THE CYANOBACTERIUM SPIRULINA-PLATENSIS CONTAINS A LONG WAVELENGTH-ABSORBING PIGMENT C-738 (F-760(77K)) AT ROOM-TEMPERATURE

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].