Aerobic anoxygenic phototrophic bacteria are ubiquitous in phyllo- and endosphere microbiomes of boreal and subarctic plants

Abstract

In addition to oxygenic photosynthetic systems, solar radiation is utilized for energy by diverse anoxygenic photosynthetic systems. Aerobic anoxygenic phototrophic bacteria (AAPB) perform photosynthesis without producing oxygen but still live in aerobic conditions. Typically they have been reported from aquatic ecosystems, but they can also be found from polar and desert soil ecosystems and primary succession communities like soil crusts. Recently, AAPB have been discovered in the metagenomic data of several plant phyllospheres. By utilizing citizen science, we screened plant foliar samples from eleven different locations in Finland for AAPB by near infrared fluorescence imaging of culturable phyllosphere and endosphere bacteria. Near infrared fluorescence reports the presence of AAPB which contain Bacteriochlorophyll a molecules, embedded in Light Harvesting 1 - Reaction Center complex. We found that AAPB were ubiquotous in phyllosphere communities of diverse plant species in all sampling locations. They were also consistently present in the endosphere of plant species with perennial leaves. Most of the AAPB isolated represent alphaproteobacterial genera Sphingomonas and Methylobacterium, but several isolates from genus Lichenihabitans as well as putative novel alphaproteobacterial taxa were also identified. Methylobacterial isolates were mostly detected in the phyllosphere with weak host specificity, while Sphingomonas AAPB were detected also in the endosphere of several plant species, with clear host specific taxa. We studied also the fluorescence spectral properties of several AAPBs. All the observed spectra resemble typical fluorescence spectral properties of Light Harvesting complex 1. Still, slight variation among each spectra could be obtained, revealing some physical difference among the complexes. Our results demonstrate for the first time, that AAPB are common in cold climate plant endophytic as well as epiphytic microbiomes and they build up substantial amounts of Bacteriochlorophyll a containing Light Harvesting complexes. Their putative role in plant adaptation to strong seasonality in light and temperature or tolerance of abiotic stressors remains to be investigated in future studies.