Role, Routes and Effects of Manganese in Crustaceans

This review provides an overview of the role, routes and effects of manganese in aquatic crustaceans. Manganese is a naturally abundant metal in marine and freshwater sediments where it is involved in a large number of chemical processes. Although sediments contain high natural concentrations of manganese, the potential danger to benthic organisms has been neglected in studies to date. Manganese bioavailability increases as the result of human impact and it accumulates in biota. Manganese may occur in toxic concentrations (10–20 mg l–1) in the bottom water of marine coastal areas after hypoxia, or more locally (e.g., close to industries) as well as in acidic lakes and aquaculture shrimp ponds. Though manganese is an essential metal, it is also an unforeseen toxic metal in the aquatic environment. Although the uptake and elimination of manganese is rapid, manganese affects processes that decrease the fitness of organisms. As manganese bioavailability increases, its uptake is predominately through the water. The midgut gland, nerve tissue, blood proteins and parts of the reproductive organs have the highest accumulation factors and are the main target tissues. The functional effects of manganese in aquatic environments are still sparsely investigated. Recent results show that the immune system, the perception of food via chemosensory organs and a normal muscle extension are affected at manganese concentrations observed in the field. Geochemical role of manganese Manganese is the 12th most common element, the fourth most abundant metal and is universally distributed in the earth’s crust and waters (Anonymous 2005). This metal is involved in a large number of chemical processes, due mainly to its redox sensitivity. The literature on manganese (Mn) geochemistry in the aquatic environment is immense (Elderfield 1976), whereas literature on the occurrence and biological effects of manganese in aquatic animals is comparatively sparse. Manganese concentrations in soil vary from 0.001–7 mg g–1 dry weight (dw), averaging 0.75 mg g–1 dw (Saric 1986). Ocean sediment concentrations vary from approximately 1–50 mg g–1 dw (Elderfield 1976). Since the 1800s an intensive and ongoing debate has been centred on the origin and amount of the manganese flux to the oceans. Three main sources have been identified: continental weathering (lithogenous origin), submarine volcanism and an upward migration in porewaters as a consequence of sediment diagenesis (Elderfield 1976). The anthropogenic supplies of manganese to aquatic biotopes derive mainly from mine tailings and from steel manufacturing industries where approximately 90% of total manganese is used as a deoxidising and desulphurising additive and as an alloying constituent (Saric 1986). Manganese (MnO2) is also widely used in dry cell batteries (Saric 1986), as a contrasting agent for nuclear magnetic resonance tomography, and as an agricultural fungicide (Gerber et al. 2002). A manganese antiknock additive (methylcyclopentadienyl manganese tricarbonyl (MMT)) was introduced to Canada in 1990 to substitute for lead in fuel,