Population dynamics of SAR11 marine bacteria across a temporally-sampled nearshore to open ocean transect

Time-series surveys of microbial genetic diversity coupled with contextual measures of the environment provide a useful approach to understand the distribution of bacterial lineages across ecological niches. In 2017, a monthly time-series sampling of surface seawater along a transect spanning Kāneʻohe Bay, Hawaiʻi and the adjoining open ocean was established. This research provides a baseline of oceanographic measurements, phytoplankton diversity, and microbial community composition for comparisons over time and space. It has also afforded the opportunity to track changes in populations of biogeochemically important taxa, such as the numerically dominant SAR11 clade, also known as the order Pelagibacterales. While this free-living lineage of marine Alphaproteobacteria is one of the most abundant and ubiquitous bacterial clades on Earth, its high genetic diversity has limited our understanding of the mechanisms and conditions responsible for ecological differentiation within this group. Using amplicon sequencing of the 16S ribosomal RNA gene, we evaluated spatial-temporal patterns of distribution for major SAR11 subgroups as well as individual amplicon sequence variants (ASVs) across a steep physiochemical gradient from nearshore to the open ocean sampled over a two-year time period. We observed distinct distributions of SAR11 subgroups over this physiochemical gradient, with an increase in the relative abundance of subgroups Ib and II and a decrease in subgroup 1a at offshore stations. While some SAR11 ASVs were ubiquitously distributed, others were restricted to specific regions of the transect (e.g. nearshore, mid-bay, open ocean). In general, SAR11 ASVs displayed an increase in alpha diversity in the open ocean. Our results provide evidence for ecological differentiation of SAR11 marine bacteria at the fine-scale of 16S ribosomal RNA ASVs, further increasing our understanding of how SAR11 genetic diversity partitions in physically adjacent yet physiochemically distinct marine systems.