Electron transport kinetics in the diazotrophic cyanobacterium Trichodesmium spp. grown across a range of light levels

The diazotrophic cyanobacterium Trichodesmium is a major contributor to marine nitrogen fixation. We analyzed how light acclimation influences the photophysiological performance of Trichodesmium IMS101 during exponential growth in semi-continuous nitrogen fixing cultures under light levels of 70, 150, 250, and 400 umol photons m-2 s-1, across diel cycles. There were close correlations between growth rate, trichome length, particulate organic carbon and nitrogen assimilation, and cellular absorbance, which all peaked at 150 umol photons m-2 s-1. Growth rate was light saturated by about 100 umol photons m-2 s-1 and was photoinhibited above 150 umol photons m-2 s-1. In contrast, the light level (I k) to saturate PSII electron transport (e - PSII-1 s-1) was much higher, in the range of 450-550 umol photons m-2 s-1, and increased with growth light. Growth rate correlates with the absorption cross section as well as with absorbed photons per cell, but not to electron transport per PSII; this disparity suggests that numbers of PSII in a cell, along with the energy allocation between two photosystems and the state transition mechanism underlie the changes in growth rates. The rate of state transitions after a transfer to darkness increased with growth light, indicating faster respiratory input into the intersystem electron transport chain.