Cyanobacterial XXXX Modulate Photosynthetic Cyclic Electron Flow to Assist Growth under Cold Stress.

@entity19765 @entity450 @entity7294 @entity9436 @entity1110 @entity13418 @entity324 @entity38 @entity855
All cyanobacterial membranes contain diesel-range C15-C19 @entity9436 at concentrations similar to @entity38 . Recently, two universal but mutually exclusive @entity7294 production pathways in cyanobacteria were discovered. We engineered a mutant of @entity324 that produces no @entity19765 , which grew poorly at low temperatures. We analyzed this defect by assessing the redox kinetics of PSI. The mutant exhibited enhanced @entity450 ( @entity450 ), especially at low temperature. @entity450 raises the @entity855 : @entity1110 ratio from photosynthesis and balances reductant requirements of biosynthesis with maintaining the redox poise of the electron transport chain. We conducted in silico flux balance analysis and showed that growth rate reaches a distinct maximum for an intermediate value of @entity450 equivalent to recycling 1 electron in 4 from PSI to the @entity13418 pool. Based on this analysis, we conclude that the lack of membrane @entity19765 causes higher @entity450 , perhaps for maintenance of redox poise. In turn, increased @entity450 reduces growth by forcing the cell to use less energy-efficient pathways, lowering the quantum efficiency of photosynthesis. This study highlights the unique and universal role of medium-chain @entity9436 in cyanobacterial thylakoid membranes: they regulate redox balance and reductant partitioning in these oxygenic photosynthetic cells under stress.
