My understanding is (1) Think of a hot day. Put a thermometer in the air (warm), then on the ground in the shade (cooler) and on the ground in full sun (hot, because you get not only the convective heating from the air but also radiation from the sun!).Another part of the documentary that bugs me is the "unexplained" florescent SPS colonies post bleaching event. Best explained was that it was protecting itself from UV rays. So why would it do that if it's been growing in that spot? Why would those colonies create some "unexplained" reaction to the Sun light? Has the Sun not been there for...ever? Or is the Sun getting brighter?
Consider the water is getting warmer (say it +10 degrees) and the sun keeps beating coral with radiation. The radiation isn't what's changing, it's just 10 degrees warmer ambiently. So the coral needs to photoprotect; it does that by fluorescing (i.e. emitting that excess energy). In order to thermally regulate to its desired level, the coral has more excess energy to emit because the water is warmer. Hence greater florescence.
But when coral get really thermally stressed, they actually downregulate gene expression of fluorescent proteins (i.e. lower fluorescent density). Why then do we see greater apparent fluorescence? Because (2) at that degree of thermal stress, the algal symbionts have been expelled. Those dinoflag's were dull colored and masking the bright fluorescence. So it's like taking your hand off a flashlight.
It's not 100% mechanistically worked out, but it's been bandied about and tested for over a decade now. edit: in controlled studies where only water temp and nothing else was changed.
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