Light control over the size of an antenna unit building block as an efficient strategy for light harvesting in photosynthesis

Femtosecond Exciton Relaxation in Chlorosomes of the Photosynthetic Green Bacterium Chloroflexus aurantiacus

Process of photosynthesis in the green bacteria Chloroflexus (Cfx.) aurantiacus starts from absorption of light by chlorosomes, peripheral antennas consisting of thousands of bacteriochlorophyll c (BChl c) molecules combined into oligomeric structures. In this case, the excited states are formed in BChl c, energy of which migrates along the chlorosome towards the baseplate and further to the reaction center, where the primary charge separation occurs. Energy migration is accompanied by non-radiative electronic transitions between the numerous exciton states, that is, exciton relaxation. In this work, we studied dynamics of the exciton relaxation in Cfx. aurantiacus chlorosomes using differential femtosecond spectroscopy at cryogenic temperature (80 K). Chlorosomes were excited by 20-fs light pulses at wavelengths in the range from 660 to 750 nm, and differential (light-dark) absorption kinetics were measured at a wavelength of 755 nm. Mathematical analysis of the obtained data revealed kinetic components with characteristic times of 140, 220, and 320 fs, which are responsible for exciton relaxation. As the excitation wavelength decreased, the number and relative contribution of these components increased. Theoretical modelling of the obtained data was carried out based of the cylindrical model of BChl c. Nonradiative transitions between the groups of exciton bands were described by a system of kinetic equations. The model that takes into account energy and structural disorder of chlorosomes turned out to be the most adequate.

Dynamic Stark effect in β and γ carotenes induced by photoexcitation of bacteriochlorophyll c in chlorosomes from Chloroflexus aurantiacus

Chlorosomes of green bacteria can be considered as a prototype of future artificial light-harvesting devices due to their unique property of self-assembly of a large number of bacteriochlorophyll (BChl) c/d/e molecules into compact aggregates. The presence of carotenoids (Cars) in chlorosomes is very important for photoprotection, light harvesting and structure stabilization. In this work, we studied for the first time the electrochromic band shift (Stark effect) in Cars of the phototrophic filamentous green bacterium Chloroflexus (Cfx.) aurantiacus induced by fs light excitation of the main pigment, BChl c. The high accuracy of the spectral measurements permitted us to extract a small wavy spectral feature, which, obviously, can be associated with the dynamic shift of the Car absorption band. A global analysis of spectroscopy data and theoretical modeling of absorption spectra showed that near 60% of Cars exhibited a red Stark shift of ~ 25 cm^-1 and the remaining 40% exhibited a blue shift. We interpreted this finding as evidence of various orientations of Car in chlorosomes. We estimated the average value of the light-induced electric field strength in the place of Car molecules as ~ 10⁶ V/cm and the average distance between Car and the neighboring BChl c as ~ 10 Å. We concluded that the dynamics of the Car electrochromic band shift mainly reflected the dynamics of exciton migration through the chlorosome toward the baseplate within ~ 1 ps. Our work has unambiguously shown that Cars are sensitive indicators of light-induced internal electric fields in chlorosomes.

Femtosecond excited-state dynamics in chlorosomal carotenoids of the photosynthetic bacterium Chloroflexus aurantiacus revealed by near infrared pump-probe spectroscopy

In photosynthetic green bacteria, chlorosomes provide light harvesting with high efficiency. Chlorosomal carotenoids (Cars) participate in light harvesting together with the main pigment, bacteriochlorophyll (BChl) c/d/e. In the present work, we studied the excited-state dynamics in Cars from Chloroflexus (Cfx.) aurantiacus chlorosomes by near infrared pump-probe spectroscopy with 25 fs temporal resolution at room temperature. The S2 state of Cars was excited at a wavelength of ∼520 nm, and the absorption changes were probed at 860-1000 nm where the excited state absorption (ESA) of the Cars S2 state occurred. Global analysis of the spectroscopy data revealed an ultrafast (∼15 fs) and large (>130 nm) red shift of the S2 ESA spectrum together with the well-known S2 → S1 IC (∼190 fs) and Cars → BChl c EET (∼120 fs). The S2 lifetime was found to be ∼74 fs. Our findings are in line with earlier results on the excited-state dynamics in Cars in vitro. To explain the extremely fast S2 dynamics, we have tentatively proposed two alternative schemes. The first scheme assumed the formation of a vibrational wavepacket in the S2 state, the motion of which caused a dynamical red shift of the S2 ESA spectrum. The second scheme assumed the presence of two potential minima in the S2 state and incoherent energy transfer between them.

