Oxygen scavenging enables microoxic survival of the marine anammox bacterium <i>Scalindua</i> sp.

Marine anammox bacteria of the genus Scalindua play a pivotal role in nitrogen loss within oceanic oxygen minimum zones (OMZs), yet their physiological resilience to oxygen exposure remains poorly characterized. Here, we show that Scalindua sp. possesses an intrinsic capacity to reduce ambient oxygen under microoxic conditions (headspace O₂ ≈ 3%), exhibiting specific oxygen reduction rates (SORRs) of ~10 nmol-O₂ min-1 mg- protein-1. Anammox activity, evidenced by 29N₂ production, resumed once dissolved oxygen (DO) declined below ~5 μM, defining the DO threshold. This oxygen-scavenging ability is likely mediated by direct oxygen reduction to water via cbb₃-type cytochrome c oxidase (CcO) and A-type flavodiiron proteins (Fdps), with minimal reactive oxygen species (ROS) generation. CcO activity increased rapidly and markedly upon nitrite addition, consistent with the immediate onset of oxygen consumption and the transcriptional induction of CcO subunit genes. Nitrite was essential for sustaining oxygen consumption, and the rapid CcO activation suggests that nitrite stimulates CcO function through an unresolved regulatory mechanism. Nitrite oxidation also supplies supplementary electrons for oxygen reduction independently of core anammox metabolism, underscoring a flexible substrate-driven detoxification mechanism that enables Scalindua sp. to cope with transient microoxia. Together, these results show that Scalindua sp. employs a multi-layered defense -moderate oxygen-reduction capacity coupled with efficient ROS detoxification- conferring high and reversible oxygen tolerance. Although laboratory conditions cannot fully replicate natural OMZ complexity, our findings indicate that this physiological flexibility reflects the potential ecological resilience of Scalindua sp. in dynamic ocean environments.