Combined resistance to oxidative stress and reduced antenna size enhance light-to-biomass conversion efficiency in Chlorella vulgaris cultures

Abstract

Background Microalgae are efficient producers of lipid-rich biomass, making them a key component in developing a sustainable energy source, and an alternative to fossil fuels. Chlorella species are of special interest because of their fast growth rate in photobioreactors. However, biological constraints still cast a significant gap between the high cost of biofuel and cheap oil, thus hampering perspective of producing CO2-neutral biofuels. A key issue is the inefficient use of light caused by its uneven distribution in the culture that generates photoinhibition of the surface-exposed cells and darkening of the inner layers. Efficient biofuel production, thus, requires domestication, including traits which reduce optical density of cultures and enhance photoprotection. Results We applied two steps of mutagenesis and phenotypic selection to the microalga Chlorella vulgaris. First, a pale-green mutant (PG-14) was selected, with a 50% reduction of both chlorophyll content per cell and LHCII complement per PSII, with respect to WT. PG-14 showed a 30% increased photon conversion into biomass efficiency vs. WT. A second step of mutagenesis of PG-14, followed by selection for higher tolerance to Rose Bengal, led to the isolation of pale-green genotypes, exhibiting higher resistance to singlet oxygen (strains SOR). Growth in photobioreactors under high light conditions showed an enhanced biomass production of SOR strains with respect to PG-14. When compared to WT strain, biomass yield of the pale green + sor genotype was enhanced by 68%. Conclusions Domestication of microalgae like Chlorella vulgaris, by optimizing both light distribution and ROS resistance, yielded an enhanced carbon assimilation rate in photobioreactor.

Potential and Challenges of Improving Photosynthesis in Algae

“…The major efforts are focused on obtaining strains with higher photosynthetic efficiency in order to decrease the unitary cost of algal biomass. The best results have been obtained by enhancing the homogeneity of light distribution within photobioreactors [82] and by decreasing their susceptibility to photodamage [89]. Besides the obvious strategy of targeting antenna systems, photoprotection was also enhanced by increasing photochemical quenching by boosting CO 2 fixation through over-expressing RuBisCO activity, and other rate-liming enzymes in the CBB cycle [109].…”

“…Additional perspectives towards enhancing the light use efficiency in algae are likely to be developed in the future based on the emerging functional diversity of individual Lhcm proteins which have been reported to be involved in state1-state2 transitions, NPQ and/or in sustained photoprotection [75,77] thus opening the perspective of enhancing such functions selectively in industrial strains. Nevertheless, it is not yet clear how engineering antennas can be combined with the well-established enhanced growth efficiency of truncated antenna strains [12,89].…”

“…Analogously, UV-mutagenesis and selection under high irradiance (2000 µmol photons m −2 s −1 ) identified the Light Responsive Signal 1 (LSR1) gene, which conferred improved resistance against exogenous ROS [93]. Recently, Dall'Osto and colleagues [89] applied two steps of mutagenesis and phenotypic selection to Chlorella vulgaris. First, they selected a strain characterized by a 50% reduction of Chl content per cell and a 30% increased photon-to-biomass conversion efficiency with respect to WT.…”

“…First, they selected a strain characterized by a 50% reduction of Chl content per cell and a 30% increased photon-to-biomass conversion efficiency with respect to WT. After a second mutagenesis cycle followed by a selection on Rose Bengal, they selected pale-green genotypes exhibiting higher resistance to singlet oxygen (strains SOR) that showed a further enhancement in biomass productivity with respect to both parental and WT strains [89].…”

Green Production and Biotechnological Applications of Cell Wall Lytic Enzymes

“…Hydrolysates from lignocellulosic materials can be used as feed for sustaining the mixotrophic growth of oleaginous microalgae such as Botryococcus braunii, Monoraphidium neglectum, and those belonging to Chlorella spp. [136][137][138]. Under mixotrophy and specific growth conditions (e.g., nitrogen starvation), these microalgae may accumulate lipids up to 60% of their dry weight [136,139].…”