Utilization of blue-green light by chlorosomes from the photosynthetic bacterium Chloroflexus aurantiacus: Ultrafast excitation energy conversion and transfer

Chlorosomes of photosynthetic green bacteria are unique molecular assemblies providing efficient light harvesting followed by multi-step transfer of excitation energy to reaction centers. In each chlorosome, 10⁴-10⁵ bacteriochlorophyll (BChl) c/d/e molecules are organized by self-assembly into high-ordered aggregates. We studied the early-time dynamics of the excitation energy flow and energy conversion in chlorosomes isolated from Chloroflexus (Cfx.) aurantiacus bacteria by pump-probe spectroscopy with 30-fs temporal resolution at room temperature. Both the S₂ state of carotenoids (Cars) and the Soret states of BChl c were excited at ~490 nm, and absorption changes were probed at 400-900 nm. A global analysis of spectroscopy data revealed that the excitation energy transfer (EET) from Cars to BChl c aggregates occurred within ~100 fs, and the Soret → Q energy conversion in BChl c occurred faster within ~40 fs. This conclusion was confirmed by a detailed comparison of the early exciton dynamics in chlorosomes with different content of Cars. These processes are accompanied by excitonic and vibrational relaxation within 100-270 fs. The well-known EET from BChl c to the baseplate BChl a proceeded on a ps time-scale. We showed that the S₁ state of Cars does not participate in EET. We discussed the possible presence (or absence) of an intermediate state that might mediates the Soret → Q_y internal conversion in chlorosomal BChl c. We discussed a possible relationship between the observed exciton dynamics and the structural heterogeneity of chlorosomes.

Q-band hyperchromism and B-band hypochromism of bacteriochlorophyll c as a tool for investigation of the oligomeric structure of chlorosomes of the green photosynthetic bacterium Chloroflexus aurantiacus

Chlorosomes of green photosynthetic bacteria are the most amazing example of long-range ordered natural light-harvesting antennae. Chlorosomes are the largest among all known photosynthetic light-harvesting structures (~ 10⁴-10⁵ pigments in the aggregated state). The chlorosomal bacteriochlorophyll (BChl) c/d/e molecules are organized via self-assembly and do not require proteins to provide a scaffold for efficient light harvesting. Despite numerous investigations, a consensus regarding the spatial structure of chlorosomal antennae has not yet been reached. In the present work, we studied hyperchromism/hypochromism in the chlorosomal BChl c Q/B absorption bands of the green photosynthetic bacterium Chloroflexus (Cfx.) aurantiacus. The chlorosomes were isolated from cells grown under different light intensities and therefore, as we discovered earlier, they had different sizes of both BChl c antennae and their unit building blocks. We have shown experimentally that the Q-/B-band hyperchromism/hypochromism is proportional to the size of the chlorosomal antenna. We explained theoretically these findings in terms of excitonic intensity borrowing between the Q and B bands for the J-/H-aggregates of the BChls. The theory developed by Gülen (Photosynth Res 87:205-214, 2006) showed the dependence of the Q-/B-band hyperchromism/hypochromism on the structure of the aggregates. For the model of exciton-coupled BChl c linear chains within a unit building block, the theory predicted an increase in the hyperchromism/hypochromism with the increase in the number of molecules per chain and a decrease in it with the increase in the number of chains. It was previously shown that this model ensured a good fit with spectroscopy experiments and approximated the BChl c low packing density in vivo. The presented experimental and theoretical studies of the Q-/B-band hyperchromism/hypochromism permitted us to conclude that the unit building block of Cfx. aurantiacus chlorosomes comprises of several short BChl c chains.This conclusion is in accordance with previous linear and nonlinear spectroscopy studies on Cfx. aurantiacus chlorosomes.

On Excitation Energy Transfer within the Baseplate BChl a-CsmA Complex of Chloroflexus aurantiacus

Recently, a hybrid approach combining solid-state NMR spectroscopy and cryo-electron microscopy showed that the baseplate in green sulfur bacterium Chlorobaculum tepidum is a 2D lattice of BChl a-CsmA dimers [Nielsen, J. T.; et al., Nat. Commun. 2016, 7, 12454-12465]. While the existence of the BChl a-CsmA subunit was previously known, the proposed orientations of the BChl a pigments had only been elucidated from spectral data up to this point. Regarding the electronic structure of the baseplate, two models have been proposed. 2D electronic spectroscopy data were interpreted as revealing that at least four excitonically coupled BChl a might be in close contact. Conversely, spectral hole burning data suggested that the lowest energy state was localized, yet additional states are sometimes observed because of the presence of the Fenna-Matthews-Olson (FMO) antenna protein. To solve this conundrum, this work studies the chlorosome-baseplate complex from Chloroflexus aurantiacus, which does not contain the FMO protein. The results confirm that in both C. tepidum and C. aurantiacus, excitation energy is transferred to a localized low-energy trap state near 818 nm with similar rates, most likely via exciton hopping.

