| Duckweed,as a small floating plant distributed in the world,is widely applied in sewage treatment because it can directly absorb water nutrients and produce high-quality biomass containing high contents of protein and starch.However,it is found that the pond system based on duckweed has prominent methane emission in the sewage treatment process,and nitrogen removal and methane emission are obviously influenced by environmental factors,such as duckweed coverage,sediment and seasons.It is supposed that the influence should be related to the microbial community in the ponds.Thus,this study was carried out to confirm the supposition and reveal its influencing process,and to provide scientific basis and technical guidance for the application of the duckweed-based ponds.The microbial community of water,sedment and duckweed samples collected from the pond systems with and without sediment(or duckweed coverage)was investigated by using traditional pure culture and molecular biology methods in this study,the main purpose was to reveal that how duckweed coverage and sediment affect nitrogen removal and CH4 emissions throuth affecting microorganisms in the pond system.The results indicate that:(1)There are abundant methanogenic archaea(containing 6 methanogenic orders)in the sediment of duckbug pond,but the composition and quantity of methanogenic bacteria are not the main influencing factors of methane production,Methane production may be influenced by the activity of methanogens.Adding sediment to duckweed pond system can reduce the abundance of epiphytic bacteria of duckweed,but has little effect on the diversity of bacteria.At the same time,the sediment has a certain effect on the epiphytic bacteria of duckweed and the bacterial community of water,but the effect is small.Comparatively speaking,the seasonal change has more important influence on water bacteria and duckweed epiphytic bacteria community.However,the stability of bacteria and archaea in the sediment was higher and less affected by season.In addition,both duckweed and water samples contained abundant methanogens.Sediment can significantly increase the abundance of Methanotrophs in water,but the effect on Methanotrophs attached to duckweed has no obvious rule.(2)Duckweed coverage increased the abundance of methanogens and the stability of the archaea community in the bottom sediment of the pond system,but the abundance of methanogens was not the main factor affecting methane production.At the same time,duckweed coverage can also increase the diversity and richness of bacteria in sediment and water in the pond system,and significantly affect the composition of sediment bacteria community,but the influence on water bacterial community was less,and the water bacterial community was more affected by seasonal differences.In terms of the abundance of Methanotrophs,duckweed coverage can increase the abundance of Methanotrophs in water.In addition,the epiphytic bacteria of duckweed contains a large number of functional flora related to nitrogen cycle and greenhouse gas emissions,such as methanotrophs(Methylophilus),denitrifying bacteria(Pseudomonas),nitrogen-fixing bacteria(Rhizobium)Its function and function need to be revealed by separation and purification and performance research.(3)Methanotrophs strain DMO-1 with strong methane oxidizing ability was isolated from the root samples of duckweed pond system.After identification,the strain belongs to the Methylomonas genus,named as Methylomonas sp.DMO-1.The strain was able to grow with methanol or methane as the only carbon source and showed high oxidation capacity at different concentrations of methane,the highest oxidation rate of CH4 can reach 0.40?mol/h/mL,higher than most reported strains.The co-culture study of DMO-1 and duckweed showed that Methylomonas sp.DMO-1 also has the role of promoting the growth of duckweed,which is the first known duckweed stimulating bacteria(PGPB)with CH4 oxidation ability.Meanwhile,duckweed can also promote the growth of DMO-1 and enhance the CH4 oxidation efficiency of DMO-1.(4)During the enrichment of aerobic denitrifying bacteria from duckweed,adding duckweed secretions or laps of different concentrations to the enrichment solution can affect the composition and diversity of bacteria in the enrichment solution.A total of 5potential denitrifying bacteria were obtained from the 6 kinds of enrichment solutions.Pseudomonas strains DN9 and DN10 showed excellent nitrate denitrification removal ability under aerobic and anaerobic conditions within 24 hours The nitrate nitrogen with a concentration of about 120 mg/L is almost completely removed from the culture medium However,with the removal of nitrate nitrogen,nitrite nitrogen accumulated and showed no downward trend,indicating that strains DN9 and DN10 could only reduce nitrate nitrogen to nitrite nitrogen,and did not have the ability to further reduce nitrite nitrogen.In addition,a dominant strain of duckweed epiphytic strain DN15 belonging to the genus Sphingorhabdus was isolated,but under the conditions of this study,it was not detected that the strain has denitrifying ability,which needs further study.In general,different environmental factors,such as sediment,duckweed coverage and seasonal differences,affect the microbial communities of various components of the pond treatment system(such as water,sediments and plants)to varying degrees,which in turn affects the nitrogen removal of duckweed pond and methane emissions.The relevant findings have certain practical guiding significance for the application of duckweed ponds.For example,the presence of sediment provides abundant methanogens for duckweed pond and thus increases the production of CH4,so it is necessary to clean the sediment in duckweed pond regularly.The presence of duckweed in the pond system can provide support for the growth and accumulation of many functional microorganisms,such as methane oxidizing bacteria,nitrifying bacteria and denitrifying bacteria,which is more conducive to the removal of nitrogen in the system and the reduction of CH4emissions,Therefore,appropriate duckweed coverage is more advantageous.In addition,the methane oxidizing bacteria and aerobic denitrifying bacteria isolated from duckweed samples in this study have strong methane oxidizing and denitrifying capabilities,which can provide important microbial resources for the microbial strengthening of duckweed pond system or other pond systems. |
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