| High nitrate concentrations can lead to serious health risks for humans.Insufficient total organic carbon in natural water can makes difficult to denitrification.Therefore,it is still a difficult problem that denitrification of water bodies with low carbon to nitrogen ratios in the field of water treatment.Compared with aerobic denitrifying bacteria,fungi can be used as new candidate for nitrogen removal in low carbon to nitrogen ratio waters because of their spores that can germinate in harsh environments and their solid cell walls that resist fluctuations in the external environment.However,most studies have focused on the denitrification of aerobic denitrifying bacteria,and there are few reports on the denitrification of aerobic denitrifying fungi.In this study,the variability of community structure of fungi and its relationship with water quality in six water reservoirs in the north and south of China by high-throughput sequencing technology were investigated.Next,the aerobic denitrifying fungi were isolated and identified,their denitrification characteristics were analysed,their optimal denitrification conditions were clarified,and the aerobic denitrifying fungi were successfully applied to the denitrification experiments of landscape raw water.Finally,two strains of aerobic denitrifying fungi with poor denitrification effect were screened out and enhanced by co-culture,and their denitrification characteristics under co-culture were analysed,and the denitrification mechanism of fungal co-culture was discussed,and finally they were applied to the denitrification experiments of reservoir raw water.The main findings of the study include:1.The geographical differences in water quality parameters,fungal populations and fungal population structure characteristics of reservoirs in the south and north were investigated by using in situ monitoring,pure culture counting and high-throughput sequencing techniques in six water source reservoirs in the south and north.The results showed that:(1)the physical and chemical parameters of reservoir water quality showed significant north-south differences.The average dissolved oxygen concentration in the northern reservoirs was 1.2 times higher than that in the southern reservoirs,and the average total nitrogen concentration in the southern reservoirs was 2.08 times higher than that in the northern reservoirs.(2)There was a north-south difference in the number of fungi.The number of fungi in the geographical distribution showed that the south was higher than the north,with the highest number in Shiyan reservoir in the south(4.3×10~2CFU/L)and the lowest in Jinpen reservoir in the north(1.4×10~2 CFU/L),with a 3-fold difference between them.(3)The community structure of fungi in north and south reservoirs showed obvious north-south differences.High-throughput sequencing analysis of fungal population succession showed that the abundance of fungi in the southern reservoirs was significantly higher than that in the north.Cystobasidium sp.and Acremonium sp.dominated in northern(50.34%)and southern reservoirs(39.18%),respectively.(4)The results of the random forest model indicated that Cryptococcus saitoi and Exophiala xenobiotica were potential keystone species within the community structure of fungi of the northern and southern reservoirs.The correlation between fungal communities and water quality parameters was analysed by correlation matrix heat map,and the results indicated that water temperature and total organic carbon were key factors affecting the structural succession of fungal populations in the north and south.The results of this study will provide a scientific basis for the geographical differences and drivers of fungal population structure in water source reservoirs.2.Three aerobic denitrifying fungi were isolated from water samples of different water source reservoirs,and their denitrification performance,optimal conditions and raw water application were investigated by using ITS fungal identification technique to identify Trichoderma viridans,Penicillium brasiliensis and Fusarium solani.The results showed that:(1)the removal rates of nitrate by the three strains were 89.93%,89.20%and 87.76%respectively,and the removal rates of total organic carbon were higher than90%.(2)The results of nitrogen balance analysis showed that the three fungal strains could convert 53.10%,52.26%and 53.77%of the initial nitrogen into gaseous nitrogen.(3)The results of single-factor experiments showed that the optimal denitrification conditions for the three fungal strains were:C/N=15,rotational speed=160 rpm,temperature of 30°C,and carbon source of sodium acetate.(4)The nitrate removal rates of the three fungal strains were increased by 1.4,1.68 and 1.46 times,respectively,after the addition of degradable plastic.(5)The results of FTIR,X-ray diffractometer and organic elemental analyser showed that Trichoderma,Penicillium and Fusarium could secrete relevant hydrolytic enzymes to break the ester bonds of degradable plastics and thus be utilized as a carbon source.(6)All three strains of fungi can remove more than80%of total nitrogen and organic matter such as humus from the lake water.The results of this study will provide a scientific basis for the application of aerobic denitrification fungi to treat raw water with low carbon to nitrogen ratio.In order to investigate the enhanced denitrification effect of aerobic denitrifying fungi co-culture,two aerobic denitrifying fungi with less than 50%nitrate removal rate,Aspergillus niger H1 and Trichoderma afroharzianum C1,were co-cultured in this study,and the electron transfer activity,intracellular ATP and The fungal co-culture was successfully used to denitrify the reservoir water by combining electron transfer activity,intracellular ATP and denitrifying enzyme activity.The results showed that(1)aerobic denitrifying fungal co-culture improved the nitrate removal rate(69.71%).(2)The results of nitrogen balance analysis showed that the co-culture strain could convert 33%of the initial nitrogen into gaseous nitrogen.(3)Single-factor experiments showed that the optimal conditions for aerobic denitrifying fungal co-culture for denitrification of low-C/N waters were C/N=5,temperature 30°C and inoculum ratio 3:1.(4)The results of electron transfer activity,intracellular ATP and denitrifying enzyme activity showed that fungal co-culture denitrification significantly increased electron transfer,intracellular ATP concentration and nitrite reductase activity.(5)The total nitrogen removal rate of the reservoir water by fungal co-culture was nearly 70%and organic matter such as fulvic acid could be removed from the water.The results of this study will provide a reference for the practical application of in situ denitrification by aerobic denitrifying fungal co-culture. |
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