Diversity and community structure of aerobic anoxygenic phototrophic bacteria are shaped by the deep chlorophyll maximum

Abstract The surface ocean exhibits strong vertical gradients in light irradiance, nutrients, and temperature, shaping the phytoplankton distribution which often defines a deep chlorophyll maximum (DCM). Aerobic anoxygenic phototrophic (AAP) bacteria inhabit the euphotic zone, with their abundances generally following the chlorophyll a variability. While AAP bacterial communities are known to differ across regions with contrasting environmental conditions, their vertical distribution remains poorly understood. We hypothesized that the diversity and community structure of AAP bacteria vary across the vertical gradient, in relation to changes in environmental variables and following the DCM profile. To test this hypothesis, we studied the composition of AAP communities at different depths along the DCM structure in the South and Central Atlantic Ocean, by means of amplicon sequencing of the pufM gene. The results show significant differences in richness, community structure, and taxonomic composition of samples from different layers of the DCM, highlighting the dependence of AAP bacteria on its structure. Remarkably, the use of primers with broad phylogenetic coverage enabled the recovery of several AAP phylogroups previously detected only through metagenomics. We show that they represent a significant fraction of marine AAP communities, provide clues about their ecological preferences, and confirm their association with the family Candidatus Luxescamonaceae.