One specific aspect of the interactions between DOM and heterotrophic prokaryotes that we are currently investigating is focused on the Red Sea mesopelagic layer, particularly warmer (22°C) that elsewhere. Trophic relationships between heterotrophic prokaryotes (bacteria and archaea) and metazoans have been largely neglected in that realm (200-1000 m depth). Different fishes migrate to the surface at night in order to feed and retreat at mesopelagic depths during daytime to escape predation. We have found indications that vertically migrating fishes of the species Benthosema pterotummay supply an overlooked source of labile DOM for heterotrophic prokaryotes. Within this project we explore this synergistic effect, previously suggested but never tested, with still unknown consequences for the deep ocean ecosystem dynamics and carbon biogeochemical cycling. We are combining diel and seasonal assessments of DOM-prokaryotes interactions in the field with laboratory incubations to determine the lability of mesopelagic DOM.
