Electron transport reactions of cyanobacterial photosynthesis and state-of-the-art in vivo measurement techniques

Cyanobacteria perform oxygenic photosynthesis using an integrated network of photosynthetic, respiratory, and auxiliary electron transport pathways embedded within the thylakoid membrane. Understanding how electrons are dynamically distributed among these interacting processes and how these flows are regulated under fluctuating environmental conditions requires approaches that can probe electron transport in vivo. In this review, we summarize the current understanding of linear, cyclic, auxiliary, respiratory, and extracellular electron transport in model cyanobacteria and highlight recent insights into the mechanisms that maintain redox balance and protect the photosynthetic apparatus. We critically assess state-of-the-art techniques used to quantify electron transport in vivo, including chlorophyll fluorescence, microscopy, membrane inlet mass spectrometry, differential absorbance spectroscopy, electrochromic shift measurements, photoelectrochemistry, and electron paramagnetic resonance spectroscopy. Finally, to address the major outstanding questions in regulation of photosynthesis, we recommend integration of techniques for simultaneous measurement of multiple processes and identify a need for non-invasive probes and modeling to achieve a systems-level understanding of cyanobacterial bioenergetics. Further study of nonmodel species is also needed to understand the diversity of cyanobacterial photosynthesis.