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

Abstract

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.

Photochemical and Photophysical Properties of Carotenoids and Reactive Oxygen Species: Contradictions Relating to Skin and Vision

“…Even though the results from Ogilby were somewhat surprising, far more surprising is the recent postulate from Moskalenko and co-workers [18] arising from photosynthetic bacteria studies. This work shows that carotenoids do not protect bacteriochlorophylls in isolated light-harvesting LH2 complexes of photosynthetic bacteria.…”

Carotenoides

“…Esto es seguido por una serie de etapas de desaturación, saturación, etc., así como la introducción de varios grupos (hidroxilo, metilo, etc.) y, como resultado, se forma un sistema de dobles enlaces conjugados en la molécula de carotenoide, que es capaz de disipar la energía recibida de la BChl en forma de calor (Makhneva et al, 2021).…”

“…Los nombres de enzimas e intermediarios de la rama β,ε están señalados en verde y los de la rama β,β en azul. GGPP, geranilgeranil difosfato; PSY, fitoeno sintasa; PDS, fitoeno desaturasa; Z-ISO, 15-cis-δ-caroteno isomerasa; ZDS, ζ-caroteno desaturasa; CRTISO, carotenoide isomerasa; LCYB, licopeno β-ciclasa; LCYE, licopeno ε-ciclasa; BCH1/2, β-caroteno hidroxilasa 1/2; VDE, violaxantina deepoxidasa; ZEP, zeaxantina epoxidasa; NSY, neoxantina sintasa (Makhneva et al, 2021). (Moyo et al, 2022).…”

Chlorophylls: A Personal Snapshot

“…As reviewed by Lokstein et al in this issue [ 49 ], they often participate in light-harvesting, transferring their excitation with good to excellent quantum yields to Chls. Additionally, Makhneva et al [ 53 ] presented an example from purple bacteria where the protective function against one ROS, e.g., singlet oxygen, seems negligible.…”