i Evidence of increased mercury methylation after soil disturbance by forest harvesting

Candidate: Moiz Syed
Supervisor: Dr. Stephanie Melles

ABSTRACT

Methylmercury is a potent neurotoxin that can have a negative impact on human health. The formation of methylmercury is a biotic process driven primarily by microorganisms possessing the hgcAB gene pair in anaerobic environments. During forest management operations, harvesting equipment can disturb forest soils, creating wheel track depressions where water collects to form pools. These pools can be quite numerous in the landscape, and they may be hotspots for mercury methylation. As part of a multi-university, industry, and government partnership project focused on improving forest harvesting practices to reduce the risk of mercury methylation (and mobilization), we sampled soil from disturbed areas and pools paired with unimpacted control soils (n = 91). Our research focused on three experimental headwater catchments in boreal forest, subject to forest management and located in the vicinity of Dryden in Northwestern Ontario. Methylmercury and total mercury were measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Cold Vapor Atomic Fluorescence Spectroscopy (CVAFS), respectively. The community structure of the soil samples’ microbiome was determined using 16S rRNA next-generation sequencing (NGS). Results of soil chemistry analysis demonstrated that methylmercury concentrations in disturbed saturated soils were up to four times higher than in undisturbed control soils. However, total mercury levels did not differ significantly between post-harvest compared with pre-harvest soil samples. Genomic analysis demonstrated that the microbiome was more diverse in disturbed soils with evidence of increased prevalence of families of potential methylators. This study sheds light on how forestry operations can impact soil mercury concentrations and microbial communities in affected areas.