Preliminary Studies On Relationship Between Microcystis Sp. And Nitrifying Bacteria

In the aquatic environment such as eutrophic lakes, complex interaction exists between microbes, especially algae and nitrogen cycle microbes. In this research, relationships between a toxic Microcystis (Microcystis sp. PCC7806) and nitrifying bacteria were preliminarily studied under different conditions in the co-culture system, from two aspects:the nitrogen chemical index and the microbial biomass index.We designed alga-bacteria co-culture light-dark experiment, cell sap of disrupted algae experiment and the interactions between Microcystis and nitrifying bacteria with different population, aiming to investigate the inhibition which may exist between Microcystis and nitrifying bacteria and the mechanism of it. In the process of experiment, the phenomenon of NO2--N absorbed by algae was noticed, so we designed the experiment to investigate Microcystis using low concentration NO2--N and the interactions between Microcystis and nitrite-oxidizing bacteria(NOB).Nitrifying bacteria and Microcystis cultured separately as control in the experiments, alga-bacteria co-cultured of light and dark groups were set up for the alga-bacteria co-culture light-dark experiment; cell sap of disrupted algae group was set up to study the influence of disrupted algae cell sap on the nitrifying bacteria growth. In the alga-bacteria co-culture light-dark experiment, nitrite nitrogen (NO2--N) significantly accumulated, Microcystis died gradually and nitrifying bacteria grew fast in the dark group, similar to the situation that nitrifying bacteria cultured alone. While in the dark group, NO2--N had no significant accumulation, Microcystis grew rapidly and nitrifying bacteria grew slowly. These results indicated that the growth of Microcystis under high population density (107cell/mL) has obvious inhibitory effect on the the growth of nitrifying bacteria. Meanwhile, in the cell sap of disrupted algae experiment, NO2-N accumulation and consumption process and nitrifying bacteria growth were no significant differences, compared with the nitrifying bacteria cultured alone. Results display that cell sap of disrupted algae has no significant effect on the the growth of nitrifying bacteria. The release of intracellular substances such as Microcystin is not the key factors that inhibit the growth of nitrifying bacteria.The results of the interactions between Microcystis and nitrifying bacteria with different population density show that, in the system of nitrifying bacteria and Microcystis co-cultured with30mg/L initial ammonia nitrogen(NH4+-N), when algae initial population density was small (106cell/mL), algae and bacteria growth simultaneously in the early period of the test, but subsequently algae growth was hampered, and gradually die, as nitrite nitrogen (NO2--N) was accumulated quickly caused by ammonia-oxidizing process. When algae initial population density was higher(107cell/mL), algae and bacteria synchronous growth until the NH4+-N was consumed to a low level, because NO2--N was used timely by algae and nitrifying bacteria. These results indicated that, Microcystis has obvious inhibitory effect on the growth of AOB, and the higher population density of Microcystis is, the more obvious inhibitory effects on the growth of AOB will be. This inhibition is mainly because the Microcystis and AOB have competitive use of NH4+-N.Low concentration NO2--N intermittent adding experiments showed that Microcystis has certain absorption in low concentration NO2--N. In certain conditions, the absorption capacity and algal cell density were positively correlated relationship.106cell/mL algae’s absorbing ability of NO2--N could reach0.6mg/L every day. In the simulation experiment of NO2--N metabolism in alga-bacteria co-culture system, NO2--N produced by AOB in co-culture system was replaced by NaNO2added to algae medium intermittently, the situation of algae utilization of NO2--N in co-culture system had been been simulated very well. And low concentration NO2--N light-dark experiments have shown that algae had no consumption of NO2--N in the darkness. Only in the light condition, algae consumed NO2--N, and got growth. It suggests that Microcystis can use low concentration NO2--N as nitrogen source in assimilation.The experiment of the interactions between Microcystis and nitrite-oxidizing bacteria(NOB) under low concentration NO2--N had shown that,107cell/mL Microcystis has inhibitory effect on the growth of NOB, because of the competitive use of NO2--N. To sum up, high population density of Microcystis growth has obvious inhibitory effect on the growth of nitrifying bacteria. The main reason is not attributed to the release of Microcystis iatracellular substances, but the competitive utilization of NH4+-N and low concentration NO2--N between Microcystis and AOB or NOB. It is concluded that NH4+-N and NO2--N as nitrogen source of Microcystis and energy source of nitrifying bacteria, the relationship between nitrifying bacteria and Microcystis is mainly the competitive utilization of nutrient in eutrophic lake. And the effect of nitrifying bacteria may be weakened when algae bloom coming.