Outdoor photoacclimation of two Chlorella strains characterized by normal and reduced light-harvesting antennas: photosynthetic activity and chlorophyll-protein organization

Photoacclimation of two Chlorella cultures - strain g-120 characterised by a reduced size of light-harvesting antenna complex (LHC) and strain R-117 with full antenna size was studied during 5-day outdoor trials. The aim was to correlate the functional and structural changes in the photosynthetic apparatus to culture growth, photochemical activity and thylakoid composition of chlorophyll (Chl)-protein complexes and corresponding polypeptides. Chlorella g-120 was characterized by a low Chl/biomass ratio (< 0.5% of dry weight), about four times lower compared to Chlorella R-117. The important observation was that the high molecular mass Chl-binding protein supercomplexes, i.e. Photosystem II (PSII) and Photosystem I (PSI) cores associated with LHCs were physically missing or negligible in Chlorella g-120. However, there were no visible changes in Chl-protein composition in the g-120 strain during its acclimation to phototrophic conditions. Measurement of the effective absorption cross-section of PSII centres confirmed a markedly reduced functional antenna size in Chlorella g-120 as compared to R-117 which coincided with the absence of the PSII-LHC supercomplexes. We demonstrated that Chlorella g-120 represents a typical reduced antenna-size strain due to its Chl-protein composition. As compared to the full-antenna Chlorella R-117 strain, the outdoor cultures of Chlorella g-120 showed significantly lower oxygen production and electron transport rate measured in-situ. On the contrary, Chlorella g-120 revealed increased futile energy dissipation via non-photochemical quenching and higher respiration compared to Chlorella R-117. Consequently, the potential use of microalgae strains with reduced LHCII for outdoor mass cultivation may not be as straightforward as anticipated from laboratory experiments.

Automated summary

The photosynthetic performance and electron transport rate of Chlorella cultures with reduced antenna size were studied during outdoor trials. The results showed that the strain with reduced antenna size had lower oxygen production and electron transport rate, but higher non-photochemical quenching and respiration compared to the strain with full antenna size. These findings suggest that the use of microalgae strains with reduced antenna size for outdoor mass cultivation may not be as effective as expected.