Electron transport in cyanobacterial thylakoid membranes: are cyanobacteria simple models for photosynthetic organisms?

Abstract

Cyanobacteria are structurally the simplest oxygenic phototrophs, but it is difficult to understand the regulation of their photosynthesis because the photosynthetic and respiratory processes share the same thylakoid membranes and cytosolic space. This review aims to summarize the molecular mechanisms and in vivo activities of electron transport in cyanobacterial thylakoid membranes based on the latest progress in photosynthesis research in cyanobacteria. Photosynthetic linear electron transport for CO2 assimilation is the dominant electron flux in the thylakoid membranes. The capacity for O2 photoreduction mediated by flavodiiron proteins is comparable to that for photosynthetic CO2 assimilation in cyanobacteria. Additionally, cyanobacterial thylakoid membranes harbour the significant electron flux of respiratory electron transport through a homologue of respiratory complex I, which is also recognized as forming part of the cyclic electron transport chain if it is coupled with photosystem I in the light. Further, O2-independent alternative electron transport through hydrogenase and nitrate reductase function with reduced ferredoxin as the electron donor. Whereas all these electron transport chains are understood individually, the regulatory complexity of the whole system remains to be uncovered in the near future.

Keywords: Alternative electron sink; cyanobacteria; cyclic electron transport; flavodiiron proteins; photosynthesis; respiration.