The cyanobacterium Spirulina platensis contains a long wavelength-absorbing pigment C738 (F76077K) 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 organisms, as shown by its 77 K fluorescence peaking at 760 nm. To exclude preparation artifacts and to exclude the possibility that long wavelength-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 spectra as well as fluorescence induction and time-resolved fluorescence decays shows that the pigment responsible for the 77 K fluorescence at 760 nm (i) has the oscillator strength of approximately one Chl a molecule, (ii) absorbs maximally at 738 nm (C-738(293K)), (iii) is present only in the antenna system of PS I, (iv) participates in light collection, and (v) does not entail a low photochemical quantum yield. Other, more abundant but less red-shifted Chl a antenna pigments lead to a significantly larger absorption cross section of the photosynthetic unit of PS I above 700 nm compared to units that would not possess these long wavelength-absorbing pigments. These results support the hypothesis that the physiological role of long wavelength-absorbing pigments is to increase the absorption cross section at wavelengths of >700 nm when in densely populated mats the spectrally filtered light is relatively more intense at these wavelengths [Trissl, H.-W. (1993) Photosynth. Res. 35, 247-263].