Written by Nereus Fellow Colin Thackray,
Mercury is found in very small concentrations in the ocean (less than a billionth of a gram per liter) yet is present in much larger concentrations in the fish we eat – especially those high up in the food chain such as swordfish and tuna. Methylmercury (MeHg), the highly bioaccumulative and toxic form of mercury, has negative effects on the health of people who consume it through eating fish. How does so much MeHg make it into the fish we eat when ocean concentrations are so small? The answer is through uptake by plankton and then bioaccumulation in the marine food web, and the first step in this process is a big one.
Mercury makes its way into the oceans mostly through the atmosphere, with a bit contributed by rivers, after being emitted mostly through human activities such as burning coal. The mercury that gets into the ocean this way is for the most part inorganic, but a portion of it gets converted to MeHg by microbes in the oceans. This small concentration of MeHg is what becomes magnified in the food web, with most of the magnification happening in the first step from seawater to plankton.
Plankton are the tiny organisms at the base of marine food webs which include tiny plants known as phytoplankton that get their energy from the sun, and the larger (but still-tiny) zooplankton which eat other plankton as their source of energy. Phytoplankton themselves come in a variety of microscopic sizes, ranging in diameter from about 1 to 100 microns (0.001-0.1 mm), with their size having a large effect on their MeHg uptake. Smaller phytoplankton end up with larger MeHg concentrations because their cell surface (through which the MeHg is passing) is bigger in proportion to their volume than in larger phytoplankton.
Through this uptake, phytoplankton end up with 500-500,000 times higher MeHg concentrations than the surrounding water, depending on if they are on the larger- or smaller-diameter ends of the scale, respectively. The structure of the phytoplankton community (i.e. how many of the phytoplankton are small, medium, or large) depends on nutrient availability, with common open ocean conditions leading to an abundance of the smallest, highly MeHg concentrated phytoplankton.
These phytoplankton are the food source for the larger but still very small zooplankton. Zooplankton ingest phytoplankton, along with any MeHg present in the phytoplankton but eliminate MeHg from their cells more slowly than they ingest it. This leads to bioaccumulation and magnification of MeHg concentrations in the zooplankton compared to the phytoplankton. Interestingly, the range of zooplankton concentrations under different conditions is not as large as the range of phytoplankton concentrations. This is because of the relationship between nutrient availability, MeHg uptake, and the total amount of phytoplankton. The conditions which cause the highest MeHg concentrations in phytoplankton are also conditions of food scarcity for their predators, leading to lower rates of consumption and therefore less zooplankton MeHg intake. The conditions associated with low phytoplankton MeHg are also the conditions where the zooplankton eat a lot; they ingest large amounts of MeHg due to the quantity of food available but are also growing more quickly and effectively diluting their own MeHg concentration through growth. This results in concentrations of MeHg in zooplankton to be 50,000-1,000,000 times higher than in the surrounding seawater. This large increase compared to the seawater, before even making it to fish, makes tiny concentrations of mercury in the ocean lead to much larger concentrations of mercury in marine food webs.
Amina T. Schartup, Asif Qureshi, Clifton Dassuncao, Colin P. Thackray, Gareth Harding, and Elsie M. Sunderland. A Model for Methylmercury Uptake and Trophic Transfer by Marine Plankton Environmental Science & Technology 2018 52 (2), 654-662 DOI: 10.1021/acs.est.7b03821