Re-evaluation of nitrogenase oxygen-protective mechanisms in the planktonic marine cyanobacterium Trichodesmium

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Abstract
There were no apparent differences in distribution of photosystems (PS) I and II between central cells and tip cells of trichomes of Trichodesmium thiebautii colonies collected in the eastern Caribbean Sea. Single cell absorption spectra, observations of phycoerythrin fluorescence via epifluorescence microscopy, and enhanced DCMU chlorophyll a fluorescence were essentially identical for both internal and peripheral regions, thus indicating that nitrogenase is not protected from O2 via the absence of PS II in the central region of the colony. The calculated DCMU-induced Fluorescence Response Index (FRI) indicates that both central and tip cell areas are similar (FRI range 0.55 to 0.65), and active in photosynthesis. Our microelectrode measurements of O2 within and outside of healthy colonies did not indicate a low O2 region in the colony center relative to concentrations at the surface. A theoretical model of oxygen diffusion and respiration in the colony indicated that respiration alone cannot account for the low O2 microzones previously observed by others. The clear sensitivity of T. thiebautii nitrogenase to O2 indicates that there must be other, probably intracellular, mechanisms of protecting nitrogenase from O2 inactivation.