The Mesopelagic Scattering Layer: A Hotspot for Heterotrophic Prokaryotes in the Red Sea Twilight Zone

Maria L Calleja,Xosé Anxelu G Morán,Luis Silva,Xabier Irigoien,Stein Kaartvedt,Anders Røstad,Mohd I Ansari

doi:10.3389/fmars.2018.00259

Abstract

The vast majority of marine dissolved organic carbon (DOC), the largest reservoir of reduced carbon on Earth, is believed to accumulate in the abyssal layers of the ocean over timescales of decades to millennia. However, evidence is growing that small animals that migrate vertically every day from the surface to mesopelagic layers are significantly contributing to the active vertical flux of organic matter. Whether that represents an important source of carbon available for microbial production and respiration at the mesopelagic realm, and its contribution to oceanic carbon budgets and energy flows, is yet to be explored. Here we present data suggesting that Red Sea migrating animals may produce an overlooked source of labile DOC (used at a mean rate of 2.1 µmol C L-1 d-1) that does not accumulate but fuels the metabolism of prokaryotic heterotrophs in the twilight zone, generating a disregarded hotspot for heterotrophic prokaryotes.

Highlights

In the extensive oligotrophic ocean, dissolved organic carbon (DOC) is produced mostly in the euphotic zone and passively mixed downward (Jiao et al, 2010; Hansell, 2013) where deep heterotrophic prokaryotes shape both its amount and composition while remineralizing most of it back to carbon dioxide (Carlson and Hansell, 2014)
With activities documented to be higher than previously assumed (Gasol et al, 2009), the mesopelagic is the region where many animals retreat during the day to find refuge from efficient visual predators (Robinson et al, 2010), making excursions to the surface waters to feed at night
Documented worldwide (Klevjer et al, 2016; Proud et al, 2017) the diel vertical migration (DVM) to the mesopelagic layer is responsible for the biggest vertical movement of biomass on Earth

Summary

Introduction

In the extensive oligotrophic ocean, dissolved organic carbon (DOC) is produced mostly in the euphotic zone and passively mixed downward (Jiao et al, 2010; Hansell, 2013) where deep heterotrophic prokaryotes (bacteria and archaea) shape both its amount and composition while remineralizing most of it back to carbon dioxide (Carlson and Hansell, 2014).

Results

Conclusion