Ultrafast excited-state dynamics in chlorosomes isolated from the photosynthetic filamentous green bacterium Chloroflexus aurantiacus

Bacteriochlorophyll (BChl) c pigments in the aggregated state are responsible for efficient light harvesting in chlorosomes of the filamentous anoxygenic photosynthetic bacterium, Chloroflexus (Cfx.) aurantiacus. Absorption of light creates excited states in the BChl c aggregates. After subpicosecond intrachlorosomal energy transfer, redistribution and relaxation, the excitation is transferred to the BChl a complexes and further to reaction centers on the picosecond time scale. In this work, the femtosecond excited state dynamics within BChl c oligomers of isolated Cfx. aurantiacus chlorosomes was studied by double difference pump-probe spectroscopy at room temperature. Difference (A^light  - A^dark ) spectra corresponding to excitation at 725 nm (blue side of the BChl c absorption band) were compared with those corresponding to excitation at 750 nm (red side of the BChl c absorption band). A very fast (time constant 70 ± 10 fs) rise kinetic component was found in the stimulated emission (SE) upon excitation at 725 nm. This component was absent at 750-nm excitation. These data were explained by the dynamical red shift of the SE due to excited state relaxation. The nature and mechanisms of the ultrafast excited state dynamics in chlorosomal BChl c aggregates are discussed.

Variability of aggregation extent of light-harvesting pigments in peripheral antenna of Chloroflexus aurantiacus

The stationary ground state and femtosecond time-resolved absorption spectra as well as spectra of circular dichroism were measured at room temperature using freshly prepared samples of chlorosomes isolated from fresh cultures of the green bacterium Chloroflexus aurantiacus. Cultures were grown by using as inoculum the same seed culture but under different light conditions. All measured spectra clearly showed the red shift of BChl c Q_y bands (up to 5 nm) for low-light chlorosomes as compared to high-light ones, together with concomitant narrowing of these bands and increasing of their amplitudes. The sizes of the unit BChl c aggregates of the high-light-chlorosomes and the low-light ones were estimated. The fit of all experimental spectra was obtained within the framework of our model proposed before (Fetisova et al., Biophys J 71:995-101, 1996). The model assumes that a unit building block of the BChl c antenna has a form of a tubular aggregate of L = 6 linear single or double exciton-coupled pigment chains within a rod element, with the pigment packing density, approximating that in vivo. The simultaneous fit of all experimental spectra gave the number of pigments in each individual linear pigment chain N = 4 and N = 6 for the high-light and the low-light BChl c unit building blocks, respectively. The size of a unit building block in the BChl c antenna was found to vary from L × N = 24 to L × N = 36 exciton-coupled BChl c molecules being governed by the growth-light intensity. All sets of findings for Chloroflexus aurantiacus chlorosomes demonstrated the biologically expedient light-controlled variability, predicted by us, of the extent of BChl c aggregation within a unit building block in this antenna.

Orientation of B798 BChl a Q y transition dipoles in Chloroflexus aurantiacus chlorosomes: polarized transient absorption spectroscopy studies

Isotropic and anisotropic pump-probe spectra of Cfx. aurantiacus chlorosomes were measured on the fs-through ps-time scales for the B798 BChl a Q y band upon direct excitation of the B798 band at T = 293 K and T = 90 K. Upon direct excitation of the B798 band, the anisotropy parameter value r(λ) was constant within the whole BChl a Q y band at any delay time at both temperatures. The value of the anisotropy parameter r decayed from r = 0.4 at both temperatures (at 200 fs delay time after excitation) to the steady-state values r = 0.1 at T = 293 K and to r = 0.09 at T = 90 K (at 30 ÷ 100 ps delay time after excitation). The results were considered within the framework of the model of uniaxial orientation distribution of BChl-a transition dipoles within a single Cfx. aurantiacus chlorosome. This implies that the B798 BChl a Q y transition dipoles, randomly distributed around the normal to the baseplate plane, form the angle θ with the plane. For this model, the theoretical dependence of the steady-state anisotropy parameter r on the angle θ was derived. According to the theoretical dependence r(θ), the angle θ corresponding to the experimental steady-state value r = 0.1 at T = 293 K was found to equal 55°. As the temperature drops to 90 K, the angle θ decreases to 54°.

Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

Truncated light-harvesting antenna 1 (TLA1) is a nuclear gene proposed to regulate the chlorophyll (Chl) antenna size in Chlamydomonas reinhardtii. The Chl antenna size of the photosystems and the chloroplast ultrastructure were manipulated upon TLA1 gene over-expression and RNAi downregulation. The TLA1 over-expressing lines possessed a larger chlorophyll antenna size for both photosystems and contained greater levels of Chl b per cell relative to the wild type. Conversely, TLA1 RNAi transformants had a smaller Chl antenna size for both photosystems and lower levels of Chl b per cell. Western blot analyses of the TLA1 over-expressing and RNAi transformants showed that modulation of TLA1 gene expression was paralleled by modulation in the expression of light-harvesting protein, reaction centre D1 and D2, and VIPP1 genes. Transmission electron microscopy showed that modulation of TLA1 gene expression impacts the organization of thylakoid membranes in the chloroplast. Over-expressing lines showed well-defined grana, whereas RNAi transformants possessed loosely held together and more stroma-exposed thylakoids. Cell fractionation suggested localization of the TLA1 protein in the inner chloroplast envelope and potentially in association with nascent thylakoid membranes, indicating a role in Chl antenna assembly and thylakoid membrane biogenesis. The results provide a mechanistic understanding of the Chl antenna size regulation by the TLA1 gene.

Photosynthetic Light-Harvesting Complexes

The light-harvesting antenna systems found in photosynthetic organisms function to collect light and transfer energy in the photon to a reaction center, where electron transfer gives rise to long-term energy storage. The antenna systems found in different types of photosynthetic organisms adapt the organisms to very different photic environments, and almost certainly have been invented multiple times during evolution. The diverse collection of photosynthetic antenna systems is described in terms of their pigment and protein components and their organization in the photosystem. The Förster theory is described as the physical basis of energy transfer in photosynthetic antennas, although in many systems it is not adequate to describe energy transfer in complexes with closely interacting pigments. Regulatory aspects of antennas are described, including the process of non-photochemical quenching.

Genetic and biochemical analysis of the TLA1 gene in Chlamydomonas reinhardtii

The Chlamydomonas reinhardtii genomic DNA database contains a predicted open reading frame (ORF-P) without an apparent stop-codon and unknown coding sequence, located in close proximity and immediately upstream of the TLA1 gene (GenBank Accession No. AF534570). The latter was implicated in the regulation of the light-harvesting chlorophyll antenna size of photosynthesis (Tetali et al. Planta 225:813-829, 2007). To provide currently lacking information on ORF-P and its potential participation in TLA1 gene expression, thus in the regulation of the chlorophyll antenna size, genetic and biochemical analyses were undertaken. The coding and UTR regions of the ORF-P were defined and delineated from those of the adjacent TLA1 gene. ORF-P is shown to encode a protein with a distinct RING-like zinc finger domain that is present in numerous eukaryotic proteins, believed to play a role in cellular ubiquitination, leading to regulation of cellular processes like signaling, growth, transcription, and DNA repair. It is further shown that the two genes share a 74-bp overlap between the 3' UTR region of ORF-P and the 5' UTR region of TLA1. However, they possess distinct start and stop codons and separate coding sequences, and transcribed as separate mRNAs without any trans-splicing between them. Complementation experiments showed that the TLA1 gene alone is sufficient to rescue the truncated chlorophyll antenna size phenotype of the tla1 mutant. Protein sequence alignments in C. reinhardtii and the colorless microalga Polytomella parva suggested that TLA1 defines the relationship between nucleus and organelle in microalgae, indirectly affecting the development of the chlorophyll antenna size.

Optical properties of microalgae for enhanced biofuels production

Research seeks to alter the optical characteristics of microalgae in order to improve solar-to-biofuels energy conversion efficiency in mass culture under bright sunlight conditions. This objective is achieved by genetically truncating the size of the light-harvesting chlorophyll arrays that serve to absorb sunlight in the photosynthetic apparatus.

