Pangenomics and ecophysiology of SAR116 bacterioplankton

Jordan Coelho, University of Southern California, Biological Sciences, Los Angeles, United States, Michael W Heson, University of Southern California, Biological Sciences, Los Angeles, CA, United States, Ben Temperton, University of Exeter, Exeter, United Kingdom and Cameron Cameron Thrash, University of Southern California, Department of Biological Sciences, Los Angeles, United States
Abstract:
Bacterioplankton of the SAR116 clade (Alphaproteobacteria) are ubiquitous in epipelagic waters. They typically represent 1%-17% of clone libraries and can reach relative abundances of 20% during a strong bloom. However, intraclade diversity and their roles in marine systems are poorly constrained. Furthermore, SAR116 taxonomy remains unresolved - they have been classified both as members of the Rhodospirillaceae and as members of their own Order, the Puniceispirillales. Using high-throughput cultivation, we recently isolated and genome-sequenced five new diverse SAR116 isolates. Here we present the findings of the first pan-genomic investigation of the SAR116 clade using a combination of metagenome-assembled genomes (MAGs, n=36), single amplified genomes (SAGs, n=15), and both existing (n=2) and our new isolate (n=5) genomes. Phylogenomic and average nucleotide identity analyses support the division of SAR116 into seven subclades hosting multiple genera and species. Subclades are associated with increasing GC content with divergence over time. Expected genome size ranges from 1.8 - 2.8 Mbp and coding densities range from 80-95%. Pangenomic analysis supports and extends previous work indicating that the clade subsists on obligately aerobic chemoorganoheterotrophic metabolism with the potential for phototrophy via rhodopsin. The clade is enriched in genes for aromatic ring cleavage, and there appears to be substrate specificity among the subclades. Two of our isolates contain genes for the high oxygen affinity cytochrome bd oxidase, and two subclades (along with three of our isolates) carry genes for thiosulfate oxidation. An early diverging subclade depends on a sodium-translocating NADH dehydrogenase, while all others use the proton-translocating NADH dehydrogenase. Future work will include subclade and gene specific abundances and spatial distributions, along with the continued investigation of the drivers of subclade structure, and subclade specific metabolisms.
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