Acetotrophic methanogens are sensitive to long-term nickel contamination in paddy soil

Paddy soil accounts for approximately one-fifth of the world's cultivated area and faces a serious threat from nickel (Ni). Ni pollution has an impact on the activity, composition and emission of methanogens in paddy, which is a major natural source of methane (CH4) emissions. We combined a high-throughput sequencing approach and laboratory incubation methods to evaluate the impact of long-term Ni pollution on the methanogenic archaeal community in paddy soil. The highest rate of CH(4)production was 697 mg kg(-1)of dry soil per d with the addition of sodium acetate at 50 mg kg(-1)of Ni, which was significantly negatively correlated with the total and available Ni (p< 0.05). While the highest CH(4)production rates were 485 and 544 mg kg(-1)of dry soil per d with the addition of sodium formate and methanol, respectively, there was no significant difference in the CH(4)production rate and maximum CH(4)accumulation between the different Ni additions. Heavy pollution with 500 mg kg(-1)of Ni unexceptionally inhibited the relative abundance of various genera of methanogens (22.2% in total). The abundance of acetotrophicMethanosaetadecreased with an increasing concentration of Ni (3.25-1.11%). The diverse nutrient types of species belonging toMethanosarcinawere the highest under treatment with Ni200 (18.0%), and lowest in the soil with 500 mg kg(-1)of Ni (2.8%). Similarly, the abundances of the most abundant hydrogenotrophic methanogens ofMethanocellaleswere relatively high with Ni200 (26.2%) compared with those with Ni500 (5.4%). ThemcrA gene was enriched under the light pollution treatment (50 mg kg(-1)of Ni, 6.73 x 10(7)+/- 9.0 x 10(6)copies per g of soil) compared with the control (4.18 x 10(7)+/- 5.1 x 10(6)copies per g of soil). These results indicate that the long-term pollution by Ni has an impact on the activity and composition of methanogens with heavy Ni pollution, and in particular, acetotrophic methanogens are sensitive to Ni pollution in paddy soil.