Abstract
Microalgae are unicellular photosynthetic organisms considered as potential alternative sources for biomass, biofuels or high value products. However, their limited biomass productivity represents a bottleneck that needs to be overcome to meet the applicative potential of these organisms. One of the domestication targets for improving their productivity is the proper balance between photoprotection and light conversion for carbon fixation. In the model organism for green algae, Chlamydomonas reinhardtii, a photoprotective mechanism inducing thermal dissipation of absorbed light energy, called Non-photochemical quenching (NPQ), is activated even at relatively low irradiances, resulting in reduced photosynthetic efficiency. Two pigment binding proteins, LHCSR1 and LHCSR3, were previously reported as the main actors during NPQ induction in C. reinhardtii. While previous work characterized in detail the functional properties of LHCSR3, few information is available for the LHCSR1 subunit. Here, we investigated in vitro the functional properties of LHCSR1 and LHCSR3 subunits: despite high sequence identity, the latter resulted as a stronger quencher compared to the former, explaining its predominant role observed in vivo. Pigment analysis, deconvolution of absorption spectra and structural models of LHCSR1 and LHCR3 suggest that different quenching efficiency is related to a different occupancy of L2 carotenoid binding site.
Highlights
Among photosynthetic organisms, microalgae have high potential with a wide range of applications of their biomass and its derivatives, from food/feed to biofuels and high-value products[1,2,3,4,5,6,7,8,9]
This conservancy is a common feature for Light Harvesting Complexes (LHC), the protein family evolved in eukaryotic organisms involved in the assembly of the external antenna systems of P hotosystems[33,34] with some exceptions: as in the case of L HCSR324, in LHCSR1 the residues involved in binding Chl 606 and Chl 614 are absent
Light Harvesting Complex Stress Related proteins (LHCSRs) subunits were reported to be able to sense the lumenal pH by protonation of specific acidic residues exposed to the lumen, leading to protein activation as a quencher of excitation energy[26,27]: all the glutamate and aspartate residues previously reported to be involved in LHCSR3 activation are conserved in LHCSR1 with the only exception of LHCSR3 D239 residue which is substituted with E (E233) in LHCSR1 (Fig. 1)
Summary
Microalgae have high potential with a wide range of applications of their biomass and its derivatives, from food/feed to biofuels and high-value products[1,2,3,4,5,6,7,8,9]. Photosynthetic organisms are exposed to variable light conditions, which can induce photodamage, affecting their photosynthetic efficiency, so they have developed various acclimation mechanisms, such as thermal dissipation of energy absorbed in excess, a process called non-photochemical quenching (NPQ)[10,13,14,15,16,17,18,19]. This mechanism is activated in excess, especially in controlled photobioreactor conditions, leading to a massive energy dissipation, up to 80% of light absorbed, through heat[10]. An in vitro study was used to elucidate LHCSR1 quenching capacity by its comparison with LHCSR3: carotenoid-binding properties were investigated by HPLC and spectral deconvolution of the absorption spectra in the 400–520 nm region (Soret region), where both chlorophylls (Chl) and carotenoids absorb, and quenching properties of LHCSR1 protein were investigated by time resolved fluorescence analysis
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