The symbiosis between corals and the dinoflagellates—zooxanthellae—is the key to a tight recycling of nutrients on reefs that generally thrive best in nutrient poor parts of the oceans. But several other mechanisms and species groups aid transmission of organic matter and energy along the numerous food chains of a reef. Viruses, bacteria, and archaea are key to the recycling of carbon and organic compounds, making the ‘microbial loop’, one key but invisible aspect to how the reef functions. Cyanobacteria, formerly blue-green algae, are a major part of the micro-benthos too, and are important primary producers. Protists are also hugely abundant—larger, single-celled organisms which are eukaryotes with cells with nuclei, and this group has species that exist in planktonic and benthic forms. Foraminifera are important protists, being abundant and having calcareous tests, so that they are significant sand producers in some areas. Finally, zooplankton provide food for numerous reef species, and indeed larvae from all species form part of the plankton temporarily too.
