Circadian rhythms in the motility (floating versus immersion) in Nereocystis luetkeana
Bull kelp, Nereocystis luetkeana, brown algae on the Pacific coast of North America in our field observations, when sunlight, actively position their stipes/blades exposed floating on the water surface, while at dusk get immersed hence invisible (with mostly the bulbs remaining observable on the surface). The suggested interpretation is of a phototropic circadian directional (vector) motility (as opposed to haphazard passive relocations due to low tides and water turbulence). This was unrecognized in the algae, generally rendered too primitive for such vector motor activity coordinated at the level of the whole organism. This mechanism is principally different from the previously recorded intracellular circadian rhythms in mitotic activity, photosynthesis and growth of juvenile sporophytes in some Laminariales. Floating exposure of the stipes/blades is maximal from sunrise till early afternoon and seems to gradually decrease later only to drop by the dusk. Those motility changes may occur as fast as within half an hour.