Heterotrophic prokaryotes have the capacity to uptake inorganic nitrogen (N) substrates. However, it remains unclear what the potential competition is between heterotrophic prokaryotes and autotrophic plankton for N in the ocean, which would shunt the flow of N supporting primary production. To date, it has been difficult to distinguish heterotrophic prokaryotic N uptake from that of autotrophic picoplankton, especially in oligotrophic oceans dominated by cyanobacteria. We carried out field-based DNA stable isotope probing incubation experiments in the South China Sea combining measurements of uptake rates of ammonium, nitrate, nitrite, and urea to estimate the taxon-specific potential N assimilation. The results indicate that phylogenetically diverse heterotrophic prokaryotes significantly incorporated multiple N sources, contributing approximately 17–41% and 19–55% of total N uptake potential in the euphotic zone of the South China Sea continental shelf and open ocean, respectively, potentially competing with cyanobacteria (mainly Prochlorococcus). Notably, heterotrophic prokaryotes made a higher contribution to bulk uptake of nitrate in the incubation systems of the open ocean relative to regenerated N, and thus there was a tendency to overestimate the f-ratio. Extrapolating our results to the oligotrophic, low-latitude ocean via a global model suggests the f-ratio would decrease ~ 18%. This suggests a more complicated biogeochemical role of heterotrophic prokaryotes in the biological carbon pump than hitherto assumed, with important implications for N and carbon cycling in the vast open ocean.

中文翻译：

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海洋异养原核生物和自养微型浮游生物对氮底物的潜在竞争

异养原核生物具有吸收无机氮 (N) 底物的能力。然而，目前尚不清楚异养原核生物和自养浮游生物之间对海洋中氮的潜在竞争是什么，这会分流支持初级生产的氮流。迄今为止，很难将异养原核生物的氮吸收与自养微型浮游生物的氮吸收区分开来，尤其是在以蓝藻为主的贫营养海洋中。我们在南海进行了基于实地的 DNA 稳定同位素探测孵化实验，结合对铵、硝酸盐、亚硝酸盐和尿素的吸收率的测量，以估计特定于分类群的潜在 N 同化。结果表明，系统发育多样化的异养原核生物显着整合了多种氮源，原绿球藻）。值得注意的是，相对于再生 N，异养原核生物对开放海洋孵化系统中硝酸盐的大量吸收做出了更高的贡献，因此存在高估f比的趋势。通过全球模型将我们的结果外推到贫营养的低纬度海洋，表明f比率将下降约 18%。这表明异养原核生物在生物碳泵中的生物地球化学作用比迄今为止假设的更复杂，对广阔公海中的氮和碳循环具有重要意义。