A piece of an article others found:
Sunlight impinging on the reef kicks off the energy cycle by promoting photosynthesis within free-swimming plankton in the water column and also within the zooxanthellae of the scleractinian corals that we obsess about in our aquaria. Photosynthesis utilizes the sunlight's energy to "fix" (= attach) inorganic carbon in the form of CO2 to organic chemical structures that eventually become carbohydrates (cf. Fig. 1). These carbohydrate building blocks are chemically manipulated further by the zooxanthellae and/or the coral host and then secreted by the coral as coral mucus (a combination of complex polysaccharides, amino acid oligomers/polymers, lipids, etc., cf. Fig. 1). The carbohydrates and amino acids within the mucus serve as food sources for the bacteria and other microbiota that comprise the foundation of the marine food pyramid (Kirchman, 1990; Rich, 1996; Weiss, 1999; Wild, 2004; Sharon, 2008). The remaining reef inhabitants, including the very corals that house the zooxanthellae, then feed on these energy rich microbes (or, in turn, on the organisms that eat the microbes, ad infinitum), thus perpetuating the reef's nutrient recycling food web.