Genome-resolved adaptation strategies of <i>Rhodobacterales</i> to changing conditions in the Chesapeake and Delaware Bays

The abundant and metabolically versatile aquatic bacterial order, <i>Rhodobacterales</i>, influences marine biogeochemical cycles. We assessed <i>Rhodobacterales</i> metagenome-assembled genome (MAG) abundance, estimated growth rates, and potential and expressed functions in the Chesapeake and Delaware Bays, two important US estuaries. Phylogenomics of draft and draft/closed <i>Rhodobacterales</i> genomes from this study and others placed 46 nearly complete MAGs from these bays into 11 genera, many were not well characterized. Their abundances varied between the bays and were influenced by temperature, salinity, and silicate and phosphate concentrations. <i>Rhodobacterales</i> genera possessed unique and shared genes for transporters, photoheterotrophy, complex carbon degradation, nitrogen, and sulfur metabolism reflecting their seasonal differences in abundance and activity. <i>Planktomarina</i> genomospecies were more ubiquitous than the more niche specialists, HIMB11, CPC320, LFER01, and MED-G52. Their estimated growth rates were correlated to various factors including phosphate and silicate concentrations, cell density, and light. Metatranscriptomic analysis of four abundant genomospecies commonly revealed that aerobic anoxygenic photoheterotrophy-associated transcripts were highly abundant at night. These <i>Rhodobacterales</i> also differentially expressed genes for CO oxidation and nutrient transport and use between different environmental conditions. Phosphate concentrations and light penetration in the Chesapeake Bay likely contributed to higher estimated growth rates of HIMB11 and LFER01, respectively, in summer where they maintained higher ribosome concentrations and prevented physiological gene expression constraints by downregulating transporter genes compared to the Delaware Bay. Our study highlights the spatial and temporal shifts in estuarine <i>Rhodobacterales</i> within and between these bays reflected through their abundance, unique metabolisms, estimated growth rates, and activity changes.<h4>Importance</h4>In the complex web of global biogeochemical nutrient cycling, the <i>Rhodobacterales</i> emerge as key players, exerting a profound influence through their abundance and dynamic activity. While previous studies have primarily investigated these organisms within marine ecosystems, this study delves into their roles within estuarine environments using a combination of metagenomic and metatranscriptomic analyses. We uncovered a range of <i>Rhodobacterales</i> genera, from generalists to specialists, each exhibiting distinct abundance patterns and gene expression profiles. This diversity equips them with the capacity to thrive amidst the varying environmental conditions encountered within dynamic estuarine habitats. Crucially, our findings illuminate the adaptable nature of estuarine <i>Rhodobacterales</i>, revealing their various energy production pathways and diverse resource management, especially during phytoplankton or algal blooms. Whether adopting a free-living or particle-attached existence, these organisms demonstrate remarkable flexibility in their metabolic strategies, underscoring their pivotal role in driving ecosystem dynamics within estuarine ecosystems.