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Abstract The Order Pelagibacterales (SAR11) is the most abundant group of heterotrophic bacterioplankton in global oceans and comprises multiple subclades with unique spatiotemporal distributions. Subclade IIIa is the primary SAR11 group in brackish waters and shares a common ancestor with the dominant freshwater IIIb (LD12) subclade. Despite its dominance in brackish environments, subclade IIIa lacks systematic genomic or ecological studies. Here, we combine closed genomes from new IIIa isolates, new IIIa MAGS from San Francisco Bay (SFB), and 460 highly complete publicly available SAR11 genomes for the most comprehensive pangenomic study of subclade IIIa to date. Subclade IIIa represents a taxonomic family containing three genera (denoted as subgroups IIIa.1, IIIa.2, and IIIa.3) that had distinct ecological distributions related to salinity. The expansion of taxon selection within subclade IIIa also established previously noted metabolic differentiation in subclade IIIa compared to other SAR11 subclades such as glycine/serine prototrophy, mosaic glyoxylate shunt presence, and polyhydroxyalkanoate synthesis potential. Our analysis further shows metabolic flexibility among subgroups within IIIa. Additionally, we find that subclade IIIa.3 bridges the marine and freshwater clades based on its potential for compatible solute transport, iron utilization, and bicarbonate management potential. Pure culture experimentation validated differential salinity ranges in IIIa.1 and IIIa.3 and provided detailed IIIa cell size and volume data. This study is an important step forward for understanding the genomic, ecological, and physiological differentiation of subclade IIIa and the overall evolutionary history of SAR11.
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This content will become publicly available on June 13, 2026
Culture-supported ecophysiology of the SAR116 clade demonstrates metabolic and spatial niche partitioning
Abstract Marine SAR116 bacterioplankton are ubiquitous in surface waters across global oceans and form their own order, Puniceispirillales, within the Alphaproteobacteria. To date no comparative physiology among diverse SAR116 isolates has been performed to capture the functional diversity within the clade, and further, diversity through the lens of metabolic potential and environmental preferences via clade-wide pangenomics continues to evolve with the addition of new genomes. Using high-throughput dilution-to-extinction cultivation, we isolated and genome sequenced five new and diverse SAR116 isolates from the northern Gulf of Mexico. Here we present a comparative physiological analysis of these SAR116 isolates, along with a pangenomic investigation of the SAR116 clade using a combination of metagenome-assembled genomes (MAGs, n = 258), single-amplified genomes (SAGs, n = 84), previously existing (n = 2), and new isolate genomes (n = 5), totaling 349 SAR116 genomes. Phylogenomic investigation supported the division of SAR116 into three distinct subclades, each with additional structure totaling 15 monophyletic groups. Our SAR116 isolates belonged to three groups within subclade I representing distinct genera with different morphologies and varied phenotypic responses to salinity and temperature. Overall, SAR116 genomes encoded differences in vitamin and amino acid synthesis, trace metal transport, and osmolyte synthesis and transport. They also had genetic potential for diverse sulfur oxidation metabolisms, placing SAR116 at the confluence of the organic and inorganic sulfur pools. SAR116 subclades showed distinct patterns in habitat preferences across open ocean, coastal, and estuarine environments, and three of our isolates represented the most abundant coastal and estuarine subclade. This investigation provides the most comprehensive exploration of SAR116 to date anchored by new culture genomes and physiology.
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- Award ID(s):
- 1945279
- PAR ID:
- 10613334
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- The ISME Journal
- ISSN:
- 1751-7362
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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