Whose study first described the role of zooxanthellae in building coral reefs?

The idea that reef-building corals live with microscopic algae was developed through work by several scientists, beginning in the 19th century. Johannes Müller and later investigators showed that these algae, now called zooxanthellae, live inside coral tissues and provide much of the energy corals need. That partnership explains why many corals live in clear, shallow tropical water where sunlight reaches the seabed. Modern reef science still relies on this discovery to understand coral bleaching and reef decline.