Kinetics of in vivo bacteriochlorophyll fluorescence yield and the state of photosynthetic apparatus of purple bacteria

Utilization of light energy in phototrophic Gemmatimonadetes

Gemmatimonas phototrophica is, so far, the only described phototrophic species of the bacterial phylum Gemmatimonadetes. Its cells contain a unique type of photosynthetic complex with the reaction center surrounded by a double ring antenna, however they can also grow in the dark using organic carbon substrates. Its photosynthesis genes were received via horizontal gene transfer from Proteobacteria. This raises two questions; how the horizontally transferred photosynthesis apparatus has integrated into the cellular machinery, and how much light-derived energy actually contributes to the cellular metabolism? To address these points, the photosynthetic reactions were studied on several levels, from photophysics of the reaction center to cellular growth. Flash photolysis measurements and bacteriochlorophyll fluorescence kinetic measurements documented the presence of fully functional type-2 reaction centers with a large light harvesting antenna. When illuminated, the bacterial cells reduced their respiration rate by 58 ± 5%, revealing that oxidative phosphorylation was replaced by photophosphorylation. Moreover, illumination also more than doubled the assimilation rates of glucose, a sugar that is mostly used for respiration. Finally, light increased the growth rates of Gemmatimonas phototrophica colonies on agar plates. All the presented data provide evidence that photosynthetic complexes are fully integrated into cellular metabolism of Gemmatimonas phototrophica, and are able to provide a substantial amount of energy for its metabolism and growth.

Unique double concentric ring organization of light harvesting complexes in Gemmatimonas phototrophica

The majority of life on Earth depends directly or indirectly on the sun as a source of energy. The initial step of photosynthesis is facilitated by light-harvesting complexes, which capture and transfer light energy into the reaction centers (RCs). Here, we analyzed the organization of photosynthetic (PS) complexes in the bacterium G. phototrophica, which so far is the only phototrophic representative of the bacterial phylum Gemmatimonadetes. The isolated complex has a molecular weight of about 800 ± 100 kDa, which is approximately 2 times larger than the core complex of Rhodospirillum rubrum. The complex contains 62.4 ± 4.7 bacteriochlorophyll (BChl) a molecules absorbing in 2 distinct infrared absorption bands with maxima at 816 and 868 nm. Using femtosecond transient absorption spectroscopy, we determined the energy transfer time between these spectral bands as 2 ps. Single particle analyses of the purified complexes showed that they were circular structures with an outer diameter of approximately 18 nm and a thickness of 7 nm. Based on the obtained, we propose that the light-harvesting complexes in G. phototrophica form 2 concentric rings surrounding the type 2 RC. The inner ring (corresponding to the B868 absorption band) is composed of 15 subunits and is analogous to the inner light-harvesting complex 1 (LH1) in purple bacteria. The outer ring is composed of 15 more distant BChl dimers with no or slow energy transfer between them, resulting in the B816 absorption band. This completely unique and elegant organization offers good structural stability, as well as high efficiency of light harvesting. Our results reveal that while the PS apparatus of Gemmatimonadetes was acquired via horizontal gene transfer from purple bacteria, it later evolved along its own pathway, devising a new arrangement of its light harvesting complexes.

The majority of life on Earth depends directly or indirectly on the sun as a source of energy. Phototrophic organisms use energy from light to power various cellular and metabolic processes. The initial step of photosynthesis is facilitated by light-harvesting complexes, which capture and transfer light energy into the reaction centers where it is used to power proton gradients or to form new chemical bonds. Here, we analyzed photosynthetic complexes in Gemmatimonas phototrophica, the only known phototrophic representative of the bacterial phylum Gemmatimonadetes. Using a combination of biochemical and spectroscopic techniques, we show that the light-harvesting complexes of G. phototrophica are organized in 2 concentric rings around the reaction center. This organization is unique among anoxygenic phototrophs. It offers both structural stability and high efficiency of light harvesting. The structural unit of both antenna rings is a dimer of photosynthetic pigments called bacteriochlorophyll. The inner ring is populated by more densely packed dimers, while the outer ring contains more distant dimers with a minimal excitation exchange. Such an arrangement modifies the spectral properties of bacteriochlorophylls in the complex and ensures efficient capture of light in the near-infrared part of the solar spectrum.

A two-component nonphotochemical fluorescence quenching in eustigmatophyte algae

Eustigmatophyte algae represent an interesting model system for the study of the regulation of the excitation energy flow due to their use of violaxanthin both as a major light-harvesting pigment and as the basis of xanthophyll cycle. Fluorescence induction kinetics was studied in an oleaginous marine alga Nannochloropsis oceanica. Nonphotochemical fluorescence quenching was analyzed in detail with respect to the state of the cellular xanthophyll pool. Two components of nonphotochemical fluorescence quenching (NPQ), both dependent on the presence of zeaxanthin, were clearly resolved, denoted as slow and fast NPQ based on kinetics of their formation. The slow component was shown to be in direct proportion to the amount of zeaxanthin, while the fast NPQ component was transiently induced in the presence of membrane potential on subsecond timescales. The applicability of these observations to other eustigmatophyte species is demonstrated by measurements of other representatives of this algal group, both marine and freshwater.

