Coral reefs are highly diverse ecosystems that thrive in nutrient-poor seas, a phenomenon frequently referred to as the “Darwin Paradox”. Nonetheless, even in nutrient-poor ocean basins, reefs can be replenished regularly with dissolved inorganic N and P from deeper, nutrient-rich water through upwelling, internal waves and vertical mixing. Substantial amounts of nutrients in their inorganic form are also released by filter feeders in the intimate vicinity of the corals or enter the reef from land-based sources. These inorganic forms of N and P constitute a key nutrient pool, which is, however, primarily accessible to the photosynthetic symbionts and not to the coral host. Accordingly, it is unclear to which extent dissolved inorganic nutrients contribute directly to the success of symbiotic corals.
We demonstrate that symbiotic coral animals can satisfy a substantial amount of their N and P demand through ‘farming’ and digestion of excess symbiont cells. Thus, we show that both the symbionts and the host gain growth-related benefits through the efficient, reciprocal exchange of the essential cellular nutrients. Since both organic and inorganic sources of N and P are overall scarce in oligotrophic tropical waters, the symbiotic lifestyle offers corals a truly competitive edge over exclusively heterotrophic animals that rely solely on N and P in organic forms, or exclusively autotrophic plants such as macroalgae that are restricted to N and P in dissolved inorganic form.
The fully closed cycle of reciprocal N and P exchange between the symbiotic partners can explain the evolutionary and ecological success of symbiotic corals in well-lit, nutrient-limited warm water habitats. It also underpins the vulnerability of symbiotic corals to disturbances of their nutrient environment.