Which form of mercury is fat soluble, thus most harmful for ecosystem?

CH₃Hg⁺ is the fat-soluble form of mercury, thus most harmful to the ecosystem.
Methyl mercury (CH₃Hg⁺) is the deadly form of mercury due to its higher membrane permeability and greater tissue fixation.
It affects the immune system, alters genetic and enzyme systems, and damages the nervous system, including coordination and the senses of touch, taste and sight.