Bioaccumulation of mercury and other potentially toxic elements in Holothuria (Halodeima) atra and Polycarpa aurata associated with gold mining in North Sulawesi, Indonesia

Coral reef species are increasingly threatened by multiple stressors, including potentially toxic elements (PTEs) pollution. Depending on the extraction methods and mineralogy of local rocks, gold mines can release PTEs into the catchments and thus reach marine habitats. Although the use of mercury in industrial mines has been eliminated following the Minamata Convention, mercury amalgamation practices in artisanal and small-scale gold mines (ASGMs) persist in several Indo-Pacific regions. The contamination of PTEs in marine sediments and suspended particulate matter and their bioaccumulation in the edible, deposit-feeder, holothurian Holothuria (Halodeima) atra and the suspension-feeder ascidian Polycarpa aurata were assessed, comparing samples collected downstream to four mine sites (two industrial and two ASGMs) and at four control sites in the North Sulawesi, Indonesia. In sediments, mean concentrations of arsenic, gold, cobalt, chromium, copper, mercury, nickel, lead, antimony, and zinc were significantly higher at sites receiving mine discharges than at control sites. Particularly, mercury overcame the upper guideline value downstream of the oldest ASGM. High mercury concentrations were also found in suspended particulate matter downstream ASGMs. Significantly higher concentrations of As, Au, Co, Cr, Cu, Hg, Pb, Sb, Sn, and Zn were found in holothurians near mines, especially ASGMs. There, the ascidians also showed significantly higher concentrations of Hg and Cu. Environment contamination and bioaccumulation varied according to mining processes, environmental matrix, and species; however, concentrations and bioaccumulation of PTEs, especially mercury, were very high, posing risks for both the environment and human health. To reduce the impact and possible long-term consequences of gold mines on marine coastal habitats, it is necessary to phase out mercury amalgamation for gold extraction, regulate artisanal mining, and enforce environmental laws; moreover, regular PTEs monitoring, possibly including “biomonitors” like this holothurian and ascidian species, must be a priority objective for many countries in the Indo-Pacific regions.