Enhanced Performance Of Algal-bacterial-based Aquaponics Based On Optimization Of Its Carbon And Nitrogen Cycles

With the upgrading of the consumption structure of residents,the aquaculture industry has been developed rapidly in recent year,resulting in shortage of breeding sites,waste of water resources and environmental pollution of external drainage bodies.The algal-bacterial-based aquaponics is one of the emerging recirculating aquaculture systems,which was considered to be a sustainable and eco-friendly aquaculture model with broad development prospects.It is of great practical significance to grasp the key ways that affect the performance of the algal-bacterial-based aquaponics,improve the utilization efficiency of the elements of the system and thereby strengthen the performance of the algal-bacterial-based aquaponics.In this paper,a laboratory-scale algal-bacterial-based aquaponics was constructed,and different types of carbon sources were added to the system.The influence of additional carbon sources on the carbon and nitrogen cycle of the system and its mechanism were resolved by means of element balance analysis and quantitative real-time PCR.The effects of antibiotics in water on the performance of the algal-bacterial-based aquaponics were evaluated from the aspects of system performance,water quality and greenhouse gas emissions.The carbon and nitrogen conversion process of the algal-bacterial-based aquaponics was preliminarily mastered,and the feasible suggestions for optimizing the performance of the algal-bacterial-based aquaponics were put forward,which laid a theoretical foundation for optimizing the design and operation of the system.The main conclusions are as follows:(1)The addition of carbon source significantly increased the algae-bacteria biomass and the chlorophyll concentration of algae,but had no significant effect on the production performance of fish.The addition of organic carbon source(OC)and inorganic carbon source(IC)could improve the distribution of carbon and nitrogen in algae-bacteria.The biomass of algae-bacteria in OC group and IC group were increased by 9.6%and 19.2%,respectively,compared with the control group.The nitrogen distribution in OC group water decreased significantly,while the nitrogen distribution in IC group water did not change significantly,but the total carbon concentration in IC group water inereased significantly.The release of greenhouse gases carbon dioxide(CO2)and nitrous oxtde(N2O)in OC group was significantly enhanced,while IC had no significant effect on the greenhouse gas release in the system.(2)The added carbon source was beneficial to the long-term maintenance of good water quality in the algal-bacterial-based aquaponics.The NH4+-N concentration in the carbon source addition group was maintained at a low level,while the NH4+-N concentration in the control group was significantly increased at the later stage of system operation.The concentration of NO3--N in OC group was significantly lower than that in IC group and control group,which was mainly due to the presence of organic carbon to enhance the absorption of nitrogen nutrient by microalgae and the denitrification capacity of photobioreactor.The quantitative real-time PCR results showed that organic carbon sources increased the nirS abundance and enhanced the denitrification process.The abundance of amoA in IC group was significantly increased,effectively improved the ammonia-oxidation rate of the system.However,the good water quality effect of OC group was at the cost of increased greenhouse gas release,the releases of N2O and CO2 in OC group were 9 times and 1.66 times of those in control group(3)The introduction of antibiotics made nitrogen and carbon more widely distributed in fish and algae-bacteria,which improved the carbon and nitrogen utilization efficiency of the algal-bacterial-based aquaponics.By enhancing the enzyme activity in the intestinal tract of fish,antibiotics enhanced the feeding behavior of fish,increased the biomass of fish by 30.0%,and increased the biomass of algae-bacteria by 2.89 times.Compared with the control group,the presence of antibiotics improved the nitrogen and carbon efficiency of the system by 31.5%and 30.3%,respectively.The results of HPLC-MS showed that the algal-bacterial-based aquaponics could buffer the adverse effects of sulfadiazine antibiotics.More than 98%of the sulfadiazine imported into the system was degraded or photolysis,only a small part existed in the water,and a small part remained in fish and algae-bacteria,the concentration of sulfadiazine residue was lower than the relevant national standards.However,antibiotics increased the abundance of sul-1,sul-2 and intl-1 resistance genes in the system to varying degrees,and the consequent environmental biological risks need to be carefully considered.(4)Antibiotics had little effect on the water quality of the algal-bacterial-based aquaponics.Although antibiotics had a short-term inhibitory effect on AOB and NOB,which delayed the peak of NO2--N concentration in the system.However,in the later period of the system operation,the concentrations of NO2--N and NO3--N maintained the same level as the control group.Although the concentration of NH4+-N increased slightly compared with the control group,there was no significant difference.It is worth noting that the total N2O release of the antibiotic group was 162.2%higher than that of the control group,the greenhouse effect caused by antibiotics should not be ignored.This may be because antibiotics reduced nosZ abundance but increased nirS abundance,which affected the normal progress of the denitrification,resulting in increased N2O emissions.