[chris (OP)]

Occasionally water, when perturbed, emits a transient glow. This is apparently down to marine algae. But what causes the algae to glow and why would they "want" to emit light in this way?

[RD]

Spotlighting what's eating them would make it more liable to predation by something higher-up the food-chain.

[chris (OP)]

Interesting theory, although that sounds slightly dubious to me. If the organisms are already inside a whale's mouth, illuminating themselves sounds fruitless...?

[Colin2B]

Occasionally water, when perturbed, emits a transient glow. This is apparently down to marine algae. But what causes the algae to glow and why would they "want" to emit light in this way?

I've always known it as phosphorescence and I thought it was a range of plankton not just algae, but including flagellates.
When I last read about it there seemed to be a variety of chemical methods used including action of enzymes.
Suggestions for an advantage include attracting a mate, to distracting a predator. The latter might work similar to the confusion experienced by predatory fish and raptors when faced by a shoal of fish or flock of birds.

[RD]

Interesting theory, although that sounds slightly dubious to me. If the organisms are already inside a whale's mouth, illuminating themselves sounds fruitless...?

The animal* eating the algae would trigger the illumination by moving in the cloud of algae. Even if it could remain stationary in the cloud, the algae close to its chomping-chin would be activated and visible , ( turbulence caused by gill & fin movement may be sufficient trigger illumination ).

So the luminescent-algae and the predator(s) eating the algae-eaters have symbiotic relationship.

[* fish rather than whale , as whales have few predators ]