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Abstract
The colonial green alga Botryococcus braunii (BB) is a potential source of biofuel due to its natural high hydrocarbon content. Unfortunately, its slow growth limits its biotechnological potential. Understanding its photosynthetic machinery could help to identify possible growth limitations. Here, we present the first study on BB light-harvesting complexes (LHCs). We purified two LHC fractions containing the complexes in monomeric and trimeric form. Both fractions contained at least two proteins with molecular weight (MW) around 25 kDa. The chlorophyll composition is similar to that of the LHCII of plants; in contrast, the main xanthophyll is loroxanthin, which substitutes lutein in most binding sites. Circular dichroism and 77 K absorption spectra lack typical differences between monomeric and trimeric complexes, suggesting that intermonomer interactions do not play a role in BB LHCs. This is in agreement with the low stability of the BB LHCII trimers as compared to the complexes of plants, which could be related to loroxanthin binding in the central (L1 and L2) binding sites. The properties of BB LHCII are similar to those of plant LHCII, indicating a similar pigment organization. Differences are a higher content of red chlorophyll a, similar to plant Lhcb3. These differences and the different Xan composition had no effect on excitation energy transfer or fluorescence lifetimes, which were similar to plant LHCII.
Highlights
Botryococcus braunii (BB) Kützing (Trebouxiophyceae, Chlorophyta) is a colonial green alga found in fresh and brackish lakes and ponds throughout different climate zones with big potential for production of biofuel, due to its natural high hydrocarbon content (Metzger and Largeau 2005) and in some strains of carbohydrates with industrial potential (Fernandes et al 1989)
The solubilized membranes were separated by ultracentrifugation in sucrose density gradients. α-DM and OG (OG is not shown) solubilization resulted in two main green bands in the sucrose gradient at molecular weight (MW) corresponding to monomeric and trimeric light-harvesting complexes (LHCs) (Fig. 1a, B2–B3)
The SDSPAGE shows that bands B2 and B3 contain at least two proteins with a molecular weight in the range of that of the LHCs of A. thaliana and C. reinhardtii (Fig. 1b)
Summary
Botryococcus braunii (BB) Kützing (Trebouxiophyceae, Chlorophyta) is a colonial green alga found in fresh and brackish lakes and ponds throughout different climate zones with big potential for production of biofuel, due to its natural high hydrocarbon content (Metzger and Largeau 2005) and in some strains of carbohydrates with industrial potential (Fernandes et al 1989). For the purification of LHCs 1% α-DM at room temperature was selected as the optimal solubilization condition (Fig. 1a), because it is a good compromise between yield and sample quality: lower concentrations yielded considerably less complexes and higher concentrations could lead to pigment loss (Ruban et al 1999). The Chl/carotenoid (Car) ratio is 4.8–5 for both fractions, much higher than in monomeric and trimeric LHCs of plants.
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