Diel dynamics of dissolved organic matter and heterotrophic prokaryotes reveal enhanced growth at the ocean’s mesopelagic fish layer during daytime

摘要: ABSTRACT Contrary to epipelagic waters, where biogeochemical processes closely follow the light and dark periods, little is known about diel cycles in ocean’s mesopelagic realm. Here, we monitored dynamics of dissolved organic matter (DOM) planktonic heterotrophic prokaryotes every 2 h for one day at 0 550 m (a depth occupied by vertically migrating fish during hours) oligotrophic waters central Red Sea. We additionally performed predator-free seawater incubations samples collected from same site both midnight noon. Comparable situ variability microbial biomass carbon concentration suggests a supply fresh DOM layers. The presence zone daytime promoted sustained, longer growth larger prokaryotic cells. specific rates were consistently higher noon experiments depths (surface: 0.34 vs. 0.18 d−1, mesopelagic: 0.16 0.09 d−1). Heterotrophic bacteria archaea layer also more efficient converting into new biomass. These results suggest that twilight receives consistent diurnal labile vertical fishes, enabling an unexpectedly active community prokaryotes.