Hydrogen Dynamics in Colonies and Their Potential Role in Mineral Iron Acquisition.

N-fixing cyanobacteria mediate H fluxes through the opposing processes of H evolution, which is a by-product of the N fixation reaction, and H uptake, which is driven by uptake hydrogenases. Here, we used microelectrodes to characterize H and O dynamics in single natural colonies of the globally important N fixer collected from the Gulf of Eilat. We observed gradually changing H dynamics over the course of the day, including both net H evolution and net H uptake, as well as large differences in H fluxes between individual colonies. Net H uptake was observed in colonies amended with H in both light and dark. Net H evolution was recorded in the light only, reflecting light-dependent N fixation coupled to H evolution. Both net H evolution and H uptake rates were higher before 2 pm than later in the day. These pronounced H dynamics in the morning coincided with strong net O uptake and the previously reported diel peak in N fixation. Later in the afternoon, when photosynthesis rates determined by O measurements were highest, and N fixation rates decrease according to previous studies, the H dynamics were also less pronounced. Thus, the observed diel variations in H dynamics reflect diel changes in the rates of O consumption and N fixation. Remarkably, the presence of H strongly stimulated the uptake of mineral iron by natural colonies. The magnitude of this effect was dependent on the time of day, with the strongest response in incubations that started before 2 pm, i.e., the period that covered the time of highest uptake hydrogenase activity. Based on these findings, we propose that by providing an electron source for mineral iron reduction in N-fixing cells, H may contribute to iron uptake in colonies.