Exploring the ecology of the mesopelagic biological pump

•A vast amount is known on role of surface ocean ecology and carbon cycle.•Large gaps in knowledge on mesopelagic ecological processes affecting carbon cycle.•Three mesopelagic systems co-exist driven by phytoplankton, zooplankton and microbes.•We define a carbon sequestration (Cseq) term estimating sequestration timescales.•Mesopelagic organic Cseq is highest in tropics, even if transfer efficiency is low.