Cyclic electron transport pathways around photosystems I and II: Working together

The processes of cyclic electron transport around photosystems I and II (CET-PSI and CET-PSII) do not lead to O₂ evolution and CO₂ assimilation and are referred to as anoxygenic photosynthesis a broad sense, in contrast to specific processes in bacteria, which are commonly referred to as anoxygenic photosynthesis in a narrow sense. CET-PSI has been studied much more extensively than CET-PSII. Attempts to quantify CET-PSI have yielded contradictory results. It is not detected in non-stressed C3-plants using photoacoustic methods but is commonly considered as being observed when using Antimycin А which was had previously been proposed as a CET-PSI inhibitor. However, most researchers ignore recent data showing that Antimycin А primarily inhibits rather CET-PSII then CET-PSI. These facts, along with others, suggest that the contribution of CET-PSI to photosynthesis of non-stressed C3-plants has been overestimated. Our analysis of the data in this field also shows the possibility of underestimating CET-PSII, as well as anoxygenic photosynthesis in total, which is not excluded from being dominated over oxygenic photosynthesis. We point out that CET-PSI and CET-PSII cannot be studied separately. The difficulties in the quantitative evaluation of CET-PSII can be solved using photoacoustic techniques, which are highly promising in studies of anoxygenic photosynthesis.