Screening lactic acid bacteria for fermentation and mineralization in aquaponic system

Lactic acid bacteria have been widely used as feed additives in aquaculture to improve the absorption of nutrients, as well as the immunity and growth performance of fish. According to the lactic acid bacteria to decompose organic matter, this study aims to improve the utilization efficiency of solid waste resources with high nutrients in aquaponics systems. Fish-derived bacteria were screened with strong resistance and better mineralization function, in order to enhance the fermentation and mineralization of fish waste. The experiments were conducted to take the biofilter media and fish from the aquaponics system as the source of lactic acid bacteria. A series of tests were then performed on the high-temperature tolerance, acid and alkali tolerance, high salt tolerance, and the mineralization performance of various nutrients in fish waste. Two strains were obtained with mineralization function and application potential, which were identified as Lactococcus lactis L1 and Lactobacillus dextrinicus L2, respectively. A comparison was also made on the growth performance of the strains at 10, 20, 30 and 40 °C. The results revealed that the growth of L1 and L2 was inhibited at 10 and 20 °C, while they proliferated normally at 30 and 40 °C. The survival rate of L1 and L2 after 30 min of heating at 50, 60, and 70 °C was examined to verify the high-temperature tolerance of the strains. It was found that the L1 was achieved the better high-temperature tolerance. The survival rate of L1 was 96.60% at 50 °C, which was significantly higher than that of L2 (80.35%). According to the range of water acidity and alkalinity in the environment of aquaculture, the tolerance of the two strains was examined at pH 5.0-9.0. The results showed that the survival rates of L1 (65.43% and 71.25%) were higher than those of L2 (31.10% and 52.22%) at both pH 5.0 and 9.0 (P<0.05). The tolerance of the strains was examined at 15, 30, and 60 g/L salinity, where the mariculture and salinity were considered after the thickening of mariculture sludge. The survival rate of the strains gradually decreased with the increase of salinity. Once the salt concentration was 60 g/L, the survival rate of L1 was 37.33%, while there was no L2 survival. In terms of tolerance to temperature, acid, and alkali, L1 was selected as the subsequent experimental object for the fish waste fermentation mineralization experiment. The control group (no lactic acid bacteria added) and the L group (with Lactococcus lactis added) were divided to last for seven days. A comparison was made on the contents of nitrogen, phosphorus and various mineral elements in the fermentation broth at days 0, 1, 3, 5 and 7. The results showed that better degradation and mineralization of organic matter were achieved in the L group. The highest concentrations of total ammonia nitrogen (TAN) and total nitrogen (TN) in the fermentation broth of the L group were 192.82 and 342.00 mg/L, respectively, which were much higher than the control group (30.29 and 79.00 mg/L, respectively). Potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), and zinc (Zn) in the fermentation broth of the L group were significantly higher than those of the control group, except for sulfur (S) and all reached the highest values (27.59%-94.67%) in the 3d. In conclusion, Lactococcus lactis presented the better resistance to heat, salt, acid, and alkali, indicating better fermentation and mineralization of fish waste. Furthermore, 3 d was the optimal cycle of fermentation and mineralization. The finding can provide a strong reference to apply the lactic acid bacteria to the mineralization of solid wastes, in order to improve the nutrient utilization efficiency in aquaponics systems.