized microalgaes was simulated by the interspecific competition model—Lotka-Voterra. The competitive inhibition parameters of A. carterae and P. helgolandica (α,β) were respectively 1.79 and 25.09. That means the degree that A. carterae inhibited by P. helgolandica was about 14 times stronger than that P. helgolandica exerted on A. carterae. In addition, the instantaneous rate of P. helgolandica was greater than the control. Above all, we found that P. helgolandica possessed a more interspecific competitive capacity. The initial inoculation biomass of microalgae has a significant impact on the interspecies competition.3. Effect of environment bacteria on the growth of two microalgaes and the physiochemical property of five environment bacteriaOn the basis of physiochemical and morphological test, strain M3-123, M3-0424, M4-129, M3-129, M2-123, which were isolated from marine environment, were identified respectively as Clostridium sp.,Flavobacterium sp.,Bacillus sp.,Citrobacter sp.,Micrococcaeae sp. according to Bacteria identification of Berger's manual.The effects of these bacteria on the growth of two marine microalgaes'axenic and nonaxenic systems were studied. The results show that: Both in the axenic and nonaxenic systems of P. helgolandica, the cell density increased with the increase of initial bacteria concentration. In the axenic system of A. carterae, it was promoted by high concentration and inhibited by low concentration; but in nonaxenic system, it was inhibited by high concentration and promoted by low concentration. Specially, strains M₃-0424, M₃-123, M₄-129 had a significant killing effect on A. carterae in nonaxenic system and the order of effect is: M3-123> M3-0424> M4-129. In addition, the concentration and growth rate of initial inoculation bacteria had a significant correlation with the inhibitory effect.To explore the possible mechanisms of three kinds of algae-lysing bacteria, by adding following five different treatments of the bacterial culture to the system of microalgae, including A: sterilization in 121.3℃, B: filtration from 0.22μm, C: centrifugal sedimentation at 9000rpm, D: fragmentation by ultrasonic, E: the control, we found that the cultivation of bacteria had obviously inhibitory effect on the growth of microalgae : strains M3-0424 secreted extracellular products which can inhibit the growth of microalgae, and the product which maybe belong to protein had no thermal stability; the cultivation both of strain M4-129 and M3-123 which had a thermal stability can inhibit the growth of A. carterae. One of the possible reasons of this effect is allelopathy.