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Abstract:
N-2-fixing cyanobacteria mediate H-2 fluxes through the opposing
processes of H-2 evolution, which is a by-product of the N-2 fixation
reaction, and H-2 uptake, which is driven by uptake hydrogenases. Here,
we used microelectrodes to characterize H-2 and O-2 dynamics in single
natural colonies of the globally important N-2 fixer Trichodesmium
collected from the Gulf of Eilat. We observed gradually changing H-2
dynamics over the course of the day, including both net H-2 evolution
and net H-2 uptake, as well as large differences in H-2 fluxes between
individual colonies. Net H-2 uptake was observed in colonies amended
with H-2 in both light and dark. Net H-2 evolution was recorded in the
light only, reflecting light-dependent N-2 fixation coupled to H-2
evolution. Both net H-2 evolution and H-2 uptake rates were higher
before 2 pm than later in the day. These pronounced H-2 dynamics in the
morning coincided with strong net O-2 uptake and the previously reported
diel peak in N-2 fixation. Later in the afternoon, when photosynthesis
rates determined by O-2 measurements were highest, and N-2 fixation
rates decrease according to previous studies, the H-2 dynamics were also
less pronounced. Thus, the observed diel variations in H-2 dynamics
reflect diel changes in the rates of O-2 consumption and N-2 fixation.
Remarkably, the presence of H-2 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-2-fixing cells, H-2 may contribute to iron uptake in Trichodesmium
colonies.
