Nitrogen fixation is critical for the biological productivity of the ocean, but clear mechanistic controls on this process remain elusive. Here, we investigate the abundance, activity, and drivers of nitrogen-fixing diazotrophs across the tropical western North Pacific. We find a basin-scale coherence of diazotroph abundances and N 2 fixation rates with the supply ratio of iron:nitrogen to the upper ocean. Across a threshold of increasing supply ratios, the abundance of nifH genes and N 2 fixation rates increased, phosphate concentrations decreased, and bioassay experiments demonstrated evidence for N 2 fixation switching from iron to phosphate limitation. In the northern South China Sea, supply ratios were hypothesized to fall around this critical threshold and bioassay experiments suggested colimitation by both iron and phosphate. Our results provide evidence for iron:nitrogen supply ratios being the most important factor in regulating the distribution of N 2 fixation across the tropical ocean.

中文翻译：

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热带西部北太平洋生物固氮的营养调节

固氮对于海洋的生物生产力至关重要，但对该过程的明确机械控制仍然难以捉摸。在这里，我们调查了热带西部北太平洋固氮重氮菌的丰度、活动和驱动因素。我们发现固氮菌丰度和 N 的流域尺度一致性2个固定率与铁：氮到上层海洋的供应比。在供应比率不断增加的门槛上，大量镍氢基因和N2个固定率增加，磷酸盐浓度降低，生物测定实验证明了 N2个固定从铁转换为磷酸盐限制。在南海北部，假设供应比率落在这个临界阈值附近，生物测定实验表明铁和磷酸盐的共同限制。我们的结果证明铁：氮供应比是调节 N 分布的最重要因素2个横跨热带海洋的固定。