Genetic diversity of aerobic anoxygenic photosynthetic bacteria in open ocean surface waters and upper twilight zones

Abstract

Aerobic anoxygenic phototrophic bacteria (AAPB) represent a widespread mixotrophic bacterial group in marine ecosystems. Here we investigated AAPB genetic diversity in the surface waters and upper twilight zones of the central Pacific, Atlantic, and Indian oceans by amplifying an AAPB marker gene (pufM, encoding photosynthetic reaction center small subunit) directly from bacterioplankton community DNA. Phylogenetic and statistical analysis of 267 pufM partial sequences in six clone libraries revealed a high diversity pattern in open ocean AAPB communities. Various AAPB subgroups belonging to Alpha- and Gamma-proteobacteria were found in both surface and upper twilight zone waters. In most samples, subgroups in which no pure culture was isolated as yet were predominant. By sampling a wide size range of bacterioplankton (0.22–200 μm) and introducing nested PCR to amplification, we retrieved abundant pufM fragments (136 sequences in 37 OTUs) directly from upper twilight zone samples. AAPB populations in upper twilight zones covered major subgroups found in surface waters and had a slightly lower diversity, higher dominance, and lower GC and GC3 contents in pufM genes than those in surface AAPB populations. These diversity data combined with previous BChl.a data in upper twilight zones support the hypothesis that AAPB may be present below euphotic zones based on the speculation that AAPB can utilize the dim light in twilight zones as a supplement to energy supply in their heterotrophic lives.