2P086
Excitation energy transfer of Arthrospira platensis cells grown in seawater medium
probed by time-resolved fluorescence spectroscopy(Graduate School of Science, Kobe Univ.1, Graduate School of Engineering, Kobe Univ. 2,JST CREST3, Molecular Photoscience Research Center, Kobe Univ. 4)○Muhammad Arba1, Shimpei Aikawa2, 3, Kenta Niki1, Makio Yokono3,4,Akihiko Kondo2, 3, Seiji Akimoto1, 3, 4

[Introduction] Large-scale cultivation of Arthrospira (Spirulina) platensis is now intensively
researched worldwide due to its high potentials in pharmaceuticals, cosmetics, energy, andnutritious food source. Cultivation of A. platensis in seawater has been regarded as an
attractive option, owing to the low cost of seawater and the limited freshwater available forlarge-scale farming. The excitation energy transfer between pigments in photosynthetic
machinery largely governs the growth of oxygen-evolving organisms. However, changes inenvironmental conditions such as cultivation medium may induce changes in excitationenergy transfer within the photosynthetic system, resulting in abnormal pigment production.
In the present work, in vivo excitation energy transfer processes in A. platensis cells grown inboth f/2 medium and SOT medium as a control were studied. The SOT medium is wellknown as one of the optimal media for A. platensis, while the use of f/2 medium was intended
to imitate seawater.
[Experiment] The cyanobacterium Arthrospira platensis NIES-39 were grown under afluorescent lamp of light intensity 50 µmol photons m-2 s-1 for 7-8 days in SOT medium ormodified f/2 medium at 303 K (~30 °C) with agitation at 100 rpm. Steady-state absorption
and fluorescence spectra were measured with a spectrometer (JASCO V-650/ISV-722) and a1

spectrofluorometer (JASCO FP-6600/PMU-183), respectively, at low temperature (77 K).
Time-resolved fluorescence spectra were measured by a time-correlated single-photon
counting system at 77 K, with excitation wavelength 400 nm. Fluorescence rise and decay
curves were analyzed by global analysis to obtain fluorescence decay-associated spectra(FDAS). Photosynthetic oxygen evolution and dark respiration were determined with aClark-type oxygen electrode controlled by a computerized oxygen monitoring system.
[Results and Discussion] Growth in f/2 medium induced changes in absorption andfluorescence spectra as well as in the energy transfer pathways. Relative amount ofPhotosystem (PS) I red chlorophylls and phycobilisome (PBS) markedly decreased (Fig. 1),
fraction of PBS not incorporated
into energy transfer processes
increased, and energy transfer
betweenPCandAPC

wasslowed in the f/2 medium. Theenergy transfer from PSII to PSI
is highly inhibited in the f/2medium, which might be resultFig. 1

from the modification of proteins

medium (solid line) and f/2 medium (dotted line).

Absorption spectra of A. platensis cells grown in SOT

under salt stress. The PBS-to-PSI in parallel to PBS-to-PSII energy transfer pathways wereobserved in the two medium, however excitation energy captured by PBS was transferred
directly to PS I in f/2 medium, instead of being first transferred to an intermediate(PBS→PSII→PSI), as observed in SOT medium. It was also found in f/2 medium that
respiration rate increased, photosynthetic rate reduced, and the delayed fluorescence (DF)
lifetime was a half from that in SOT medium, which is probably result from inhibition ofelectron transfer in PSII and/or the decrease in antenna efficiency.
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