Mesopelagic prokaryotic bulk and single-cell heterotrophic activity and community composition in the NW Africa–Canary Islands coastal-transition zone

Mesopelagic prokaryotic communities have often been assumed to be relatively inactive in comparison to those from epipelagic waters, and therefore unresponsive to the presence of nearby upwelled waters. We have studied the zonal (shelf-ocean), latitudinal, and depth (epipelagic–mesopelagic) variability of microbial assemblages in the NW Africa–Canary Islands coastal-transition zone (CTZ). Vertical profiles of bacterial bulk and single-cell activity through the epi- and mesopelagic waters were combined with point measurements of bacterial respiration, leucine-to-carbon conversion factors and leucine-to-thymidine incorporation ratios. The overall picture that emerges from our study is that prokaryotes in the mesopelagic zone of this area are less abundant than in the epipelagic but have comparable levels of activity. The relationship between prokaryotes and heterotrophic nanoflagellates, their main predators, remains constant throughout the water column, further contradicting the assumption that deep ocean bacterial communities are mostly inactive. Both bulk and single-cell activity showed clear differences between stations, with higher mesopelagic activities closer to the shelf or affected by upwelling features. We also tested whether differences in microbial function between stations could be related to differences in bacterial community structure, and conclude that bacterial communities are very similar at similar depths in the deep ocean, even if the stations present order-of-magnitude differences in bacterial function.