I follow a few environmental blogs, and for a while I was occasionally seeing posts about the possibility of using iron fertilization for carbon sequestration. The idea (as I understand it) is that scientists would dissolve a bunch of iron near the ocean surface, phytoplankton would consume the iron (causing the phytoplankton to flourish), the phytoplankton would inhale a bunch of carbon dioxide, and the phytoplankton would sink to the bottom of the ocean, taking the CO2 with it, forever.
So they tried it.
They dumped a bunch of iron in the ocean, and the phytoplankton dutifully multiplied (and presumably inhaled a lot of CO2). But before the phytoplankton could sink, it ended up at the bottom of the food chain of a series of increasingly large sea creatures that live near the surface.