"Coral reefs are home to the greatest density of species in shallow marine waters, including unusually diverse microbial communities. However, almost everything we know about reef microbial communities is confined to prokaryotes and viruses: next to nothing is known about the microbial eukaryotes, with the exception of the symbiont dinoflagellate Symbiodinium. Coral reefs are like most other marine environments, where protists are consistently overlooked. Chromera and Vitrella are two reef-associated algae assumed to have a symbiotic relationship similar to that of Symbiodinium. As photosynthetic relatives of apicomplexan parasites, they held answers to long-debated questions about plastid evolution, but equally important questions about their functional relationship to corals and the reef community have hardly been asked. It has been observed that bacterial sequence surveys are heavily ‘contaminated’ with eukaryotic plastid sequences. The apicomplexanrelated lineages (ARLs) are the richest source of still-unidentified plastid diversity, and virtually all of this diversity is restricted to coral reefs. We know they exist, but have no direct information on their biology or role in ecosystems whatsoever. ARL-V is the most common lineage, but we cannot even say whether the organisms are photosynthetic symbionts or intracellular parasites. To address these questions, an ecological approach was needed."