Heterogeneity of nitrogen fixation in the mesopelagic zone of the South China Sea

Biological dinitrogen (N 2 ) fixation, the energetically expensive conversion of N 2 gas to ammonia, plays an essential role in balancing the nitrogen budget in the ocean. Accumulating studies show detectable N 2 fixation rates below the euphotic zone in various marine systems, revealing new insights of marine N 2 fixation. However, the reported rates are highly variable and frequently fall close to detection limits, raising the question of the ubiquity and significance of N 2 fixation in the global dark ocean. Using highly sensitive isotopic labeling incubation including a set of control incubations, we confirm the occurrence of mesopelagic N 2 fixation in the South China Sea. Interestingly, we consistently observed that ca. 30% of samples show a significant elevation of 15 N in the particulate nitrogen after incubation at most depths (200 - 1000 m). Although this approach does not allow accurate quantification of N 2 fixation rates, our data suggest the occurrence of dark N 2 fixation yet with highly heterogeneous signals in the mesopelagic zone of the South China Sea. A data compilation of reported N 2 fixation in the global dark ocean further reveals that such heterogeneity has also been observed elsewhere, unveiling the ubiquitous heterogeneity in mesopelagic N 2 fixation. Thus, we call for more observations to constrain mesopelagic N 2 fixation budgets and to understand the underlying mechanism for such heterogeneity.