Questioning Dark Oxygen Production in the Deep-sea Ferromanganese Nodule Field

Previous studies have concluded that the natural process where oxygen is consumed as decomposition of organic matter that supplied from shallow waters occurs on the deep-sea floor. Sweetman et al.1 presented the surprising observation that deep-sea ferromanganese nodules generate oxygen, which they labelled as dark oxygen production. The authors claimed that oxygen was generated through the electrolysis of seawater by ferromanganese nodules. If true, it represents the discovery of a significant unknown energy source. Consequently, it may have an impact comparable to that of the discovery of submarine hydrothermal systems and the ecosystems that live there. However, regarding this interesting discovery and claim, I raise the following three main concerns: (1) the lack of evidence for electrolysis, (2) the improbability of the energy source, and (3) inconsistency with existing research.