Investigation on chlorosomal antenna geometries: tube, lamella and spiral-type self-aggregates

Molecular mechanics calculations and exciton theory have been used to study pigment organization in chlorosomes of green bacteria. Single and double rod, multiple concentric rod, lamella, and Archimedean spiral macrostructures of bacteriochlorophyll c molecules were created and their spectral properties evaluated. The effects of length, width, diameter, and curvature of the macrostructures as well as orientations of monomeric transition dipole moment vectors on the spectral properties of the aggregates were studied. Calculated absorption, linear dichroism, and polarization dependent fluorescence-excitation spectra of the studied long macrostructures were practically identical, but circular dichroism spectra turned out to be very sensitive to geometry and monomeric transition dipole moment orientations of the aggregates. The simulations for long multiple rod and spiral-type macrostructures, observed in recent high-resolution electron microscopy images (Oostergetel et al., FEBS Lett 581:5435-5439, 2007) gave shapes of circular dichroism spectra observed experimentally for chlorosomes. It was shown that the ratio of total circular dichroism intensity to integrated absorption of the Q(y) transition is a good measure of degree of tubular structures in the chlorosomes. Calculations suggest that the broad Q(y) line width of chlorosomes of sulfur bacteria could be due to (1) different orientations of the transition moment vectors in multi-walled rod structures or (2) a variety of Bchl-aggregate structures in the chlorosomes.

Development of the light-harvesting chlorophyll antenna in the green alga Chlamydomonas reinhardtii is regulated by the novel Tla1 gene

The Chlamydomonas reinhardtii tla1 (truncated light-harvesting chlorophyll antenna size) mutant was generated upon DNA insertional mutagenesis and shown to specifically possess a smaller than wild type (WT) chlorophyll antenna size in both photosystems. Molecular and genetic analysis revealed that the exogenous plasmid DNA was inserted at the end of the 5' UTR and just prior to the ATG start codon of a hitherto unknown nuclear gene (termed Tla1), which encodes a protein of 213 amino acids. The Tla1 gene in the mutant is transcribed with a new 5' UTR sequence, derived from the 3' end of the transforming plasmid. This replacement of the native 5' UTR and promoter regions resulted in enhanced transcription of the tla1 gene in the mutant but inhibition in the translation of the respective tla1 mRNA. Transformation of the tla1 mutant with WT Tla1 genomic DNA successfully rescued the mutant. These results are evidence that polymorphism in the 5' UTR of the Tla1 transcripts resulted in the tla1 phenotype and that expression of the Tla1 gene is a prerequisite for the development/assembly of the Chl antenna in C. reinhardtii. A blast search with the Tla1 deduced amino acid sequence

Optimal spectral coordination of subantennae in natural antennae as an efficient strategy for light harvesting in photosynthesis

This work continues a series of our investigations on efficient strategies of functioning of natural light-harvesting antennae, initiated by a concept of rigorous optimization of photosynthetic apparatus by functional criterion, and deals with the problem of an optimal spectral coordination of subantennae in photosynthetic superantenna of the green bacterium Oscillochloris trichoides from a new family of green bacteria Oscillochloridaceae based in 2000. At present, two subantennae were identified surely: chlorosomal BChl c subantenna B750 and membrane BChl a subantennae B805-860. Some indirect experiments indicated on the presence of minor amounts of BChl a in isolated chlorosomes which allowed us to propose on the existence of an intermediate-energy subantenna which can connect the chlorosomal BChl c and the membrane BChl a ones. However, in the absorption spectra of isolated chlorosomes, this BChl a subantenna was not visually identified. This promoted us to perform a theoretical analysis of the optimality of spectral coordination of Oscillochloris trichoides subantennae. Using mathematical modeling for the functioning of the natural superantenna, we showed that an intermediate-energy subantenna, connecting B750 and B805-860 ones, allows one to control superantenna efficiency, i.e. to optimize the excitation energy transfer from B750 to B805 by functional criterion, and hence, the existence of such intermediate-energy subantenna is biologically expedient.

tla1, a DNA insertional transformant of the green alga Chlamydomonas reinhardtii with a truncated light-harvesting chlorophyll antenna size

DNA insertional mutagenesis and screening of the green alga Chlamydomonas reinhardtii was employed to isolate tla1, a stable transformant having a truncated light-harvesting chlorophyll antenna size. Molecular analysis showed a single plasmid insertion into an open reading frame of the nuclear genome corresponding to a novel gene ( Tla1) that encodes a protein of 213 amino acids. Genetic analysis showed co-segregation of plasmid and tla1 phenotype. Biochemical analyses showed the tla1 mutant to be chlorophyll deficient, with a functional chlorophyll antenna size of photosystem I and photosystem II being about 50% and 65% of that of the wild type, respectively. It contained a correspondingly lower amount of light-harvesting proteins than the wild type and had lower steady-state levels of Lhcb mRNA. The tla1 strain required a higher light intensity for the saturation of photosynthesis and showed greater solar conversion efficiencies and a higher photosynthetic productivity than the wild type under mass culture conditions. Results are discussed in terms of the tla1 mutation, its phenotype, and the role played by the Tla1 gene in the regulation of the photosynthetic chlorophyll antenna size in C. reinhardtii.