The Study Of Nitrite Degradation Mechanism By Lactic Acid Bacteria In Aquaculture Water

With the expanding of the scale and increasing intensivism of freshwater aquaculture, it has become the urgent problem that the content of nitrite in aquaculture water was likely to seriously exceed the normal level in China.Lactic acid bacteria can improve aquaculture water environment, in recent years, it has been widely used for nitrite degradation in the aquaculture water. However, its mechanism is still unclear.This study was based on seven strains lactic acid bacteria of the laboratory as the research object and studied on the nitrite degradation. The results were showed as below:1) The experimental results showed that lactic acid bacteria could decompound the nitrite in the rich nutrition medium, and achieved better performance in removing Lactobacillus than Lactococcus. In simulated natural breeding water environment, lactic acid bacteria could also decompound the nitrite, in the case that there was a right amount of nutrients.2) It was also shown that carbon sources determined the survival of lactic acid bacteria. The presence or absence of carbon sources obviously affected the ability of the lactic acid bacteria to actively perform the degradation. As long as there is a carbon source, lactic acid bacteria was able to work. On this basis, the higher the nitrogen concentration was, the better the removal could be further achieved. The lactic acid bacteria could decompound the nitrite when the carbon concentration reached up to0.01%. When both the carbon and nitrogen concentrations were0.01%, respectively, the rate for nitrite removal could reach to23%in pure culture environment containing only carbon and nitrogen two days later.3) The cause of nitrite degradation was also studied. It was shown that some of the main ingredients in the lactic acid bacteria medium could react with nitrite. The higher the concentration was, the more effective reaction was observed. Such reaction was proven to be very obvious in the high temperature and pressure conditions, but under normal conditions, the reaction was negligible. In addition, intracellular enzymolysis and lactic acid could also remove nitrite. 4) It was found that the ability of lactic acid in nitrite degradation involved two different modes, namely direct chemical reaction and indirect effect of promoting microbial denitrification when the lactic acid was added into the sterilized water and sterilized aquaculture water. The lactic acid bacteria metabolites, lactic acid, could decompound the nitrite, not only through the direct chemical reaction of2H++3NO2-=NO3-+2NO↑+H2O, but also through the indirect effect by promoting the microbial denitrification. The latter one mainly took effect in the aquaculture water. Further analysis also showed that the reason of denitrification promotion by lactic acid was that it provides the easy-to-use organic carbon (energy material) for denitrifying bacteria.5) It was found that when citric acid and hydrochloric acid were treated as the alternationa of lactic acid, the citric acid with the same concentration as lactic acid also shown a comparable effect in nitrite removal in the aquaculture water, but the removal process is much slower, and the minimal required concentration was also higher than that for the lactic acid. The hydrochloric acid did not exhibit significant removal effect. To some extents, the organic acids can act as energy material of denitrification, inorganic acid can not.