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.