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Abstract SAR86 is one of the most abundant groups of bacteria in the global surface ocean. However, since its discovery over 30 years ago, it has remained recalcitrant to isolation and many details regarding this group are still unknown. Here, we report the cellular characteristics from the first SAR86 isolate brought into culture, Magnimaribacter mokuoloeensis strain HIMB1674, and use its closed genome in concert with over 700 environmental genomes to assess the phylogenomic and functional characteristics of this order-level lineage of marine Gammaproteobacteria. The SAR86 order Magnimaribacterales invests significant genomic resources into the capacity for $$\beta$$-oxidation, which is present in most genomes with high gene copy numbers. This cyclical set of reactions appears to be fed by components of cell membranes that include lipids such as phosphatidylcholine, phosphatidylethanolamine, glycolipids, and sulfolipids. In addition to the widespread capacity to degrade the side chain of steroidal compounds via $$\beta$$-oxidation, several SAR86 sublineages also appear able to fully degrade the steroid polycyclic ring structure as well as other aromatic, polycyclic, and heterocyclic molecules. Read recruitment from publicly available metagenomes reveals that the Magnimaribacterales compose up to 6% of the global surface ocean microbial community. Only a subset of genera drives these high relative abundances, with some more globally dominant and others restricted to specific oceanic regions. This study provides an unprecedented foundation through which to understand this highly abundant yet poorly understood lineage of marine bacteria and charts a path to bring more representatives of this order into laboratory culture. 

Abstract Islands in the tropical Pacific supply elevated nutrients to nearshore waters that enhance phytoplankton biomass and create hotspots of productivity in otherwise nutrient‐poor oceans. Despite the importance of these hotspots in supporting nearshore food webs, the spatial and temporal variability of phytoplankton enhancement and changes in the underlying phytoplankton communities across nearshore to open ocean systems remain poorly understood. In this study, a combination of flow cytometry, pigment analyses, 16S rRNA gene amplicons, and metagenomic sequencing provides a synoptic view of phytoplankton dynamics over a 4‐yr, near‐monthly time series across coastal Kāneʻohe Bay, Hawaiʻi, spanning from an estuarine Indigenous aquaculture system to the adjacent offshore environment. Through comparisons with measurements taken at Station ALOHA located in the oligotrophic North Pacific Subtropical Gyre, we observed a sharp and persistent transition between picocyanobacterial communities, fromSynechococcusclade II abundant in the nearshore toProchlorococcushigh‐light adapted clade II (HLII) proliferating in offshore and open ocean waters. In comparison to immediately adjacent offshore waters and the surrounding open ocean, phytoplankton biomass within Kāneʻohe Bay was dramatically elevated. Members of the phytoplankton community revealed strong seasonal patterns, while nearshore phytoplankton biomass positively correlated with wind speed, rainfall, and wind direction, and not water temperatures. These findings elucidate the spatiotemporal dynamics underlying transitions in ocean biogeochemistry and phytoplankton dynamics across estuarine to open ocean waters in the tropical Pacific and provide a foundation for quantifying deviations from baseline conditions due to ongoing climate change. 

Abstract After decades studying the microbial “deep biosphere” in subseafloor oceanic crust, the growth and life strategies in this anoxic, low energy habitat remain poorly described. Using both single cell genomics and metagenomics, we reveal the life strategies of two distinct lineages of uncultivated Aminicenantia bacteria from the basaltic subseafloor oceanic crust of the eastern flank of the Juan de Fuca Ridge. Both lineages appear adapted to scavenge organic carbon, as each have genetic potential to catabolize amino acids and fatty acids, aligning with previous Aminicenantia reports. Given the organic carbon limitation in this habitat, seawater recharge and necromass may be important carbon sources for heterotrophic microorganisms inhabiting the ocean crust. Both lineages generate ATP via several mechanisms including substrate-level phosphorylation, anaerobic respiration, and electron bifurcation driving an Rnf ion translocation membrane complex. Genomic comparisons suggest these Aminicenantia transfer electrons extracellularly, perhaps to iron or sulfur oxides consistent with mineralogy of this site. One lineage, called JdFR-78, has small genomes that are basal to the Aminicenantia class and potentially use “primordial” siroheme biosynthetic intermediates for heme synthesis, suggesting this lineage retain characteristics of early evolved life. Lineage JdFR-78 contains CRISPR-Cas defenses to evade viruses, while other lineages contain prophage that may help prevent super-infection or no detectable viral defenses. Overall, genomic evidence points to Aminicenantia being well adapted to oceanic crust environments by taking advantage of simple organic molecules and extracellular electron transport.