Circadian rhythm of photosynthesis in Kappaphycus alvarezii (Rhodophyta): independence of the cell cycle and possible photosynthetic clock targets
Various processes in the output pathway of the circadian clock are thought to act as important clock targets resulting in the circadian rhythms of photosynthesis observed in various algae. Examples of such processes are synchronization of the cell cycle, pigmentation, and light or dark reaction of photosynthesis. The newly detected, robust photosynthetic circadian rhythm in the red macroalga Kappaphycus alvarezii was investigated in more detail with respect to rhythmically changing components within the photosynthetic apparatus. The following major results were obtained; (1) The growing tips of Kappaphycus (0 - 2 cm) were found to exhibit a diurnal and circadian rhythm of nuclear division like many other algal species, (2) The circadian photosynthetic rhythm was apparent in the actively growing and dividing tip portions (0 - 2 cm) as well as in older portions (2 - 4 cm) with little remaining mitotic activity. The Kappaphycus rhythm seems therefore to be independent of the cell cycle, at least in the older portions of the thallus, (3) During real (L : D cycle) or subjective (LL) dark phases, net photosynthetic capacity (Pmax) dropped drastically in young (tip) parts of the thallus, and α, the 'light affinity' parameter, decreased likewise. The net result of these two changes was an increase in the light saturation point, Ek. Dark respiration did not change rhythmically from one circadian maximum to the next circadian minimum. ΔF/Fm′ dropped during real and subjective night phases, while non-photochemical quenching (NPQ) increased. Low temperature (77 K) emission spectra with an excitation wavelength of 580 nm exhibited a larger increase of the 720 : 685 nm as well as the 720 : 696 nm emission quotients compared with spectra at 440 nm excitation for samples harvested in the middle of the subjective dark phase indicating changes in energy trapping from the phycobilisomes to the photosystems.
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