B. Koehne et Hw. Trissl, THE CYANOBACTERIUM SPIRULINA-PLATENSIS CONTAINS A LONG WAVELENGTH-ABSORBING PIGMENT C-738 (F-760(77K)) AT ROOM-TEMPERATURE, Biochemistry, 37(16), 1998, pp. 5494-5500
Spirulina platensis is a cyanobacterium which usually lives under high
-light conditions. Nonetheless, it is thought to contain the most red-
shifted antenna pigment of all known Chl a-containing phototrophic org
anisms, as shown by its 77 K fluorescence peaking at 760 nm. To exclud
e preparation artifacts and to exclude the possibility that long wavel
ength-absorbing pigments form only when the temperature is lowered to
77 K, we carried out experiments with whole cells at room temperature,
The combined analysis of stationary absorption and fluorescence spect
ra as well as fluorescence induction and time-resolved fluorescence de
cays shows that the pigment responsible for the 77 K fluorescence at 7
60 nm (i) has the oscillator strength of approximately one Chl a molec
ule, (ii) absorbs maximally at 738 nm (C-738(293K)), (iii) is present
only in the antenna system of PS I, (iv) participates in light collect
ion, and (v) does not entail a low photochemical quantum yield. Other,
more abundant but less red-shifted Chl a antenna pigments lead to a s
ignificantly larger absorption cross section of the photosynthetic uni
t of PS I above 700 nm compared to units that would not possess these
long wavelength-absorbing pigments. These results support the hypothes
is that the physiological role of long wavelength-absorbing pigments i
s to increase the absorption cross section at wavelengths of >700 nm w
hen in densely populated mats the spectrally filtered light is relativ
ely more intense at these wavelengths [Trissl, H.-W. (1993) Photosynth
. Res. 35, 247-263].