The meadow makes the poison
In the turquoise waters of the Myeik Archipelago, a sprawling ecosystem of sea grass meadows has a complex biogeochemical process. These underwater fields, covering an estimated 878 acres, are habitats; they actively facilitate the conversion of inorganic mercury into its much more toxic organic form, methylmercury. This potent neurotoxin is the form of mercury that readily enters food webs and accumulates in organisms, including the fish that are a staple for local communities.
The source of the base mercury can be both atmospheric deposition from global industrial activity and local sources like gold mining. This inorganic mercury settles in the marine sediments. In the low-oxygen conditions of the sediment beneath the dense sea grass, specific types of microbes, particularly sulfate-reducing bacteria, thrive. These bacteria are the primary agents of mercury methylation. In a process linked to their respiration, they convert the relatively benign inorganic mercury into the dangerous methylmercury (MeHg). The dense root systems of sea grass create a stable, organic-rich environment ideal for these methylating bacteria, effectively turning the sea grass beds into hotspots for methylmercury production.
From microbe to meal
Once formed in the sediments, methylmercury moves up the food chain. It is first absorbed by microorganisms and phytoplankton at the base of the food web. This initial step from seawater into phytoplankton can concentrate methylmercury by a factor of up to one million. Zooplankton then consume the phytoplankton, and small fish eat the zooplankton. At each step, the methylmercury is transferred and becomes more concentrated in the predator's tissues—a process called biomagnification.
Studies have shown that over 95% of the mercury found in fish is in the form of methylmercury. Research in the Myeik Archipelago indicates a direct relationship between the density of sea grass meadows and the levels of mercury in local fish. Species living in or near dense meadows show higher concentrations of the toxin, irrespective of their distance from known pollution point sources. This suggests the habitat itself is the dominant factor driving contamination. Fish species commonly found and consumed in the region, such as groupers (Epinephelus tauvina) and seabass (Lates calcarifer), are part of this food web and can accumulate significant levels of mercury.
The archipelago hosts at least 11 species of sea grass. Dominant species include Cymodocea rotundata and Thalassia hemprichii, which form extensive meadows that shelter juvenile fish and invertebrates, while also driving the cycle of mercury toxicity.