Blue light screening reduce blue light photosynthesis effeciency of cyanolichens compared with chlorolichens

Cyanolichens have phycobiliproteins that mainly absorb light in the green part of the spectrum. Thereby, phycobiliproteins enhance the utilization of light transmitted through a canopy. The combination of phycobiliproteins and chl a may thus improve photosynthesis in shaded forest sites. We compared the chlorolichens Lobaria pulmonaria and Peltigera leucophlebia with the cyanolichens Lobaria hallii and Peltigera praetextata by measuring light response curves for photosynthetic CO2 uptake, O2 evolution, as well as photosystem II efficiency in blue, green and red light, respectively. Maximal photosynthetic CO2 uptake was slightly higher for both cyanolichens than for the chlorolichens in green light. In red light there was no difference in maximal CO2 uptake, whereas both cyanolichens had substantially lower photosynthesis in blue light. The same trend occurred for photosynthetic O2 evolution. Apparent electron transport rate (ETR) did not differ between red and green light in any of the species. For the cyanolichens, ETR showed no sign of light saturation in blue light, indicating that little blue light absorbed is used in photosynthesis. Reflectance spectra showed that green light was less reflected in the cyanolichens, which may partly explain the slightly higher cyanobacterial photosynthetic CO2 uptake in green light. At the same time, the reflectance patterns in the blue region cannot explain the reduced photosynthesis in cyanolichens in blue light. Transmittance of light through the combined cortex and photobiont layer indicated that also the blue light was efficiently absorbed. Screening was estimated indirectly by comparing chlorophyll fluorescence ratios between chlorophyll fluorescence excited with blue and red light. Much lower blue/red ratios occurred in the cyanolichens L. hallii and P. praetextata than in the chlorolichens L. pulmonaria and P. leucophlebia, indicating that screening of blue light in the cyanolichens inhibited blue light from reaching the photosynthetic apparatus. Cyanobacteria may contain the UV and blue light absorbing compound scytonemin. HPLC analyses showed that L. hallii contained some scytonemin that partly may explain the blue light screening, whereas P. praetextata contained no scytonemin, Therefore, the mechanism for low cyanolichen photosynthesis in blue light remains unknown