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.

Despite the ecological importance of viruses, our understanding of their evolutionary dynamics in natural environments remains limited. This gap is particularly pronounced for giant dsDNA viruses of the phyla Nucleocytoviricota and Mirusviricota. Knowledge on their population genetic dynamics is mostly derived from a small number of laboratory-based experiments, while patterns in nature are rarely observed. To overcome this limitation, we traced the genetic structure and transcription status of Heterosigma akashiwo virus (HaV) using high-frequency, time-resolved sampling during a host bloom in a coastal area of Japan by integrating cell counting, metabarcoding, and metagenomic and metatranscriptomic sequencing. The results obtained revealed that HaV dominated the giant virus community in most samples, with relative abundance up to 56%. Despite its high abundance, the HaV population exhibited a low level of microdiversity, but had a higher pN/pS ratio than other giant viruses in the study site. Microdiversity increased during the early sampling period, peaked mid-sampling, and decreased during the later period, consistent with rapid diversification during viral expansion, which may be driven by both in situ mutations and the succession of pre-existing minor variants. Several accessory genes, including a glycosyltransferase and an endonuclease, were highly expressed, providing functional evidence consistent with host interaction-driven selective pressure during the bloom. Collectively, these results indicate that HaV population dynamics during algal blooms are shaped by host-driven selection acting on standing genetic variations.