Light-harvesting complexes from purple sulfur bacteria Allochromatium minutissimum assembled without carotenoids

Light-Quality-Adapted Carotenoid Photoprotection in the Photosystem of Roseiflexus castenholzii

The photosystem of filamentous anoxygenic phototroph Roseiflexus (Rfl.) castenholzii comprises a light-harvesting (LH) complex encircling a reaction center (RC), which intensely absorbs blue-green light by carotenoid (Car) and near-infrared light by bacteriochlorophyll (BChl). To explore the influence of light quality (color) on the photosynthetic activity, we compared the pigment compositions and triplet excitation dynamics of the LH-RCs from Rfl. castenholzii was adapted to blue-green light (bg-LH-RC) and to near-infrared light (nir-LH-RC). Both LH-RCs bind γ-carotene derivatives; however, compared to that of nir-LH-RC (12%), bg-LH-RC contains substantially higher keto-γ-carotene content (43%) and shows considerably faster BChl-to-Car triplet excitation transfer (10.9 ns vs 15.0 ns). For bg-LH-RC, but not nir-LH-RC, selective photoexcitation of Car and the 800 nm-absorbing BChl led to Car-to-Car triplet transfer and BChl-Car singlet fission reactions, respectively. The unique excitation dynamics of bg-LH-RC enhances its photoprotection, which is crucial for the survival of aquatic anoxygenic phototrophs from photooxidative stress.

Culture optimization to enhance carotenoid production of a selected purple nonsulfur bacterium and its activity against acute hepatopancreatic necrosis disease-causing Vibrio parahaemolyticus

Purple nonsulfur bacteria (PNSB) were investigated for their carotenoid production and anti-vibrio activity against acute hepatopancreatic necrosis disease (AHPND)-causing Vibrio parahaemolyticus. To test carotenoid production, selected strains were cultivated in basic isolation medium (BIM), glutamate acetate medium, G5 medium and artificial acetic acid wastewater (AAW) medium. From 144 PNSB, Rhodopseudomonas palustris KTSSG46 was selected to produce carotenoids under microaerobic light conditions in BIM. When the culture medium was optimized, strain KTSSG46 grown in BIM modified with l-glutamate at 1 g/L more effectively inhibited AHPND-causing V. parahaemolyticus strains than standard BIM with 1 g/L (NH₄ )₂ SO₄ . BIM was further modified with 1.23 g/L MgSO₄ ·7H₂ O and carotenoid production increased 40.22%. Carotenoid production at day 2 by strain KTSSG46 grown in BIM modified with l-glutamate at 1 and 1.23 g/L MgSO₄ ·7H₂ O was the same as production in BIM modified with monosodium glutamate (MSG). Culture supernatants from all BIM formulations showed similar activity against the resistant AHPND strain SR2. Based on high-performance liquid chromatography, carotenoids of strain KTSSG46 might be canthaxanthin. Grown in BIM modified with MSG, strain KTSSG46 could produce inexpensive carotenoids and release anti-vibrio compounds that, applied as shrimp feed additive, would prevent AHPND strains.

Carotenoids Do Not Protect Bacteriochlorophylls in Isolated Light-Harvesting LH2 Complexes of Photosynthetic Bacteria from Destructive Interactions with Singlet Oxygen

The effect of singlet oxygen on light-harvesting (LH) complexes has been studied for a number of sulfur (S+) and nonsulfur (S-) photosynthetic bacteria. The visible/near-IR absorption spectra of the standard LH2 complexes (B800-850) of Allochromatium (Alc.) vinosum (S+), Rhodobacter (Rba.) sphaeroides (S-), Rhodoblastus (Rbl.) acidophilus (S-), and Rhodopseudomonas (Rps.) palustris (S-), two types LH2/LH3 (B800-850 and B800-830) of Thiorhodospira (T.) sibirica (S+), and an unusual LH2 complex (B800-827) of Marichromatium (Mch.) purpuratum (S+) or the LH1 complex from Rhodospirillum (Rsp.) rubrum (S-) were measured in aqueous buffer suspensions in the presence of singlet oxygen generated by the illumination of the dye Rose Bengal (RB). The content of carotenoids in the samples was determined using HPLC analysis. The LH2 complex of Alc. vinosum and T. sibirica with a reduced content of carotenoids was obtained from cells grown in the presence of diphenylamine (DPA), and LH complexes were obtained from the carotenoidless mutant of Rba. sphaeroides R26.1 and Rps. rubrum G9. We found that LH2 complexes containing a complete set of carotenoids were quite resistant to the destructive action of singlet oxygen in the case of Rba. sphaeroides and Mch. purpuratum. Complexes of other bacteria were much less stable, which can be judged by a strong irreversible decrease in the bacteriochlorophyll (BChl) absorption bands (at 850 or 830 nm, respectively) for sulfur bacteria and absorption bands (at 850 and 800 nm) for nonsulfur bacteria. Simultaneously, we observe the appearance of the oxidized product 3-acetyl-chlorophyll (AcChl) absorbing near 700 nm. Moreover, a decrease in the amount of carotenoids enhanced the spectral stability to the action of singlet oxygen of the LH2 and LH3 complexes from sulfur bacteria and kept it at the same level as in the control samples for carotenoidless mutants of nonsulfur bacteria. These results are discussed in terms of the current hypothesis on the protective functions of carotenoids in bacterial photosynthesis. We suggest that the ability of carotenoids to quench singlet oxygen (well-established in vitro) is not well realized in photosynthetic bacteria. We compared the oxidation of BChl850 in LH2 complexes of sulfur bacteria under the action of singlet oxygen (in the presence of 50 μM RB) or blue light absorbed by carotenoids. These processes are very similar: {[BChl + (RB or carotenoid) + light] + O2} → AcChl. We speculate that carotenoids are capable of generating singlet oxygen when illuminated. The mechanism of this process is not yet clear.

