Vertical distribution and light energy capture by AAP bacteria along contrasted areas of productivity in the Atlantic Ocean

Aerobic anoxygenic phototrophic (AAP) bacteria are a common part of microbial communities in the sunlit ocean. They contain bacteriochlorophyll a-based photosystems that harvest solar energy for their metabolism. Across different oceanic areas and regimes, AAP bacteria seem to be more abundant in eutrophic areas, associated to high chlorophyll concentrations. While most of these studies are based on surface samples, little information is available on their vertical distribution in euphotic zones of the major ocean basins. We hypothesize that AAPs will follow a similar structure to the chlorophyll depth profile across areas with different degrees of stratification. To test this hypothesis, we enumerated AAP cells and determined bacteriochlorophyll a concentrations along the photic zone of a latitudinal transect in the South and Mid Atlantic Ocean. We show that the distribution of AAP bacteria is highly correlated to the chlorophyll a concentration and the abundance of picophytoplankton across vertical and horizontal gradients. Furthermore, we estimate the light energy captured by AAP bacteria across the water column and find that, while they share a common latitudinal pattern of light capture with the picophytoplankton, they display a unique vertical arrangement with highest photoheterotrophic activity is in the surface ocean.