Stage-dependent shifts in bacterial communities and nitrogen cycling during Pyropia cultivation on the west coast of Korea.

As seaweed aquaculture expands globally, nitrogen (N) availability is becoming central to understanding both crop productivity and the biogeochemical role of coastal farming systems. Yet how bacterial assemblages and predicted N-transformation potential vary across cultivation phases remains poorly constrained. Here, we examined how environmental conditions and bacterial N-cycling potential varied across a Pyropia cultivation cycle on the central west coast of Korea. Early cultivation occurred under stronger freshwater influence and relatively high dissolved inorganic nitrogen (DIN) availability, with bacterial assemblages including taxa associated with particle-rich and surface-associated habitats. Mid-cultivation was marked by sharp DIN and dissolved organic carbon depletion, maximum dissolved N2O, reduced bacterial diversity, and a shift toward heterotrophic lineages associated with algal-derived organic matter. Tax4Fun2 profiles indicated lower predicted N2 fixation and nitrification potentials during this phase, but higher predicted ammonification potential and greater predicted abundance of downstream NO2- reduction biomarkers, including denitrification- and DNRA-related markers. After cultivation, partial DIN recovery coincided with a distinct bacterial assemblage and continued reconfiguration of predicted N-cycling biomarker profiles. These stage-resolved patterns suggest that nutrient depletion during cultivation was associated with shifts in the predicted potential for different N pathways, rather than demonstrating direct redistribution of N fluxes. Although direct rate measurements and functional gene validation are needed, our results identify mid-cultivation as a key biogeochemical window in which nutrient depletion, elevated N2O, bacterial assemblage reorganization, and predicted N cycle reconfiguration converge in a shallow Pyropia farming system.