Nitrogen availability limits marine productivity across large ocean regions. Diazotrophs can supply new nitrogen to the marine environment via nitrogen (N 2 ) fixation, relieving nitrogen limitation. The distributions of diazotrophs and N 2 fixation have been hypothesized to be generally controlled by temperature, phosphorus, and iron availability in the global ocean.However, even in the North Atlantic where most research on diazotrophs and N 2 fixation has taken place, environmental controls remain contentious. Here we measure diazotroph composition, abundance, and activity at high resolution using newly developed underway sampling and sensing techniques. We capture a diazotrophic community shift from Trichodesmium to UCYN-A between the oligotrophic, warm (25-29°C) Sargasso Sea and relatively nutrient-enriched, cold (13-24°C) subpolar and eastern American coastal waters. Meanwhile, N 2 fixation rates measured in this study are among the highest ever recorded globally and show significant increase with phosphorus availability across the transition from the Gulf Stream into subpolar and coastal waters despite colder temperatures and higher nitrate concentrations. Transcriptional patterns in both Trichodesmium and UCYN-A indicate phosphorus stress in the subtropical gyre. Over this iron-replete transect spanning the western North Atlantic, our results suggest that temperature is the major factor controlling the diazotrophic community structure while phosphorous drives N 2 fixation rates. Overall, the occurrence of record-high UCYN-A abundance and peak N 2 fixation rates in the cold coastal region where nitrate concentrations are highest (~200 nM) challenges current paradigms on what drives the distribution of diazotrophs and N 2 fixation.
1We report the cultivation of Chlorella sorokiniana str. SLA-04 in media containing trace 2 amounts of Ca and Mg. The differences in productivities of biomass, lipids and carbohydrates 3 were assessed relative to cultures grown in standard media (BG-11) that contain approximately 4 8× higher concentration of Ca and 30× higher concentration of Mg. Culture performance in N-5 limited standard media was also investigated. In addition to growth and accumulation of storage 6 products (lipid and carbohydrate), we measured the utilization of N, Ca and Mg and monitored 7 changes in cell size and photosynthetic activity. Our results showed that limitation of Ca or Mg 8 did not inhibit cell replication and culture growth. On the contrary, Ca-limited (Ca L ) limited 9 cultures had ~30% higher biomass productivity relative to the control with excessive nutrients 10 possibly due to improvement in cell wall flexibility and cell division. We also observed that Ca L 11 and Mg-limited (Mg L ) cultures had nearly 3-fold higher lipid concentration (measured as fatty 12 acid methyl ester) and 50% higher carbohydrate concentration than the nutrient excess control 13 cultures. Simultaneous culture growth and lipid accumulation in Ca L and Mg L cultures suggest 14 that de novo synthesis was the primary mechanism for lipid accumulation in Ca/Mg-limited 15 media. Overall, our study demonstrates that micronutrient optimization, in addition to 16 optimization of macronutrients, could significantly improve microalgal biorefinery yields. 17
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