Influences of high-level atmospheric gaseous elemental mercury on methylmercury accumulation in maize (Zea mays L.)

Maize (Zea mays L.) leaves play an important role in stomatal uptake and surface adsorption of atmospheric mercury (Hg). However, the influence of atmospheric gaseous elemental mercury (GEM) on methylmercury (MeHg) accumulation in maize plants is poorly understood. In this study, we conducted a field open-top chambers (OTCs) experiment and a soil Hg-enriched experiment to investigate the response of MeHg accumulation in maize tissues to different GEM levels in the air. Maize upper leaves had a higher average MeHg concentration (0.21 ± 0.08 ng g−1) than bottom leaves (0.15 ± 0.05 ng g−1) in the OTCs experiment, which was inconsistent with that in the soil Hg-enriched experiment (maize upper leaves: 0.41 ± 0.07 ng g−1, maize bottom leaves: 0.60 ± 0.05 ng g−1). Additionally, significantly positive correlations were found between MeHg concentrations in maize leaves and air Hg levels, suggesting that elevated air Hg levels enhanced MeHg accumulation in maize leaves, which was possibly attributed to methylation of Hg on leaf surfaces. Mature maize grains from the OTCs experiment had low MeHg concentrations (0.12–0.23 ng g−1), suggesting a low accumulation capability of MeHg by maize grains. Approximately 93–96% of MeHg and 51–73% of total Hg in maize grains were lost from the grain-filling stage to the grain-ripening stage at all GEM level treatments, implying that self-detoxification in maize grains occurred. MeHg concentrations in maize roots showed a significant linear relationship (R2 = 0.98, p < 0.01) with soil Hg levels, confirming that MeHg in maize roots is primarily from soil. This study provides a new finding that elevated air GEM levels could enhance MeHg accumulation in maize leaves, and self-detoxification may occur in maize grains. Further studies are needed to clarify these mechanisms of Hg methylation on maize leaf surfaces and self-detoxification of Hg by maize grains.