Overexpression of mitochondrial?CAL1 reveals a unique photoprotection mechanism in intertidal resurrection red algae through decreasing photorespiration

There is no clear evidence of an operative xanthophyll cycle in species containing phycobilisomes, including red algae and cyanobacteria. In particular, intertidal red algae are subjected to adverse conditions such as high light (HL) stress periodically. Therefore, how red algae avert light-induced damage for photoprotection has attracted much interest. Photosynthetic oxygen evolution (POE) and photosystem II (PSII) activity analysis showed that Neopyropia yezoensis, an important economic red algae in the intertidal zone, possessed relatively high photosynthetic activity despite being under light intensity beyond the required of growth. However, such HL intensity has forced many algae to experience photoinhibition. Through systematic analysis, we found that gamma-type carbonic anhydrase-like1 (gamma CAL1) protein in mitochondria plays a key photoprotective role. We propose that the CO2 produced by photorespiration under HL is transformed into bicarbonate, through gamma CAL1, to prevent the escape of CO2. Bicarbonate is converted back into CO2, which is enriched near the Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), causing an increase in the partial pressure of CO2. To verify this result, we constructed gamma CAL1 overexpressing strains. It is interesting to discover the decreasing photorespiration flux in the transgenic strain, compared with the wild type (WT), is consistent with the findings reported in higher plants. Based on the above results, it is clear that promoting the efficiency of the Calvin cycle is an important mechanism for N. yezoensis to resist HL stress. During this process, sufficient CO2 can be supplied by photorespiration, where gamma CAL1 directly participates. Therefore, our research provides new evidence for revealing the strategies of phycobilisome-containing resurrection algae tolerating excess light energy.

Automated summary

There is no clear evidence of an operative xanthophyll cycle in species containing phycobilisomes, including red algae and cyanobacteria. However, research has found that the gamma-type carbonic anhydrase-like1 (gamma CAL1) protein in mitochondria plays a key photoprotective role in the red algae Neopyropia yezoensis, by transforming CO2 produced by photorespiration into bicarbonate to prevent CO2 escape and increase the partial pressure of CO2 near Rubisco. This mechanism promotes the efficiency of the Calvin cycle and helps red algae resist high light stress.