Reef Coral Nutrition
Reefs corals are best described as adaptively polytrophic, which means they can acquire energy from a variety of sources. Up to 95% of the inorganic carbon fixed by zooxanthellae is translocated to the coral host in the form of glycerol. Energy that the coral host acquires by metabolizing this glycerol is supplemented by opportunistic heterotrophic feeding.
Corals can capture food by phagotrophy (tentacles grab passing food items and pass to the mouth, where they are ingested), and by ciliary feeding (the mucus sheath covering the coral traps tiny organic particles which are wafted to the mouth by small hairs called cilia). Corals probably also take up dissolved organics from seawater for use as an energy substrate. A summary of energy gains and losses is found here.
In turn, the zooxanthellae receive vital inorganic nutrients from the coral host, which are passed along to the zooxanthellae as animal waste products. Some inorganic nutrients are also obtained from seawater. See summary of energy transfer here.
Carbon fixation (primary productivity) on coral reefs ranks these ecosystems as the most biologically productive on the planet (reef flats produce around 3.5kgC/m2/yr, compared to around 2kgC/m2/yr for seagrass beds and tropical rainforests, and around 1kgC/m2/yr for temperate deciduous forests).
The extremely tight recycling of nutrients within the reef coral symbiosis explains why they are so well adapted to life in low nutrient environments. They simply outcompete most other benthic lifeforms for space on the reef (which the reef corals themselves are actively building as they compete with each other). Increases in the concentration of nutrients in reef environments can have severely detrimental effects on reef corals.
The degree to which corals need to feed heterotrophically to supplement the carbon translocated from their symbionts depends on how actively the symbionts are photosynthesizing. If photosynthesis (P) by the zooxanthellae exceeds the respiration requirements (R) of both the coral host and its zooxanthellae (i.e., if P:R>1) the coral is fully autotrophic and requires no supplementary feeding. When photosynthesis drops (P:R<1), the coral requires and additional food source. As a result, deep water corals need to feed more than those in shallow water.