Mercury is one of the most hazardous contaminants that may be present in the aquatic environment, but its ecological and toxicological effects are strongly dependent on the chemical species present. Species distribution and transformation processes in natural aquatic systems are controlled by various physical, chemical, and biological factors. Depending on the prevailing environmental conditions, inorganic mercury species may be converted to many times more toxic methylated forms such as methylmercury, a potent neurotoxin that is readily accumulated by aquatic biota. Despite a considerable amount of literature on the subject, the behavior of mercury and many of the transformation and distribution mechanisms operating in the natural aquatic environment are still poorly understood. This review examines the current state of knowledge on the physicochemical behavior of mercury in the aquatic environment, and in particular the environmental factors influencing its transformation into highly toxic methylated forms.

Mercury in the Aquatic Environment: A Review of Factors Affecting Methylation

Bacterial resistance to inorganic and organic mercury compounds (HgR) is one of the most widely observed phenotypes in eubacteria. Loci conferring HgR in Gram-positive or Gram-negative bacteria typically have at minimum a mercuric reductase enzyme (MerA) that reduces reactive ionic Hg(II) to volatile, relatively inert, monoatomic Hg(0) vapor and a membrane-bound protein (MerT) for uptake of Hg(II) arranged in an operon under control of MerR, a novel metal-responsive regulator. Many HgR loci encode an additional enzyme, MerB, that degrades organomercurials by protonolysis, and one or more additional proteins apparently involved in transport. Genes conferring HgR occur on chromosomes, plasmids, and transposons and their operon arrangements can be quite diverse, frequently involving duplications of the above noted structural genes, several of which are modular themselves. How this very mobile and plastic suite of proteins protects host cells from this pervasive toxic metal, what roles it has in the biogeochemical cycling of Hg, and how it has been employed in ameliorating environmental contamination are the subjects of this review.

Bacterial mercury resistance from atoms to ecosystems

In this paper, we synthesize available information on the links between changes in ecosystem loading of inorganic mercury (Hg) and levels of methylmercury (MeHg) in fish. Although it is widely hypothesized that increased Hg load to aquatic ecosystems leads to increases in MeHg in fish, there is limited quantitative data to test this hypothesis. Here we examine the available evidence from a range of sources: studies of ecosystems contaminated by industrial discharges, observations of fish MeHg responses to changes in atmospheric load, studies over space and environmental gradients, and experimental manipulations. A summary of the current understanding of the main processes involved in the transport and transformation from Hg load to MeHg in fish is provided. The role of Hg loading is discussed in context with other factors affecting Hg cycling and bioaccumulation in relation to timing and magnitude of response in fish MeHg. The main conclusion drawn is that changes in Hg loading (increase or decrease) will yield a response in fish MeHg but that the timing and magnitude of the response will vary depending of ecosystem-specific variables and the form of the Hg loaded.

Recovery of Mercury-Contaminated Fisheries

Publisher Summary This chapter reviews achievements in mercury microbiology research during the past 35 years and identifies areas in which more information is needed to complete our understanding of how microbes in the diverse ecological niches that exist on earth interact with mercury. Mercury is a potent toxic substance, the toxicity of which is elicited at very low concentrations. Although all chemical forms of mercury are toxic, public health concerns are focused on methylmercury (MeHg). The major routes of human exposure to this toxic element are through the consumption of contaminated fish, where mercury is mostly present in its methylated form. This is the result of the bioaccumulation and biomagnification of MeHg in the aquatic food chain. MeHg is a neurotoxin that causes pathologies ranging from mild numbness of the extremities to blindness, loss of balance, and death. Because MeHg is more toxic than other forms of mercury, and mercury is mostly deposited in the environment in its ionic form, the biogeochemical cycling of mercury in the environment plays a key role in modulating mercury toxicity. Furthermore, microbial transformations play critical roles in the mercury geochemical cycle, and understanding the mechanisms of these transformations is essential for controlling mercury transport and accumulation in the biosphere.

Microbial transformations of mercury: potentials, challenges, and achievements in controlling mercury toxicity in the environment.

Speciation of mercury in surface and deep-sea waters in the Mediterranean Sea

Mercury biogeochemistry in the Idrija River, Slovenia, from above the Mine into the Gulf of Trieste

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Mercury biogeochemistry in the Idrija River, Slovenia, from above the Mine into the Gulf of Trieste