Like certain of their marine relatives, green hydra possess a population of symbiotic algae: an average of twenty or so green single-celled Chlorella lodged in vacuoles inside each of the digestive cells of the hydra's endoderm, like individually-wrapped apples in supermarket baskets. The Chlorella release to their host about half of the carbon they fix by photosynthesis, in the form of the simple sugar maltose; the hydra supplies nitrogen and phosphate. But as far as the hydra is concerned, it is not an obligate relationship. If you expose green hydra to intense light in the presence of the herbicide DCMU, which specifically disrupts the chain of reactions by which photosynthesis converts light energy to chemical energy, the Chlorella cells are internally poisoned and, dead or dying, are expelled by their hosts. The hydra are, to use the term coined by the inventor of this method, bleached. Turned into algae-free albinos that provide useful controls for experiment that probe the symbiotic relationship.
Maybe you can see where I'm going with this. Currently, reef corals in the Great Barrier Reef and elsewhere around the world are bleaching. Spitting out symbiotic algae (brown zooxanthellae rather than green Chlorella) whose photosynthetic capabilities have been adversely affected by a rise in sea temperature above the normal seasonal maximum. It's the third mass global bleaching event in less than two decades (the first, the first ever, was observed in 1998; the second in 2010). Unlike green hydra in the laboratory, reef corals need their symbionts to survive. Some corals can recover, reacquiring zooxanthellae from the environment or from remnant populations in their tissues. The rest die. In the areas of the Great Barrier Reef affected by this bleaching event, about 50% of the reef corals are expected to be lost.
It's a signal event in global warming. It shows us that the effects of human activity can reach inside the cells of reef corals, reach into the chloroplasts of their symbiotic algae, and disrupt their normal activity. A vast uncontrolled experiment, with no planning or endpoint, in the only biosphere we possess; the biosphere we must also inhabit. There are no controls.