Light-harvesting by Botryococcus braunii

Abstract

The colonial green alga Botryococcus braunii is a promising candidate for biofuel production due to its high hydrocarbon content. However, slow growth limits its potential. We studied its photosynthetic machinery and photoacclimation to identify possible growth limitations. In chapter 2 we purified two light-harvesting complex (LHC) fractions in monomeric and trimeric forms, both containing at least two proteins with a molecular weight of around 25 kDa. We found that the Chlorophyll composition is similar to that of LHCII of plants and that the spectral properties of B. braunii LHCII are most similar to those of plant Lhcb3. However, B. braunii LHC trimers are less stable than plant LHCII trimers and bind loroxanthin in the central xanthophyll binding sites. Chapter 3 investigated the regulation of photosynthesis and photoprotection during photoacclimation for B. braunii and made a comparison with the properties of the single cellular species C reinhardtii. We found that B. braunii shares some high-light photoacclimation strategies with C. reinhardtii and other frequently studied green algae. Additionally, B. braunii has unique HL photoacclimation strategies, related to its colonial form, including strong internal shading by an increase in colony size and the accumulation of extracellular echinenone. Chapter 4 investigated the PSI-LHCI supercomplex of B. braunii using functional analysis and single-particle electron microscopy of the isolated PSI-LHCI supercomplexes. We established that the largest purified PSI-LHCI supercomplex contains ten LHCIs, but electron microscopy showed heterogeneity in the particles and a total of 13 unique binding sites for the LHCIs around the PSI core. Chapter 5 describes a light-driven xanthophyll cycle operating in C. reinhardtii, which involves the xanthophylls Lutein and Loroxanthin. We found that the Lo/(L+Lo) ratio in the cells varies by a factor of ten between cells grown in low or high light, leading to a change in the Lo/(L+Lo) ratio in trimeric LHCII from 0.5 in low light to 0.07 in HL. We found that trimeric LhcbMs binding Loroxanthin have 5 ± 1% higher excitation energy transfer from carotenoid to Chlorophyll. This thesis provides valuable insights into the photosynthetic machinery of B. braunii and C. reinhardtii and highlights the diversity of photoacclimation strategies in photosynthetic organisms.