Ph.D. student Alexandra Wen uses Pulse-Amplitude-Modulation (PAM) fluorometry to measure photosynthetic activity in bleaching elkhorn corals off the coast of Key Largo, Florida, USA. These data help scientists understand coral stress levels in real-time and document how extreme conditions impact reef health. An unprecedented marine heatwave in summer 2023 caused mass bleaching of many species of corals. Under extreme temperatures, corals turn white by expelling their tiny microscopic algae partners in a process called "bleaching". This deprives the corals of their main food source, since the photosynthetic algae provide most of the coral's nutrition. Without their algae, corals can starve and die in a matter of weeks or months. Untold numbers of Florida corals died due to severe bleaching in 2023. Sadly, the elkhorn corals in this photo (taken in mid-August) were all dead by late September.