Artificial humic substances activate a rapid anoxic microbial carbon pump

The accumulation of organic carbon in anoxic environments is a critical component of the global carbon cycle, though typically considered a slow process. Here, we use Artificial Humic Substances (AHS) — a substrate with engineered functional complexity containing both labile and complex molecular fractions - to activate and accelerate the Microbial Carbon Pump (MCP). Amending anoxic sediment mesocosms with AHS resulted in the accretion of 18.5 g·L-1 of organic carbon in the surface layer over six weeks, representing a carbon retention efficiency of 73% within the organic pool. This rapid transformation is driven by a specialized, low-diversity microbial consortium, dominated by Firmicutes and sulfate-reducing bacteria, which efficiently convert labile AHS fractions into a new, stable pool of microbial necromass. Advanced molecular analyses, including ultra-high-resolution mass spectrometry, confirm the de novo synthesis of this carbon pool, whose chemical signature is determined by microbial processing, not the initial substrate. These results demonstrate that substrate engineering can activate a high-velocity mode of the MCP, offering a new principle for designing and implementing carbon storage strategies in anoxic systems.