Hansel Colleen M, Lentini Chris J, Tang Yuanzhi, Johnston David T, Wankel Scott D, Jardine Philip M
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA.
School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
ISME J. 2015 Nov;9(11):2400-12. doi: 10.1038/ismej.2015.50. Epub 2015 Apr 14.
A central tenant in microbial biogeochemistry is that microbial metabolisms follow a predictable sequence of terminal electron acceptors based on the energetic yield for the reaction. It is thereby oftentimes assumed that microbial respiration of ferric iron outcompetes sulfate in all but high-sulfate systems, and thus sulfide has little influence on freshwater or terrestrial iron cycling. Observations of sulfate reduction in low-sulfate environments have been attributed to the presumed presence of highly crystalline iron oxides allowing sulfate reduction to be more energetically favored. Here we identified the iron-reducing processes under low-sulfate conditions within columns containing freshwater sediments amended with structurally diverse iron oxides and fermentation products that fuel anaerobic respiration. We show that despite low sulfate concentrations and regardless of iron oxide substrate (ferrihydrite, Al-ferrihydrite, goethite, hematite), sulfidization was a dominant pathway in iron reduction. This process was mediated by (re)cycling of sulfur upon reaction of sulfide and iron oxides to support continued sulfur-based respiration--a cryptic sulfur cycle involving generation and consumption of sulfur intermediates. Although canonical iron respiration was not observed in the sediments amended with the more crystalline iron oxides, iron respiration did become dominant in the presence of ferrihydrite once sulfate was consumed. Thus, despite more favorable energetics, ferrihydrite reduction did not precede sulfate reduction and instead an inverse redox zonation was observed. These findings indicate that sulfur (re)cycling is a dominant force in iron cycling even in low-sulfate systems and in a manner difficult to predict using the classical thermodynamic ladder.
微生物生物地球化学的一个核心原则是，微生物代谢遵循基于反应能量产率的可预测的末端电子受体序列。因此，人们常常认为，除了高硫酸盐系统外，在所有环境中，微生物对三价铁的呼吸作用都优于对硫酸盐的呼吸作用，因此硫化物对淡水或陆地铁循环的影响很小。在低硫酸盐环境中观察到的硫酸盐还原现象，被归因于假定存在高度结晶的铁氧化物，使得硫酸盐还原在能量上更有利。在这里，我们确定了在含有淡水沉积物的柱体中，在低硫酸盐条件下的铁还原过程，这些沉积物用结构多样的铁氧化物和促进厌氧呼吸的发酵产物进行了改良。我们发现，尽管硫酸盐浓度很低，且无论铁氧化物底物是什么（水铁矿、铝水铁矿、针铁矿、赤铁矿），硫化作用都是铁还原的主要途径。这个过程是由硫化物与铁氧化物反应时硫的（再）循环介导的，以支持持续的基于硫的呼吸作用——这是一个涉及硫中间体生成和消耗的隐秘硫循环。虽然在用结晶度更高的铁氧化物改良的沉积物中没有观察到典型的铁呼吸作用，但一旦硫酸盐被消耗，在存在水铁矿的情况下，铁呼吸作用就会占主导地位。因此，尽管能量学上更有利，但水铁矿的还原并没有先于硫酸盐的还原，反而观察到了相反的氧化还原分区。这些发现表明，即使在低硫酸盐系统中，硫的（再）循环也是铁循环中的主导力量，而且其方式难以用经典的热力学阶梯来预测。