Fluorescence relaxation in intact cells of photosynthetic bacteria: donor and acceptor side limitations of reopening of the reaction center

The dark relaxation of the yield of variable BChl fluorescence in the 10(-5)-10 s time range is measured after laser diode (808 nm) excitation of variable duration in intact cells of photosynthetic bacteria Rba. sphaeroides, Rsp. rubrum, and Rvx. gelatinosus under various treatments of redox agents, inhibitors, and temperature. The kinetics of the relaxation is complex and much wider extended than a monoexponential function. The longer is the excitation, the slower is the relaxation which is determined by the redox states, sizes, and accessibility of the pools of cytochrome [Formula: see text] and quinone for donor and acceptor side-limited bacterial strains, respectively. The kinetics of fluorescence decay reflects the opening kinetics of the closed RC. The relaxation is controlled preferentially by the rate of re-reduction of the oxidized dimer by mobile cytochrome [Formula: see text] in Rba. sphaeroides and Rsp. rubrum and by the rate constant of the [Formula: see text] interquinone electron transfer, (350 μs)(-1) and/or the quinol/quinone exchange at the acceptor side in Rvx. gelatinosus. The commonly used acceptor side inhibitors (e.g., terbutryn) demonstrate kinetically limited block of re-oxidation of the primary quinone. The observations are interpreted in frame of a minimum kinetic and energetic model of electron transfer reactions in bacterial RC of intact cells.

The reaction center is the sensitive target of the mercury(II) ion in intact cells of photosynthetic bacteria

The sensitivity of intact cells of purple photosynthetic bacterium Rhodobacter sphaeroides wild type to low level (<100 μM) of mercury (Hg²⁺) contamination was evaluated by absorption and fluorescence spectroscopies of the bacteriochlorophyll-protein complexes. All assays related to the function of the reaction center (RC) protein (induction of the bacteriochlorophyll fluorescence, delayed fluorescence and light-induced oxidation and reduction of the bacteriochlorophyll dimer and energization of the photosynthetic membrane) showed prompt and later effects of the mercury ions. The damage expressed by decrease of the magnitude and changes of rates of the electron transfer kinetics followed complex (spatial and temporal) pattern according to the different Hg²⁺ sensitivities of the electron transport (donor/acceptor) sites including the reduced bound and free cytochrome c₂ and the primary reduced quinone. In contrast to the RC, the light harvesting system and the bc₁ complex demonstrated much higher resistance against the mercury pollution. The 850 and 875 nm components of the peripheral and core complexes were particularly insensitive to the mercury(II) ions. The concentration of the photoactive RCs and the connectivity of the photosynthetic units decreased upon mercury treatment. The degree of inhibition of the photosynthetic apparatus was always higher when the cells were kept in the light than in the dark indicating the importance of metabolism in active transport of the mercury ions from outside to the intracytoplasmic membrane. Any of the tests applied in this study can be used for detection of changes in photosynthetic bacteria at the early stages of the action of toxicants.

Absorbance changes accompanying the fast fluorescence induction in the purple bacterium Rhodobacter sphaeroides

The authors present a study of the fluorescence and absorbance transients occurring in whole cells of purple nonsulfur bacterium Rhodobacter sphaeroides on the millisecond timescale under pulsed actinic illumination. The fluorescence induction curve is interpreted in terms of combination of effects of redox changes in the reaction center and the membrane potential. The results of this study support the view that the membrane potential act predominantly to increase the fluorescence yield. Advantages of the pulsed actinic illumination for study of the operation of the electron transport chain in vivo are discussed.

Early detection of mercury contamination by fluorescence induction of photosynthetic bacteria

The induction (sudden dark-to-light transition) of fluorescence of photosynthetic bacteria has proved to be sensitive tool for early detection of mercury (Hg(2+)) contamination of the culture medium. The major characteristics of the induction (dark, variable and maximum fluorescence levels together with rise time) offer an easier, faster and more informative assay of indication of the contamination than the conventional techniques. The inhibition of Hg(2+) is stronger in the light than in the dark and follows complex kinetics. The fast component (in minutes) reflects the damage of the quinone acceptor pool of the RC and the slow component (in hours) is sensitive to the disintegration of the light harvesting system including the loss of the structural organization and of the pigments. By use of fluorescence induction, the dependence of the diverse pathways and kinetics of the mercury-induced effects on the age and the metabolic state of the bacteria were revealed.

Kinetic bacteriochlorophyll fluorometer

A pump and probe fluorometer with a laser diode as single light source has been constructed for measurement of fast induction and relaxation of the fluorescence yield in intact cells, chromatophores and isolated reaction centers of photosynthetic bacteria. The time resolution of the fluorometer is limited by the repetition time of the probing flashes to 20 micros. The apparatus offers high sensitivity, excellent performance and can become a versatile device for a range of demanding applications. Some of them are demonstrated here including fast and easy investigation of the (1) organization and redox state of the photosynthetic apparatus of the intact cells of different bacterial strains and mutants and (2) electron transfer reactions on donor and acceptor sides of isolated reaction centers. The compact design of the mechanics, optics, electronics, and data processing makes the device easy to use as outdoor instrument or to integrate into larger measuring systems.

The redox state of the plastoquinone pool directly modulates minimum chlorophyll fluorescence yield in Chlamydomonas reinhardtii

The effect of the plastoquionone (PQ) pool oxidation state on minimum chlorophyll fluorescence was studied in the green alga Chlamydomonas reinhardtii. In wild type and a mutant strain that lacks both photosystems but retains light harvesting complexes, oxygen depletion induced a rise in minimum chlorophyll fluorescence. An increase in minimum fluorescence yield is also observed when the PQ pool becomes reduced in the presence of oxygen and after application of an ionophore that collapses the transmembrane proton gradient. Together these results indicate that minimum chlorophyll fluorescence is modulated by the PQ oxidation state.