Eichner Meri, Basu Subhajit, Gledhill Martha, de Beer Dirk, Shaked Yeala
Microsensor Group, Max Planck Institute for Marine Microbiology, Bremen, Germany.
The Freddy & Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel.
Front Microbiol. 2019 Jul 10;10:1565. doi: 10.3389/fmicb.2019.01565. eCollection 2019.
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.
固氮蓝细菌通过固氮反应的副产物氢气释放过程以及由吸氢酶驱动的氢气摄取过程介导氢通量。在此，我们使用微电极来表征从埃拉特湾采集的全球重要固氮菌单个自然菌落中的氢和氧动态。我们观察到一天中氢动态逐渐变化，包括净氢气释放和净氢气摄取，以及各个菌落之间氢通量的巨大差异。在光照和黑暗条件下添加氢气的菌落中均观察到净氢气摄取。仅在光照下记录到净氢气释放，这反映了与氢气释放相关的光依赖固氮作用。净氢气释放和氢气摄取速率在下午2点之前均高于当天晚些时候。早晨这些明显的氢动态与强烈的净氧摄取以及先前报道的固氮作用日变化峰值相吻合。下午晚些时候，根据氧气测量确定的光合作用速率最高，而根据先前研究固氮速率下降，此时氢动态也不太明显。因此，观察到的氢动态日变化反映了氧消耗和固氮速率的日变化。值得注意的是，氢气的存在强烈刺激了自然菌落对矿物铁的摄取。这种效应的大小取决于一天中的时间，在下午2点之前开始的培养中反应最强，即涵盖最高吸氢酶活性时间的时段。基于这些发现，我们提出，通过为固氮细胞中的矿物铁还原提供电子源，氢气可能有助于菌落在铁摄取中发挥作用。
