Domestication of the green alga Chlorella sorokiniana: reduction of antenna size improves light-use efficiency in a photobioreactor

Abstract

BackgroundThe utilization of biomass from microalgae for biofuel production is one of the key elements for the development of a sustainable and secure energy supply. Among the different microalgae, Chlorella species are of interest because of their high productivity, high lipid content, and resistance to the high light conditions typical of photobioreactors. However, the economic feasibility of growing algae at an industrial scale is yet to be realized, in part because of biological constraints that limit biomass yield. A key issue is the inefficient use of light due to uneven light distribution, and the dissipation of excess absorbed light as heat. The successful implementation of biofuel production facilities requires the development of algal strains with enhanced light use efficiency in photobioreactors. Such domestication strategies include decreasing the absorption cross section in order to enhance light penetration, increasing the size of metabolic sinks per chlorophyll and minimizing feedback energy dissipation.ResultsIn this work we applied random mutagenesis and phenotypic selection to the thermotolerant, fast-growing Chlorella species, C. sorokiniana. Truncated antenna mutants (TAMs) were selected that exhibited a lower fluorescence yield than the wild-type (WT) strain. Six putatively interesting mutants were selected by high throughput fluorescence video imaging, two of which, TAM-2 and TAM-4, were found to have approximately half the chlorophyll content per cell and LHCII complement per PSII with respect to the WT. In batch culture, TAM-2 showed an increased photon use efficiency, yielding a higher Pmax at saturating irradiances with respect to the WT. Cultivation of TAM-2 in both laboratory-scale and outdoor photobioreactors showed higher productivity than WT, with a 30% higher biomass yield in dense cell suspensions typical of industrial photobioreactors.ConclusionsThese results suggest that generation of mutants with low chlorophyll content can significantly improve the light-to-biomass conversion efficiency of C. sorokiniana under mass culture conditions. However, owing to the lack of sexual reproduction in this species, the presence of additional mutations might affect growth rate, suggesting that selection should include evaluation of multiple independent mutants for each desired phenotype.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-014-0157-z) contains supplementary material, which is available to authorized users.

The algal chloroplast as a synthetic biology platform for production of therapeutic proteins

“…However, this increase in yield needs to be coupled with improvements in phototrophic algal biomass production in order to make the platform commercially competitive. Such improvements will come from a combination of media optimization [66], improvements in photobioreactor (PBR) design [67] and strain domestication, such as selection for reduced light-antenna mutants that show higher productivity in PBRs as a consequence of greater light penetration [68]. Alternatively, it might be more cost-effective to switch to mixotrophic production in PBRs, or heterotrophic cultivation in fermenters, where much higher biomass productivity can be achieved [69].…”

Strategies for Optimizing Algal Biology for Enhanced Biomass Production

“…To achieve an intermediate antenna size, the synthesis of Chl b, which is bound only to the peripheral light-harvesting antenna complex proteins, was reduced by inhibiting the synthesis of Chl a oxygenase, the enzyme that converts Chl a into Chl b, using RNAi technology (Perrine et al, 2012). Most recently, this approach to reduce antenna size, and thus increase biomass productivity, was performed in C. sorokiniana, where UV mutagenesis was utilized to isolate truncated antenna mutants (Cazzaniga et al, 2014). PSII antenna reduction (48% of LHCII protein content compared to www.frontiersin.org wild-type) resulted in a 32% increased mean biomass productivity over wild-type strains when grown in photobioreactors.…”

Antenna size reduction in microalgae mass culture

“…In many cases wrong definitions are used for 'productivity' (Cazzaniga et al 2014) and 'growth rate' (Perrine et al 2012). This has led to the often quoted claim that antenna size mutant technology could triple the biomass productivity compared to wild type cells (Melis 2009), which is not-substantiated.…”

“…Microalgae have been reported to radically increase their NPQ capacity even at moderate light intensities (Bonente et al 2012, Cazzaniga et al 2014 2014), though in one study, it was shown that the total contribution of these screening pigments to total NPQ was limited (Masojídek et al 1999). We did not measure a relative change in the absorption spectrum after a transfer from high light to mass culture.…”

“…The lower biomass specific light absorption rate leads to a higher productivity of mutant cultures than that of wild type cultures under high light conditions (Mussgnug et al 2007). In practice, some mutants were indeed reported to demonstrate improved growth characteristics under specific light conditions (Mussgnug et al 2007, Mitra et al 2008, Cazzaniga et al 2014.…”