Phytofluene as a Highly Efficient UVA Photosensitizer of Singlet Oxygen Generation

Phytoene and phytofluene - uncolored C₄₀ carotenoids with short chain of conjugated double bonds (3 and 5, respectively) - are known to be universal precursors in biosynthesis of colored carotenoids in photosynthesizing organisms. It is commonly recognized that C₄₀ carotenoids are photoprotectors of cells and tissues. We have shown that phytofluene is an exception to this rule. By measuring photosensitized phosphorescence of singlet oxygen (¹O₂) we found out that phytofluene was very effective photosensitizer of ¹O₂ formation in aerated solutions under UVA irradiation (quantum yield of 85 ± 5%), whereas phytoene was almost inactive in this process. It was demonstrated that both carotenoids quench singlet oxygen in the dark. The obtained quenching rate constants [(4 ± 1) × 10⁶ M^-1·s^-1 for phytoene and (2 ± 0.5) × 10⁷ M^-1·s^-1 for phytofluene] were smaller than the rate constant of the diffusion-controlled reactions by 3-4 orders of magnitude. Thus, both carotenoids displayed rather weak protector properties. Moreover, phytofluene due to its high photosensitizing activity might be considered as a promoter of cell photodamage and a promising UVA photosensitizer for medical purposes.

The Influence of the Number of Conjugated Double Bonds in Carotenoid Molecules on the Energy Transfer Efficiency to Bacteriochlorophyll in Light-Harvesting Complexes LH2 from Allochromatium vinosum Strain MSU

Seven different carotenoids with the number of conjugated double bonds (N) from 5 to 11 were incorporated in vitro into carotenoidless complexes LH2 of the sulfur bacterium Allochromatium vinosum strain MSU. The efficiency of their incorporation varied from 4 to 99%. The influence of N in the carotenoid molecules on the energy transfer efficiency from these pigments to bacteriochlorophyll (BChl) in the modified LH2 complexes was studied for the first time. The highest level of energy transfer was recorded for rhodopin (N = 11) and neurosporene (N = 7) (37 and 51%, respectively). In the other carotenoids, this parameter ranged from 11 to 33%. In the LH2 complexes studied, we found no direct correlation between the decrease in N in carotenoids and increase in the energy transfer efficiency from these pigments to BChl.

Identification of Carotenoids and Isoprenoid Quinones from Asaia lannensis and Asaia bogorensis

The aim of the study was to identify and quantitatively assess of carotenoids and isoprenoid quinones biosynthesized by six different strains of acetic acid bacteria, belonging to genus Asaia, that are common beverage-spoiling bacteria in Europe. Bacterial cultures were conducted in a laboratory liquid culture minimal medium with 2% sucrose. Carotenoids and isoprenoid quinones were investigated using UHPLC-DAD-ESI-MS analysis. In general, tested strains of Asaia spp. were able to produce 10 carotenoids and 3 isoprenoid quinones: menaquinone-7, menaquinone-8, and ubiquinone-10. The main identified carotenoids in Asaia lannensis strains were phytofluene, neurosporene, α-carotene, while for Asaia bogorensis, neurosporene, canthaxanthin, and zeaxanthin were noted. What is more, tested Asaia spp. were able to produce myxoxanthophyll, which has so far been identified primarily in cyanobacteria. The results show that A. lannensis are characterized by statistically higher concentrations of produced carotenoids, as well as a greater variety of these compounds. We have noted that carotenoids were not only accumulated by bacterial cells, but also some strains of A. lannensis produced extracellular carotenoids.

