Unicellular photoautotrophic diazotrophs such as Crocosphaera spp. are ubiquitous in many oligotrophic and N-limited oceans, as they can reduce N₂ into bioavailable ammonia. The Mediterranean Sea is potentially an ideal environment for photoautotrophic diazotrophic activity, and yet N₂-fixation rates measured in the last two decades are typically very low and no diazotrophic blooms have been recorded in its offshore waters. Previous studies suggest that diazotrophs, as well as nondiazotrophic phytoplankton and heterotrophic bacteria, may be P-limited, hence their low biomass and activity. Here, we amended surface seawater from six stations across a nutrient gradient in the Mediterranean Sea (east to west transect) with dissolved inorganic phosphorus (DIP), and with seawater-acclimated inocula of Crocosphaera watsonii, a unicellular cyanobacterial diazotroph, to examine if DIP can stimulate diazotrophy. Our results demonstrate that C. watsonii are poor competitors for DIP relative to native nondiazotrophic heterotrophic microbial populations, especially in the ultraoligotrophic eastern Mediterranean basin, resulting in low N₂-fixation rates. Moreover, the results indicate that when the ambient DIP concentrations are < 35 nmol L⁻¹, unicellular photoautotrophic diazotrophs such as C. watsonii will likely be outcompeted for this macronutrient in the Mediterranean Sea, whereas above 35 nmol L⁻¹ diazotrophy can be stimulated. Our findings support the “bypass theory” stating that photoautotrophs may be outcompeted by heterotrophic bacteria for DIP in nutrient-poor regions such as the Mediterranean Sea.

中文翻译：

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异养细菌在地中海的正磷酸盐竞争中胜过固氮菌

单细胞光合自养固氮菌，例如Crocosphaera spp。在许多贫营养和氮限制海洋中普遍存在，因为它们可以将 N ₂还原成生物可利用的氨。地中海可能是光合自养固氮活动的理想环境，但 N ₂-过去二十年测量的固定率通常非常低，并且在其近海水域没有记录到固氮水华。以前的研究表明，固氮菌以及非固氮浮游植物和异养细菌可能是 P 限制的，因此它们的生物量和活性较低。在这里，我们用溶解无机磷 (DIP) 和海水驯化的Crocosphaera watsonii接种物（一种单细胞蓝藻固氮菌）修正了地中海营养梯度（从东到西横断面）的六个站点的地表海水，以检查是否 DIP可刺激固氮作用。我们的结果表明C. watsonii与原生非固氮异养微生物种群相比，DIP 是较差的竞争者，特别是在超贫营养的东地中海盆地，导致 N ₂固定率低。此外，结果表明，当环境 DIP 浓度 < 35 nmol L ^-1时，单细胞光合自养固氮菌（如C. watsonii ）可能会在地中海中对这种常量营养素的竞争超过，而高于 35 nmol L ^-1固氮菌可以被刺激. 我们的研究结果支持“旁路理论”，即在地中海等营养贫乏地区，光合自养生物可能会被异养细菌竞争 DIP。