Grox
The Grox is a large single cell, sustained by its many nuclei. It lives a few centimeters deep in marine sediment in dark, cold conditions. Grox bodies are like watery, squishy, gummy cakes: they're 2 mm long, and 0.7 mm (700 pm) thick.
Groxes, in essence, eat detritus and breathe manganese. More precisely, the Grox's electron donors and carbon source are various organic compounds found in detritus, and its electron acceptors are manganese compounds.
A Grox takes up manganese compounds and stores it as microscopic crystals of manganese compounds inside its body, which makes its body pink. It then passes electrons to the crystals to extract energy. Since it carries manganese inside its body, it effectively has an "air supply" that lets it "breathe" when not next to manganese sources. While Groxes cannot breathe oxygen, they are able to tolerate very low oxygen levels. (<5% oxygen) It's an essential trick, since, although it lives a few centimeters deep in marine sediment, the surrounding cold water holds a lot of oxygen.
The tiny needle inside its body is made of bixbyite, a manganese-iron mineral. The needle makes it harder for predators to eat Groxes, pricks the mouths of predators, and often lodges in predators’ gastrointestinal systems. The needles can scrape the insides of predators’ guts, or, in higher amounts, temporarily block the passage of food through predators’ guts. The microscopic manganese crystals in its body also give the Grox a somewhat gritty texture, like a ripe pear. With enough time, chomping down on these crystals wears away the teeth of smaller, weaker predators, leading to their starvation. At time of evolution, predators either learn to avoid eating Groxes or eat them only sparingly. While its defenses are imperfect, the fact Groxes live hidden in silt and often reproduce quickly means they do not really need better defenses.
Its metabolism and growth rate varies drastically depending on how much food there is, as well as its environment's temperature and oxygen levels. Groxes move very slowly, with an irregular outline. When the levels of manganese, organic matter, oxygen are just right, Groxes can reproduce so rapidly as to stain the soil pink.