Diel study reveals increased nighttime bacterial activity and its connection to organic compounds in the ocean.

The linkages between dissolved organic matter (DOM) dynamics and bacterial activity (BAct) are key to regulating carbon fluxes in marine food webs. While transcriptional activities have shown diel patterns, the connections between BAct and DOM on the diel timescale remain unclear. This study explored how bacterioplankton transform DOM over diel cycles in the euphotic and upper twilight zones (to 300 m) of the North Atlantic subtropical gyre. BAct peaked at night in the euphotic zone, following daytime maxima in chlorophyll fluorescence, suggesting a delayed bacterial response to photosynthetic activity. Our data show nighttime BAct exceeded daytime rates 34-47% under dark incubations. Total dissolved amino acids (TDAA) concentrations were lowest at night in the upper twilight waters, indicating nighttime DOM consumption by bacterioplankton. Additionally, we observed that diel vertical migrators could contribute to the oscillation of labile DOM, such as TDAA, although their irregularity requires more detailed studies. BAct shifted DOM composition from fresher to more degraded forms, likely driven by bacterioplankton lineages such as SAR11, Rhodospirillaceae, and SAR202. Our findings show that daytime photosynthesis drives DOM production, while enhanced nighttime heterotrophic BAct facilitates its consumption and transformation, highlighting notable fluctuations in microbial processes and carbon cycling on a diel scale.