Free-living autotrophs drive a carbon fixation peak within the Orca Basin chemocline while heterotrophs dominate total activity from seawater to brine.

DIC fixation and total anabolic activity peak at the oxic–anoxic interface in Orca Basin, with autotrophs driving DIC fixation while heterotrophs drive total activity.

• Why it matters: Stratified DHABs like Orca Basin span steep chemical and salinity gradients, but how microbial lifestyle and spatial niche shape carbon cycling across the chemocline is unclear.
• What they did: They quantified total anabolic activity and DIC fixation across ten depths using dual-isotope labeling plus nanoSIMS, separating free-living versus particle-associated autotrophs and heterotrophs.
• The result: DIC fixation and total activity peak at the interface, particle-associated heterotrophs are more active than free-living there, and heterotrophs derive 4% to 10% of biomass from DIC while contributing 2% of peak DIC fixation.

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