Dissolved organic matter and heterotrophic prokaryotes diel patterns reveal enhanced growth at the mesopelagic fish layer during daytime

Contrary to epipelagic waters, where biogeochemical processes closely follow the light and dark periods, little is known about diel cycles in the mesopelagic realm. Here, we monitored the dynamics of dissolved organic matter (DOM) and planktonic heterotrophic prokaryotes every 2 h for one day at the surface and 550 m (a depth occupied by mesopelagic fish during light hours) in oligotrophic waters of the central Red Sea. We additionally performed predator-free seawater incubations of samples collected from the same site at midnight and noon. Comparable variability in microbial biomass and dissolved organic carbon in situ suggests a diel supply of fresh DOM in both layers. The presence of mesopelagic fishes during daytime promoted a sustained, longer growth of larger prokaryotic cells, with specific growth rates consistently higher in both noon experiments (surface: 0.34 vs. 0.18 d-1, deep: 0.16 vs. 0.09 d-1). Heterotrophic prokaryotes in the mesopelagic fish layer were also more efficient at converting DOM into new biomass. These results suggest that the ocean9s twilight zone receives a continuous diurnal supply of labile DOM from diel vertical migrating fishes, enabling an unexpectedly active community of heterotrophic prokaryotes.