Algae

Algae are not plants, and that is not a technicality. A seaweed has no roots, no vessels, no leaves and no flowers, and never had any. The pad that holds it to the rock is an anchor and nothing more, taking up neither water nor food; the whole body feeds itself out of the water it is sitting in. Nor are they one lineage. Green, red and brown seaweeds arrived at photosynthesis by different routes, and a brown alga runs its chloroplasts on machinery acquired second hand, from a red alga that one of its ancestors swallowed and kept.

The reef runs on them, which is easy to miss because a healthy reef looks as though it has almost no weed on it. The low fuzz on dead coral is turf algae, among the fastest-growing things in this sea, and it is eaten as fast as it appears by parrotfish, surgeonfish and urchins. Cage off a patch so nothing can graze it and within weeks it is shaggy. Lose the grazers across a whole reef and the same thing happens at that scale, with the coral losing the space it needs to settle.

Some of them are stone. Coralline algae lay calcium carbonate down inside their cell walls and grow as hard pink and purple crusts over dead coral and rubble. They are the mortar of the building: they cement loose pieces into a structure that holds against surf, and they carry the chemical cue that tells a drifting coral larva it has found somewhere worth settling. A reef stripped of its coralline crusts has trouble rebuilding itself.

Red algae reach deeper than anything else that photosynthesizes anywhere on Earth. Their accessory pigments capture the blue light that is all that survives at depth, and crusts of them grow far below the last living coral, on rock where a diver's torch is the only light that has been there in a long time.