Mercury is a heavy metal, which is liquid at room temperature. And while we may only have seen it trapped in glass in thermometers, marine animals, unfortunately, have had a much worse experience. Being a part of the heavy metal family means that mercury stays where it reaches. It is not destroyed whatsoever and simply accumulates in whatever reservoir it is being poured. Mercury is an extremely poisonous substance and is hazardous in many ways when it accumulates. Now where does waste mercury come from, and how does it get to the sea? The answer to the question is as follows.
First, mercury is an important reagent in several heavy chemical industry processes. Waste from such processes is flushed out and eventually reaches the sea.
Second, while being toxic, mercury has no specific utilization of biochemistry pathways. Most organisms never had to deal with high quantities of heavy metals before, hence never evolved a way to render them harmless. This leads to a) organisms just keep getting more and more mercury, unable to remove it, and slowly dying from poisoning and b) mercury staying in water-soluble and bioaccessible form.
To remove mercury from the food chain, it needs to be converted to an insoluble form, for example, mercury sulfide, which is the main mercury ore, cinnabar. The problem is that we can’t do this on our own, just like we can’t convert excess CO2 in the atmosphere back to coal and oxygen. This is because we don’t have some bacteria to eat it, and by its own, the reaction of free mercury with free sulfur would take literally ages.
It is not only dangerous for the fish in whose system it enters, but it also becomes hazardous for the whole food chain. As there is no way to digest or convert mercury into less-harmless forms, it accumulates inside the body of fishes, either killing them or becoming food for larger, bigger species. Eventually, the mercury accumulates at the end of the food chain and causes chronic mercury poisoning. As of now, we are saved by the fact that the ocean is huge and mercury gets diluted below dangerous levels – as long as we do not dump more waste right where we fish.