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The second field campaign of the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program was conducted in the late spring of 2021 within the vicinity of the Porcupine Abyssal Plain (49.0°N, 16.5°W) in the North Atlantic Ocean. Observations from EXPORTS support previous characterizations of this system as highly productive and organic matter rich, with the majority of primary production occurring in large cells (≥ 5 µm) such as diatoms that are primarily utilizing nitrate. Rates of total euphotic zone depth-integrated net primary production ranged from 36.4 to 146.6 mmol C m−2d−1, with an observational period average f-ratio of 0.74, indicating predominantly new production. Substantial variability in the contribution of small (

The type VI secretion system (T6SS) is an interbacterial weapon composed of thousands of protein subunits and predicted to require significant cellular energy to deploy, yet a fitness cost from T6SS use is rarely observed. Here, we identify host-like conditions where the T6SS incurs a fitness cost using the beneficial symbiont,Vibrio fischeri, which uses its T6SS to eliminate competitors in the natural squid host. We hypothesized that a fitness cost for the T6SS could be dependent on the cellular energetic state and used theoretical ATP cost estimates to predict when a T6SS-dependent fitness cost may be apparent. Theoretical energetic cost estimates predicted a minor relative cost for T6SS use in fast-growing populations (0.4-0.45% of total ATP used cell-1), and a higher relative cost (3.1-13.6%) for stationary phase cells. Consistent with these predictions, we observed no significant T6SS-dependent fitness cost for fast-growing populations typically used for competition assays. However, the stationary phase cell density was significantly lower in the wild-type strain, compared to a regulator mutant that does not express the T6SS, and this T6SS-dependent fitness cost was between 11 and 23%. Such a fitness cost could influence the prevalence and biogeography of T6SSs in animal-associated bacteria. While the T6SS may be required in kill or be killed scenarios, once the competitor is eliminated there is no longer selective pressure to maintain the weapon. Our findings indicate an evolved genotype lacking the T6SS would have a growth advantage over its parent, resulting in the eventual dominance of the unarmed population.

Synechococcusare the most abundant cyanobacteria in high latitude regions and are responsible for an estimated 17% of annual marine primary productivity. Despite their biogeochemical importance,Synechococcuspopulations have been unevenly sampled across the ocean, with most studies focused on low-latitude strains. In particular, the near absence ofSynechococcusgenomes from high-latitude, High Nutrient Low Chlorophyll (HNLC) regions leaves a gap in our knowledge of picocyanobacterial adaptation to iron limitation and their influence on carbon, nitrogen, and iron cycles. We examinedSynechococcuspopulations from the subarctic North Pacific, a well-characterized HNLC region, with quantitative metagenomics. Assembly with short and long reads produced two near completeSynechococcusmetagenome-assembled genomes (MAGs). Quantitative metagenome-derived abundances of these populations matched well with flow cytometry counts, and theSynechococcusMAGs were estimated to comprise >99% of theSynechococcusat Station P. Whereas the Station PSynechococcusMAGs contained multiple genes for adaptation to iron limitation, both genomes lacked genes for uptake and assimilation of nitrate and nitrite, suggesting a dependence on ammonium, urea, and other forms of recycled nitrogen leading to reduced iron requirements. A global analysis ofSynechococcusnitrate reductase abundance in the TARA Oceans dataset found nitrate assimilation genes are also lower in other HNLC regions. We propose nitrate and nitrite assimilation gene loss inSynechococcusrepresents an adaptation to severe iron limitation in high-latitude regions where ammonium availability is higher. Our findings have implications for models that quantify the contribution of cyanobacteria to primary production and subsequent carbon export.SignificanceThe cyanobacteriumSynechococcusis a major contributor to ocean primary production and biogeochemistry. Here, we used quantitative metagenomics to assemble and enumerate twoSynechococcusgenomes from an iron-limited, High Nutrient Low Chlorophyll region. We show these genomes represent the majority ofSynechococcuscells at the site and are the first knownSynechococcusunable to assimilate either nitrate or nitrite. This gene loss is likely due to the high iron quota of these proteins and predominant availability of recycled forms of nitrogen.Synechococcus’loss of nitrate assimilation affects their role in elemental cycles (e.g., carbon, nitrogen, and iron), limits their potential for carbon export, and enhances our understanding ofSynechococcusevolution in response to nutrient limitation and competition.

The NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) program was established to better quantify the pathways of the biological carbon pump in order to gain a more comprehensive understanding of global carbon export efficiency. The summer 2018 field campaign in the vicinity of Ocean Station Papa (Station P; 50°N, 145°W) in the Northeast Pacific Ocean yielded evidence of low phytoplankton biomass and primary productivity dominated by small cells (