Question

### Passage

Coral Bleaching Mechanisms

Coral bleaching occurs when coral polyps expel the symbiotic algae (*Symbiodinium*) living in their tissues, turning the coral completely white. Three scientists discuss the primary driver of this phenomenon.

Scientist 1
Coral bleaching is primarily triggered by elevated sea surface temperatures (SST). When SST exceeds the local summer maximum by $1^\circ\text{C}$ or more, thermal stress disrupts the photosynthetic pathways of the algae, producing toxic reactive oxygen species. To survive, the coral host must expel the algae. While increased solar radiation (UV exposure) can exacerbate bleaching, elevated temperature is the essential and direct cause of the algal expulsion.

Scientist 2
The primary driver of coral bleaching is ocean acidification, not thermal stress. As atmospheric $\text{CO}_2$ dissolves in oceans, seawater pH declines. The reduced availability of carbonate ions stresses the coral’s calcification process, destabilizing the cellular connection between the host and the algae. Elevated sea surface temperature is not the direct trigger; rather, warmer water increases the metabolic rate of the coral, which accelerates the bleaching process only after acidification has already weakened the coral host (when pH falls below $7.9$).

Scientist 3
Coral bleaching is an infectious disease process caused by opportunistic bacterial pathogens, such as *Vibrio coralliilyticus*. These bacteria are abundant in marine environments but only become virulent at higher temperatures. Under normal conditions, the coral's immune system prevents infection. However, elevated sea surface temperatures suppress the coral’s immune response and increase bacterial toxin production, leading to infection and subsequent algal expulsion. Thus, elevated temperature is a necessary environmental catalyst, but bacterial infection is the direct cause of bleaching.

Question

Based on the passage, all three scientists would agree with which of the following statements regarding the role of elevated sea surface temperatures in coral bleaching?