There is a yellow worm, called Paralvinella hessleri, that lives near hydrothermal vents in the deep ocean. Its body weight is about 1% arsenic by mass; a ridiculous concentration. But instead of killing the worm, the worm “neutralizes” it and turns it into a yellow pigment. Inside the worm’s epithelial cells, arsenic is shuttled into vacuoles by protein transporters. At the same time, hydrogen sulfide (also abundant near hydrothermal vents) diffuses into the cells and binds to intracellular hemoglobin proteins, which carry it into the vacuoles. There, in the vacuoles, these two toxins react to form orpiment (As₂S₃), a bright, yellow mineral. The orpiment molecules form huge crystals; about 0.8–1.3 μm in diameter, or the size of an E. coli cell. Orpiment was used to make yellow paint during the Middle Ages and Renaissance. The Italian painter Cennino Cennini, in Florence, wrote about grinding up orpiment to make his yellow paints -- which was super toxic! When I first read through this paper, I wondered why the worms are bringing in arsenic in the first place. Why not just exclude it from entering into cells? Well, it seems like arsenic sneaks into cells through phosphate transporters, because they look similar, and evolution hasn’t found a way to exclude one but not the other. This also happens in plants, and it's known as "adventitious uptake." Poison sneaks into the organism through a protein that is required for survival! Organisms need phosphate to build DNA, make energy molecules, and so on. Finally, the authors did a proteomics experiment to figure out which proteins are expressed in these worms. They found 2,379 different proteins in the yellow granules, including a membrane protein that is evolutionarily similar to the Multidrug Resistant-associated Proteins found in microbes. (Microbes use those proteins to pump out antibiotics, for example.) It seems the worms adapted this protein to sit on each vacuoles' membrane and pump arsenic into them. They basically evolved a way to neutralize a poison with a poison.