Excitation energy transfer from the bacteriochlorophyll Soret band to carotenoids in the LH2 light-harvesting complex from Ectothiorhodospira haloalkaliphila is negligible

Pathways of intramolecular conversion and intermolecular electronic excitation energy transfer (EET) in the photosynthetic apparatus of purple bacteria remain subject to debate. Here we experimentally tested the possibility of EET from the bacteriochlorophyll (BChl) Soret band to the singlet S₂ level of carotenoids using femtosecond pump-probe measurements and steady-state fluorescence excitation and absorption measurements in the near-ultraviolet and visible spectral ranges. The efficiency of EET from the Soret band of BChl to S₂ of the carotenoids in light-harvesting complex LH2 from the purple bacterium Ectothiorhodospira haloalkaliphila appeared not to exceed a few percent.

Embedding carotenoids of spheroidene-branch biosynthesis into antenna complexes of sulfur photosynthetic bacteria

The possibility of embedding the carotenoids of spheroidene-branch biosynthesis (spheroidene and spheroidenone) from non-sulfur bacteria into the diphenylamine antenna complexes (DPA-complexes) from the sulfur bacteria Allochromatium minutissimum and Ectothiorhodospira haloalkaliphila with carotenoid synthesis inhibited by diphenylamine (DPA) was studied for the first time. It was found that spheroidene was embedded into the DPA-complexes from these bacteria at a level of 75-87%, with spheroidene embedding efficiency being 41-68% for the LH1-RC DPA-complexes and 71-89% for the LH2 DPA-complexes. The energy transfer efficiency from carotenoids to bacteriochlorophyll was shown to depend not only on the type of carotenoid but also on the very structure on the antenna complex.

Characterisation of the LH2 spectral variants produced by the photosynthetic purple sulphur bacterium Allochromatium vinosum

This study systematically investigated the different types of LH2 produced by Allochromatium (Alc.) vinosum, a photosynthetic purple sulphur bacterium, in response to variations in growth conditions. Three different spectral forms of LH2 were isolated and purified, the B800-820, B800-840 and B800-850 LH2 types, all of which exhibit an unusual split 800 peak in their low temperature absorption spectra. However, it is likely that more forms are also present. Relatively more B800-820 and B800-840 are produced under low light conditions, while relatively more B800-850 is produced under high light conditions. Polypeptide compositions of the three different LH2 types were determined by a combination of HPLC and TOF/MS. The B800-820, B800-840 and B800-850 LH2 types all have a heterogeneous polypeptide composition, containing multiple types of both α and β polypeptides, and differ in their precise polypeptide composition. They all have a mixed carotenoid composition, containing carotenoids of the spirilloxanthin series. In all cases the most abundant carotenoid is rhodopin; however, there is a shift towards carotenoids with a higher conjugation number in LH2 complexes produced under low light conditions. CD spectroscopy, together with the polypeptide analysis, demonstrates that these Alc. vinosum LH2 complexes are more closely related to the LH2 complex from Phs. molischianum than they are to the LH2 complexes from Rps. acidophila.

The LH2 complexes are assembled in the cells of purple sulfur bacterium Ectothiorhodospira haloalkaliphila with inhibition of carotenoid biosynthesis

The effect of the inhibitor of carotenoid (Car) biosynthesis, diphenylamine (DPA), on the cells of the purple sulfur bacterium Ectothiorhodospira (Ect.) haloalkaliphila has been studied. There occurs an inhibition of the biosynthesis of colored Cars (≥99 %) at 71 μM DPA. Considering "empty" Car pockets (Moskalenko and Makhneva 2012) the content of Cars in the DPA-treated samples is first calculated more correctly. The total content of the colored Cars in the sample at 71 μM DPA does not exceed 1 % of the wild type. In the DPA-treated cells (membranes) a complete set of pigment-protein complexes is retained. The LH2 complex at 71 μM DPA is isolated, which is identical to the LH2 complex of the wild type in near IR absorption spectra. This suggests that the principles for assembling this LH2 complex in vivo in the absence of colored Cars remain the same. These results are in full agreement with the data obtained earlier for Allochromatium (Alc.) minutissimum (Moskalenko and Makhneva 2012). They are as follows: (1) DPA almost entirely inhibits the biosynthesis of the colored Cars in Ect. haloalkaliphila cells. (2) In the DPA-treated samples non-colored Cars are detected at 53.25 μM DPA (as traces) and at 71 μM DPA. (3) DPA may affect both phytoene synthase (at ≤71 μM DPA) and phytoene desaturase (at ≥53.25 μM DPA). (4) The assembly of LH2 complex does occur without any colored Cars.