Oxic Subseafloor Microbial Communities Retain Anabolic Capacity under both Oxic and Oxygen-depleted Conditions.

Marine subseafloor sediments underlying open ocean regions are predominantly low-energy environments; however, they harbor vast microbial populations. How these communities persist and whether they retain anabolic capacity during transient oxygen depletion remain unclear. We herein investigated oxic sediments from the South Atlantic Gyre (SAG; IODP Site U1559D, core 7X-1) using paired incubations under oxic and anoxic (oxygen-depleted) conditions with defined C/N amendments. Sediments were amended with a 13C-amino acid mix+15N-ammonium, 13C-bicarbonate+15N-ammonium, or 15N-ammonium only and then incubated for 10, 30, and 75 d. Single-cell nanoscale secondary ion mass spectrometry (NanoSIMS) confirmed the assimilation of isotope-labeled substrates under oxic and anoxic conditions. Pronounced 13C enrichment occurred primarily in amino acid-amended incubations, whereas 15N incorporation from ammonium was widespread across all treatments, including ammonium-only incubations without added organic carbon, indicating assimilation supported by endogenous sedimentary carbon pools. Biomass-based growth responses varied among amendments and redox regimes, demonstrating that oxic-sourced communities may transition into measurable growth even under oxygen depletion. Amplicon-based community profiles exhibited directional, time-dependent shifts with limited segregation by substrate identity,‍ ‍suggesting that incubation time and shared successional dynamics dominated over amendment identity at the whole-community level. Collectively, these results indicate that SAG subseafloor communities retain a flexible anabolic potential that may be expressed under both oxic and oxygen-depleted conditions when substrates and/or oxidants become available.