Abstract
We have investigated the photosynthetic properties of Acaryochloris marina, a cyanobacterium distinguished by having a high level of chlorophyll d, which has its absorption bands shifted to the red when compared with chlorophyll a. Despite this unusual pigment content, the overall rate and thermodynamics of the photosynthetic electron flow are similar to those of chlorophyll a-containing species. The midpoint potential of both cytochrome f and the primary electron donor of photosystem I (P(740)) were found to be unchanged with respect to those prevailing in organisms having chlorophyll a, being 345 and 425 mV, respectively. Thus, contrary to previous reports (Hu, Q., Miyashita, H., Iwasaki, I. I., Kurano, N., Miyachi, S., Iwaki, M., and Itoh, S. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 13319-13323), the midpoint potential of the electron donor P(740) has not been tuned to compensate for the decrease in excitonic energy in A. marina and to maintain the reducing power of photosystem I. We argue that this is a weaker constraint on the engineering of the oxygenic photosynthetic electron transfer chain than preserving the driving force for plastoquinol oxidation by P(740), via the cytochrome b(6)f complex. We further show that there is no restriction in the diffusion of the soluble electron carrier between cytochrome b(6)f and photosystem I in A. marina, at variance with plants. This difference probably reflects the simplified ultrastructure of the thylakoids of this organism, where no segregation into grana and stroma lamellae is observed. Nevertheless, chlorophyll fluorescence measurements suggest that there is energy transfer between adjacent photosystem II complexes but not from photosystem II to photosystem I, indicating spatial separation between the two photosystems.
Highlights
Not surprisingly, the unusual absorption spectrum of A. marina, due to the high level of Chl d, influences the spectroscopic signatures of the various cofactors involved in the primary photochemistry
Some specific strategies may have been developed by this organism to cope with this peculiar situation and maintain a photosynthetic activity compatible with its propyl-p-benzoquinone; P700 and P740, chlorophyll dimer bearing the long-lived cation in photosystem I in Chl a and Chl d organisms, respectively; PC, plastocyanin; PSI and PSII, photosystem I and II, respectively; RP, radical pair; MK, menaquinone; PQ, plastoquinone; MES, 4-morpholineethanesulfonic acid
Energetics of PS1 in A. marina—Our analyses of the kinetics and thermodynamic properties of the electron flow chain in A. marina reveals a scenario that is very similar to that found in Chl a-containing organisms
Summary
The unusual absorption spectrum of A. marina, due to the high level of Chl d, influences the spectroscopic signatures of the various cofactors involved in the primary photochemistry.
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