Climate-driven deoxygenation promoted potential mercury methylators in the past Black Sea water column

Abstract Deoxygenation in aquatic ecosystems threatens biodiversity at all levels of functional and genetic diversity. Recent studies have shown the prevalence of microorganisms that transform mercury into neurotoxic methylmercury (mercury methylators— hgcA + prokaryotes) in oxygen-deficient water columns. As climate warming expands coastal oxygen minimum zones, ongoing and near-future changes may ultimately lead to increased methylmercury formation. However, little is known about the presence of aquatic mercury methylators before the Industrial Revolution, marked by increased mercury emissions and deposition in the environment. Here we have detected hgcA genes in Black Sea sedimentary archives, with the highest abundance 9,000–5,500 years ago when anoxic conditions were documented in the water column. Historical sedimentary and modern water column data on mercury methylators provide valuable insights for projecting future methylmercury production in aquatic ecosystems impacted by ongoing deoxygenation. It also underscores the potential impacts of climate change on human exposure to methylmercury from mercury-contaminated seafood.