This KAUST Discovery item summarizes the findings of the publication of Calleja et al. (2018): Calleja, M.L., Ansari, M.I., Røstad, A., da Silva, L.R., Kaartvedt, S. Irigoien, X. & Morán, X.A.G. The mesopelagic scattering layer: a hotspot for heterotrophic prokaryotes in the Red Sea twilight zone. Frontiers in Marine Science 5 259 (2018)​