Second, while toxic, mercury has no specific utilization biochemistry pathways. Most organisms never before had to deal with high quantities of heavy metals and 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 food chain, it needs to be converted to insoluble form, for example, mercury sulfide, which is main mercury ore, cinnabar. Problem is, we can’t do this ourselves just as we can’t simply convert excess CO2 in atmosphere back to coal and oxygen, we don’t have some bacteria to eat it and by itself reaction of free mercury with free sulfur would take literally ages.
As of now we are saved by the fact that ocean is huge and mercury gets diluted below dangerous levels – as long as we do not dump more waste right where we fish.