Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543.
Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA 02139.
Proc Natl Acad Sci U S A. 2020 Feb 18;117(7):3433-3439. doi: 10.1073/pnas.1912313117. Epub 2020 Feb 3.
The balance between sources and sinks of molecular oxygen in the oceans has greatly impacted the composition of Earth's atmosphere since the evolution of oxygenic photosynthesis, thereby exerting key influence on Earth's climate and the redox state of (sub)surface Earth. The canonical source and sink terms of the marine oxygen budget include photosynthesis, respiration, photorespiration, the Mehler reaction, and other smaller terms. However, recent advances in understanding cryptic oxygen cycling, namely the ubiquitous one-electron reduction of O to superoxide by microorganisms outside the cell, remains unexplored as a potential player in global oxygen dynamics. Here we show that dark extracellular superoxide production by marine microbes represents a previously unconsidered global oxygen flux and sink comparable in magnitude to other key terms. We estimate that extracellular superoxide production represents a gross oxygen sink comprising about a third of marine gross oxygen production, and a net oxygen sink amounting to 15 to 50% of that. We further demonstrate that this total marine dark extracellular superoxide flux is consistent with concentrations of superoxide in marine environments. These findings underscore prolific marine sources of reactive oxygen species and a complex and dynamic oxygen cycle in which oxygen consumption and corresponding carbon oxidation are not necessarily confined to cell membranes or exclusively related to respiration. This revised model of the marine oxygen cycle will ultimately allow for greater reconciliation among estimates of primary production and respiration and a greater mechanistic understanding of redox cycling in the ocean.
海洋中分子氧的源和汇之间的平衡自产氧光合作用进化以来，对地球大气的组成产生了巨大影响，从而对地球的气候和（亚）地表的氧化还原状态产生了关键影响。海洋氧收支的典型源和汇术语包括光合作用、呼吸作用、光呼吸、Mehler 反应和其他较小的术语。然而，人们对隐氧循环（即微生物在细胞外将氧单电子还原为超氧化物的普遍现象）的理解最近取得了进展，但尚未将其作为全球氧动态的潜在参与者进行探索。在这里，我们表明，海洋微生物的暗细胞外超氧化物产生代表了一个以前未被考虑的全球氧通量和汇，其规模与其他关键术语相当。我们估计，细胞外超氧化物的产生代表了一个总氧汇，占海洋总氧气产生的约三分之一，净氧汇相当于总氧气产生的 15%到 50%。我们进一步证明，这种海洋暗细胞外超氧化物总通量与海洋环境中超氧化物的浓度一致。这些发现强调了海洋中活性氧的丰富来源，以及一个复杂和动态的氧循环，其中氧消耗和相应的碳氧化不一定局限于细胞膜或仅与呼吸作用有关。这种海洋氧循环的修正模型最终将允许在初级生产和呼吸作用的估计之间更好地协调，并更好地理解海洋中的氧化还原循环。
