They are particularly damaging to vertebrates that live at or near the top of food chains where bioaccumulation causes an increase in concentration of any substance that cannot be excreted by animals. With each new level of predator there is a further multiplication of the concentration of PCBs by about ten times. Diatoms are the smallest plants in the ocean. They are also the most numerous and constitute as well the greatest total biomass of any plants on earth. All life in the oceans, from the tiniest planktonic animals to the largest whales, is absolutely dependent on diatoms to trap energy from the sun.
Diatoms are single cells encased in silica shells.
Silica is basically sand and so these outer shells are really constructed of what we think of as stone. Diatoms would sink like stones and end up in the dark depths of the ocean out of the light they need to nourish them if they didn't have something to prevent this from happening and to keep them up near the sunlight where they can trap its energy. The thing they use for flotation is a tiny droplet of oil. But when any molecule of, say, a PCB touches the droplet of oil in a diatom, it will immediately go into solution in that oil-that is, in the diatom. Because the diatom has no way to break down the PCB molecule, it just stores it.