Understanding changes in the abundance and diversity of microbial communities is a necessary condition for evaluating the role of microorganisms in the environment (Lyautey et al. 2021). Methanogens, as common microorganisms, are widely distributed in various environments, including soil (Kim et al. 2017, Heděnec et al. 2018), water sediments (McKay et al. 2017, Wang 2019) and animal digestive tracts (Moraes et al. 2014, van Lingen et al. 2017). Ponds and lakes are important natural emission sources of methane, and methane generation is closely related to methanogens community. At present, three main metabolic pathways have been described for methane production, namely hydrogenotrophic (convert H2 plus CO2 to CH4), aceticlastic (convert acetate to CH4 and CO2) and methylotrophic (generate CH4 by methanol, methylamine, dimethylamine and other mechanisms), which involve the diversity of methanogens (Conrad 2007, Özbayram 2012, Evans et al. 2019). In freshwater sediments, methane production is regulated by different environmental factors, such as hypoxia (Lehours et al. 2005), quality and quantity of organic matter (Bastviken et al. 2008, Schwarz et al. 2008), temperature (Duc et al. 2010), etc. Temperature variation is likely to be one of the factors affecting CH4 production capacity in the shallowest areas of deep lakes or shallow lakes (Fuchs et al. 2016). In a certain range, the increase of temperature has an obvious promotion effect on the metabolic capacity of microorganisms, which is beneficial to improve the rate of gas production. In addition, the production capacity of CH4 is closely related to the community abundance of fermentation microorganisms (Yang 2017). Therefore, it is helpful to clarify the relationship between aquaculture and greenhouse effect to study the community characteristics of methanogens in aquaculture water and intestinal tract of aquatic animals.
In this study, we applied high-throughput sequencing technology to methanogens in aquaculture water and aquatic animal intestines. Our results showed that a total of 5 genera were identified from methanogens, among which Methanosarcina, Methanocorpusculum and Methanobacterium were the three genera with the highest relative abundance. Methanosarcina is hydrogen and acetic acid mixotrophic methanogens, Methanocorpusculum and Methanobacterium are hydrogenotrophic methanogens (Yang 2017). The results showed that CH4 was produced by H2 reduction of CO2 and acetic acid degradation, and mainly by hydrogen reduction of CO2. In SY sample, Methanocorpusculum, Methanosarcina and Methanobacterium were the dominant bacteria, which is consistent with the characteristics of methanogenic bacteria community in wetland (Parkes et al. 2012, Zhang et al. 2020). In QY and CY samples, Methanosarcina and Methanocorpusculum were the dominant bacteria genera. In addition, Methanosarcina has the highest abundance in CY and Methanocorpusculum has the highest abundance in QY, which may be related to the feeding habits of grass carp and black carp.
Grass carp is an herbivorous freshwater fish, which feeds on the stems and leaves of aquatic plants, and its food riched in cellulose and polysaccharide. Black carp is a carnivorous freshwater fish, which feeds on snails, clams and other mollusks, and its food riched in protein and fat. The intestinal bacteria of grass carp and black carp are mainly Firmicutes (69% vs 37.5%), Proteobacteria (6.9% vs 37.5%) and Actinobacteria (6.9% vs 16.7%), which are highly similar to the bacterial community in cultured water (Wu et al. 2012, Zhang et al. 2013, Ding et al. 2021). Firmicutes, Proteobacteria and Actinomycetes belong to hydrolytic fermentation bacteria (Zhang et al. 2014). Among them, Clostridium in Firmicutes is a typical cellulose-decomposing bacteria with the function of fermenting monosaccharides to produce organic acids, while Streptococcus in Firmicutes is a typical protein-decomposing bacteria (Lawson 2016, Whitman 2015). Vibrio in Proteobacteria is the dominant lipopolysis bacteria (Zhang et al. 2004). In addition, the intestinal tract of grass carp is rich in amylase and cellulase (Wu et al. 2020), and the intestinal tract of black carp is rich in protease and lipase (Zhou et al. 2021). These facts indicate that metabolic matrix of methanogens in ponds and lakes mainly comes from hydrolytic fermentative bacteria, and methanogens can effectively use the H2 and CO2 generated by these hydrolytic fermentative bacteria to produce CH4, which also fully demonstrates that ponds and lakes are important natural emission sources of methane, and are dominated by hydrogenotrophic methanogens.
In conclusion, we revealed the community structure and richness characteristics of methanogens in the intestinal tract of black carp and grass carp and aquaculture water for the first time, and clarified the relationship between intestinal methanogens and aquaculture and greenhouse effect. These results will provide reference for the relationship between intestinal methanogens and aquaculture and greenhouse effect.