Abstract Aerobic anoxygenic phototrophic (AAP) bacteria play an important role in ocean biogeochemistry by degrading organic matter and capturing light energy through bacteriochlorophyll a. However, how their genetic and ecological traits vary with lifestyle remains underexplored. We investigated free-living (FL) and particle-associated (PA) AAP communities and their responses to environmental variability in the northern Alboran Sea. Surface seawater was collected during seven cruises conducted in spring, autumn, and winter, each covering four coast-to-offshore transects. AAP abundance was quantified by infrared epifluorescence microscopy, and community diversity was assessed using pufM gene amplicon sequence variants (ASVs). AAP abundance was significantly higher in spring than in winter, although no consistent spatial pattern was observed. PA communities exhibited greater diversity than FL communities. Around 65% of ASVs were unique to the PA fraction, although most of these were rare. Distinct AAP phylogroups displayed clear niche preferences: PA-associated groups were more common in coastal waters, whereas FL groups predominated offshore. Both communities showed pronounced seasonality, but spatial structuring along the west-to-east gradient was more apparent in FL than in PA communities. Environmental drivers also differed between lifestyles, with salinity primarily shaping FL communities and chromophoric dissolved organic matter influencing PA communities. Our results indicate that lifestyle (FL vs. PA) is a major determinant of AAP bacterial diversity, spatiotemporal patterns, and environmental responses.
Santos-Bruña et al. (Tue,) studied this question.