Search

Abstract Several strains of archaea have the ability to methylate or resist mercury (Hg), and the paddy field is regarded to be conducive to Hg methylation. However, our knowledge of Hg-methylating or Hg-resistant archaea in paddy soils is very limited so far. Therefore, the distribution of archaea and bacteria in the rhizosphere (RS) and bulk soil (BS) of the rice growing in Xiushan Hg-mining area of southwest China was investigated. Bacterial and archaeal 16S rRNA gene amplicon sequencing of the rice rhizosphere along the Hg gradient was conducted. THg concentrations in RS were significantly higher than that in BS at site S1 and S2, while MeHg concentrations in RS was always higher than that in BS, except S6. Bacterial species richness estimates were much higher than that in archaea. The bacterial α-diversity in high-Hg sites was significant higher than that in low-Hg sites based on ACE and Shannon indices. At the genus level, Thiobacillus , Xanthomonas , Defluviicoccus and Candidatus Nitrosoarchaeum were significantly more abundant in the rhizosphere of high-Hg sites, which meant that strains in these genera might play important roles in response to Hg stress. Hg-methylating archaea in the paddy field could potentially be affiliated to strains in Methanosarcina , but further evidence need to be found. The results provide reference to understand archaeal rhizosphere community along an Hg gradient paddy soils. Graphical abstract Unlabelled Image Highlights • Communities of bacteria and archaea in Hg-polluted rice rhizosphere were studied firstly. • Bacteria between high and low-Hg sites clustered together respectively, so did archaea. • merA abundance was significant higher in high-Hg sites than low-Hg sites. • Methanogen Hg-methylators could potentially be affiliated to Methanosarcina. [ABSTRACT FROM AUTHOR]

Methylmercury (MeHg) in rice is presumed to be derived from MeHg formed in the soil, although it is still controversial. Moderate soil mercury (Hg) concentration can affect the diversity of soil microorganisms and may also impact the physiological changes and MeHg absorption of rice. In this study, the pot experiment was conducted to explore the effects of Hg concentration gradients (0, 0.3, 3, and 30 mg kg−1) stress on Hg transformation in the rhizosphere, Hg translocation in rice, and physiological changes in rice leaves during the whole rice growing season. Moderate soil Hg concentration (3 mg kg−1) greatly increased the MeHg/THg (1.69%) of rhizosphere, while 30 mg kg−1 soil Hg concentration sharply reduced the MeHg/THg (0.29%) of rhizosphere. Highest MeHg/THg of the four groups all appeared at the blooming or filling stage. There was a significant positive correlation between Fe2+ in rhizosphere and MeHg/THg, but no significant correlation between SO 4 2− and MeHg/THg was observed. Although the 3 mg kg−1 soil Hg concentration significantly enhanced MeHg concentrations in seeds, it considerably reduced the bioaccumulation factors of MeHg in roots, stalks, old leaves and young leaves. Soil Hg concentration of 30 mg kg−1, to a certain extent, curtailed MeHg concentrations in seeds, while MeHg concentrations in the husk were significantly increased. Consistent with the result that there was no significant difference for THg concentrations in old and young leaves among the four Hg treatment groups, the content of chlorophyll, H 2 O 2 , malondialdehyde and antioxidant substances, and the activities of antioxidant enzyme in old and young leaves varied indistinctly among groups. Moderate soil mercury concentration (3 mg kg−1) could extremely enhance MeHg production in the rhizosphere soil and its accumulation in rice; MeHg production in the rhizosphere soil increased greatly at the blooming or filling stage, whereas little effect on antioxidant systems in leaves was observed. Unlabelled Image • Highest MeHg/THg occurred with soil mercury concentration of 3 mg kg−1. • Highest MeHg/THg appeared at the blooming or filling stage. • MeHg concentrations in seeds reduced to some extent under Hg stress of 30 mg kg−1. • Little effect on the antioxidant system in leaves with mercury gradient was observed. [ABSTRACT FROM AUTHOR]