Carotenoid responds to excess energy dissipation in the LH2 complex from Rhodoblastus acidophilus

Quenching of bacteriochlorophyll a triplet state by carotenoids in the chlorosome baseplate of green bacterium Chloroflexus aurantiacus

To capture weak light fluxes, green photosynthetic bacteria have unique structures - chlorosomes, consisting of 104-5 molecules of bacteriochlorophyll (BChl) c, d, e. Chlorosomes are attached to the cytoplasmic membrane through the baseplate, a paracrystalline protein structure containing BChl a and carotenoids (Car). The most important function of Car is the quenching of triplet states of BChl, which prevents the formation of singlet oxygen and thereby provides photoprotection. In our work, we studied the dynamics of the triplet states of BChl a and Car in the baseplate of Chloroflexus aurantiacus chlorosomes using picosecond differential spectroscopy. BChl a of the baseplate was excited into the Qy band at 810 nm, and the corresponding absorption changes were recorded in the range of 420-880 nm. It was found that the formation of the Car triplet state occurs in ∼1.3 ns, which is ∼3 times faster than the formation of this state in the peripheral antenna of C. aurantiacus according to literature data. The Car triplet state was recorded by the characteristic absorption band T1 → Tn at ∼550 nm. Simultaneously with the appearance of absorption T1 → Tn, there was a bleaching of the singlet absorption of Car in the region of 400-500 nm. Theoretical modeling made it possible to estimate the characteristic time of formation of the triplet state of BChl a as ∼0.5 ns. It is shown that the experimental data are well described by the sequential scheme of formation and quenching of the BChl a triplet state: BChl a* → BChl aT → CarT. Thus, carotenoids from green bacteria effectively protect the baseplate from possible damage by singlet oxygen.

Reinvestigation on primary processes of PSII-dimer from Thermosynechococcus vulcanus by femtosecond pump-probe spectroscopy

Cyanobacterial photosynthetic apparatus efficiently capture sunlight, and the energy is subsequently transferred to photosystem I (PSI) and II (PSII), to produce electrochemical potentials. PSII is a unique membrane protein complex that photo-catalyzes oxidation of water and majorly contains photosynthetic pigments of chlorophyll a and carotenoids. In the present study, the ultrafast energy transfer and charge separation dynamics of PSII from a thermophilic cyanobacterium Thermosynechococcus vulcanus were reinvestigated by femtosecond pump-probe spectroscopic measurements under low temperature and weak intensity excitation condition. The results imply the two possible models of the energy transfers and subsequent charge separation in PSII. One is the previously suggested "transfer-to-trapped limit" model. Another model suggests that the energy transfers from core CP43 and CP47 antennas to the primary electron donor ChlD1 with time-constants of 0.71 ps and 3.28 ps at 140 K (0.17 and 1.33 ps at 296 K), respectively and that the pheophytin anion (PheoD1-) is generated with the time-constant of 43.0 ps at 140 K (14.8 ps at 296 K) upon excitation into the Qy band of chlorophyll a at 670 nm. The secondary electron transfer to quinone QA: PheoD1-QA → PheoD1QA- is observed with the time-constant of 650 ps only at 296 K. On the other hand, an inefficient β-carotene → chlorophyll a energy transfer (33%) occurred after excitation to the S2 state of β-carotene at 500 nm. Instead, the carotenoid triplet state appeared in an ultrafast timescale after excitation at 500 nm.

Direct observation of triplet energy transfer between chlorophylls and carotenoids in the core antenna of photosystem I from Thermosynechococcus elongatus

Quenching of chlorophyll triplet states by carotenoids is an essential photoprotective process, which prevents formation of reactive singlet oxygen in photosynthetic light-harvesting complexes. The process is usually very efficient in oxygenic organisms under physiological conditions, thus preventing any observable accumulation of chlorophyll triplets. However, it subsequently prevents also the determination of the triplet transfer rate. Here we report results of nanosecond transient absorption spectroscopy on photosystem I core complexes, where a major part of chlorophyll a triplet states (~60 %) accumulates on a nanosecond time scale at ambient temperature. As a consequence, the triplet energy transfer could be resolved and the transfer time was determined to be about 24 ns. A smaller fraction of chlorophyll a triplet states (~40 %) is quenched with a faster rate, which could not be determined. Our analysis indicates that these chlorophylls are in direct contact with carotenoids. The overall chlorophyll triplet yield in the core antenna was estimated to be ~0.3 %, which is a value two orders of magnitude smaller than in most other photosynthetic light-harvesting complexes. This explains why slower quenching of chlorophyll triplet states is sufficient for photoprotection of photosystem I. Nevertheless, the core antenna of photosystem I represents one of only few photosynthetic complexes of oxygenic organisms in which the quenching rate of the majority of chlorophyll triplets can be directly monitored under physiological temperature.

Carotenoid-Mediated Long-Range Energy Transfer in the Light Harvesting-Reaction Center Complex from Photosynthetic Bacterium Roseiflexus castenholzii

The light harvesting-reaction center complex (LH-RC) of Roseiflexus castenholzii binds bacteriochlorophylls a (BChls a), B800 and B880, absorbing around 800 and 880 nm, respectively. We comparatively investigated the interband excitation energy transfer (EET) dynamics of the wild-type LH-RC (wt-LH-RC) of Rfl. castenholzii and its carotenoid (Car)-less mutant (m-LH-RC) and found that Car can boost the B800 → B880 EET rate from (2.43 ps)-1 to (1.75 ps)-1, accounting for 38% acceleration of the EET process. Interestingly, photoexcitation of wt-LH-RC at 800 nm induced pronounced excitation dynamics of Car despite the insufficient photon energy for direct Car excitation, a phenomenon which is attributed to the BChl-Car exciplex 1[B800(↑↑)···Car(↓↓)]*. Such an exciplex is suggested to play an essential role in promoting the B800 → B880 EET process, as corroborated by the recently reported cryo-EM structures of wt-LH-RC and m-LH-RC. The mechanism of Car-mediated EET will be helpful to deepen the understanding of the role of Car in bacterial photosynthesis.

Characterisation of the photosynthetic complexes from the marine gammaproteobacterium Congregibacter litoralis KT71

Possibly the most abundant group of anoxygenic phototrophs are marine photoheterotrophic Gammaproteobacteria belonging to the NOR5/OM60 clade. As little is known about their photosynthetic apparatus, the photosynthetic complexes from the marine phototrophic bacterium Congregibacter litoralis KT71 were purified and spectroscopically characterised. The intra-cytoplasmic membranes contain a smaller amount of photosynthetic complexes when compared with anaerobic purple bacteria. Moreover, the intra-cytoplasmic membranes contain only a minimum amount of peripheral LH2 complexes. The complexes are populated by bacteriochlorophyll a, spirilloxanthin and two novel ketocarotenoids, with biophysical and biochemical properties similar to previously characterised complexes from purple bacteria. The organization of the RC-LH1 complex has been further characterised using cryo-electron microscopy. The overall organisation is similar to the complex from the gammaproteobacterium Thermochromatium tepidum, with the type-II reaction centre surrounded by a slightly elliptical LH1 antenna ring composed of 16 αβ-subunits with no discernible gap or pore. The RC-LH1 and LH2 apoproteins are phylogenetically related to other halophilic species but LH2 also to some alphaproteobacterial species. It seems that the reduction of light-harvesting apparatus and acquisition of novel ketocarotenoids in Congregibacter litoralis KT71 represent specific adaptations for operating the anoxygenic photosynthesis under aerobic conditions at sea.