When we view corals we should not just be thinking of coral. What do i mean? You should be viewing it as the coral holobiont. To me that includes, the coral itself, the zooxanthellae AND the whole host of bacteria living in the coral(mucus, tissue) Why are corals interesting. Well, they live in really nutrient poor environments. Think of it like a desert. So how Do corals get the nutrition they need to even survive. Let alone thrive. Corals are the ultimate nitrogen conserving org