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Abstract Marine N2-fixing cyanobacteria, including the unicellular genus Crocosphaera, are considered keystone species in marine food webs. Crocosphaera are globally distributed and provide new sources of nitrogen and carbon, which fuel oligotrophic microbial communities and upper trophic levels. Despite their ecosystem importance, only one pelagic, oligotrophic, phycoerythrin-rich species, Crocosphaera watsonii, has ever been identified and characterized as widespread. Herein, we present a new species, named Crocosphaera waterburyi, enriched from the North Pacific Ocean. C. waterburyi was found to be phenotypically and genotypically distinct from C. watsonii, active in situ, distributed globally, and preferred warmer temperatures in culture and the ocean. Additionally, C. waterburyi was detectable in 150- and 4000-meter sediment export traps, had a relatively larger biovolume than C. watsonii, and appeared to aggregate in the environment and laboratory culture. Therefore, it represents an additional, previously unknown link between atmospheric CO2 and N2 gas and deep ocean carbon and nitrogen export and sequestration. 

Summary In the surface waters of the warm oligotrophic ocean, filaments and aggregated colonies of the nitrogen (N)‐fixing cyanobacteriumTrichodesmiumcreate microscale nutrient‐rich oases. These hotspots fuel primary productivity and harbour a diverse consortium of heterotrophs. Interactions with associated microbiota can affect the physiology ofTrichodesmium, often in ways that have been predicted to support its growth. Recently, it was found that trimethylamine (TMA), a globally abundant organic N compound, inhibits N₂fixation in cultures ofTrichodesmiumwithout impairing growth rate, suggesting thatTrichodesmiumcan use TMA as an alternate N source. In this study,¹⁵N‐TMA DNA stable isotope probing (SIP) of aTrichodesmiumenrichment was employed to further investigate TMA metabolism and determine whether TMA‐N is incorporated directly or secondarily via cross‐feeding facilitated by microbial associates. Herein, we identify two members of the marineRoseobacterclade (MRC) of Alphaproteobacteria as the likely metabolizers of TMA and provide genomic evidence that they converted TMA into a more readily available form of N, e.g., ammonium (NH₄⁺), which was subsequently used byTrichodesmiumand the rest of the community. The results implicate microbiome‐mediated carbon (C) and N transformations in modulating N₂fixation and thus highlight the involvement of host‐associated heterotrophs in global biogeochemical cycling. 

Summary Synechococcus, a genus of unicellular cyanobacteria found throughout the global surface ocean, is a large driver of Earth's carbon cycle. Developing a better understanding of its diversity and distributions is an ongoing effort in biological oceanography. Here, we introduce 12 new draft genomes of marineSynechococcusisolates spanning five clades and utilize ~100 environmental metagenomes largely sourced from the TARA Oceans project to assess the global distributions of the genomic lineages they and other reference genomes represent. We show that five newly provided clade‐II isolates are by far the most representative of the recoveredin situpopulations (most ‘abundant’) and have biogeographic distributions distinct from previously available clade‐II references. Additionally, these isolates form a subclade possessing the smallest genomes yet identified of the genus (2.14 ± 0.05Mbps; mean ± 1SD) while concurrently hosting some of the highest GC contents (60.67 ± 0.16%). This is in direct opposition to the pattern inSynechococcus’s nearest relative,Prochlorococcus– wherein decreasing genome size has coincided with a strongdecreasein GC content – suggesting this new subclade ofSynechococcusappears to have convergently undergone genomic reduction relative to the rest of the genus, but along a fundamentally different evolutionary trajectory.