Picocyanobacteria are widespread and globally significant primary producers. In brackish waters, picocyanobacterial populations are composed of diverse species with both single‐cell and colony‐forming lifestyles. Compared to their marine counterparts, brackish picocyanobacteria are less well characterized and the focus of research has been weighted toward single‐cell picocyanobacteria. Here, we investigate the uptake dynamics of single and colony‐forming picocyanobacteria using incubations with dual carbon‐13 and inorganic (ammonium and nitrate) or organic (urea and amino acids) nitrogen‐15 sources during August and September 2020 in the central Baltic Sea. Phytoplankton community and group‐specific uptake rates were obtained using an elemental analyzer isotope ratio mass spectrometer (EA‐IRMS) and nano secondary‐ion mass spectrometry (NanoSIMS). Picocyanobacteria contributed greater than one third of the ammonium, urea, amino acids, and inorganic carbon community uptake/fixation in September but < 10% in August when phytoplankton biomass was higher. Overall, single‐cell ammonium and urea uptake rates were significantly higher for single‐celled compared to colonial picocyanobacteria. In a 6‐yr offshore central Baltic Sea time series (2015–2020), summer abundances of colonial picocyanobacteria reached up to 105 cells mL−1 and represented > 5% of the average phytoplankton biomass, suggesting that they are periodically important for the ecosystem. Colonial strain identification was not distinguishable using 16S rRNA gene amplicon data, highlighting a need for refined tools for identification of colonial forms. This study shows the significance of single‐celled brackish picocyanobacteria to nutrient cycling and the importance of considering uptake and lifestyle strategies when assessing the role of picocyanobacteria in aquatic ecosystems.

中文翻译：

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波罗的海微咸海的菌落形成和单细胞微微蓝藻氮获取策略和碳固定


微微蓝细菌是广泛存在且具有全球重要意义的初级生产者。在咸水中，微微蓝藻种群由具有单细胞和菌落形成生活方式的不同物种组成。与海洋细菌相比，咸水微微蓝藻的特征还不太清楚，研究重点集中在单细胞微微蓝藻上。在这里，我们于 2020 年 8 月和 9 月在波罗的海中部使用双碳 13 和无机（铵和硝酸盐）或有机（尿素和氨基酸）氮 15 源进行培养，研究了单个和菌落形成的微微蓝细菌的吸收动态。使用元素分析仪同位素比质谱仪（EA-IRMS）和纳米二次离子质谱仪（NanoSIMS）获得浮游植物群落和群体特异性摄取率。 9 月份，微微蓝藻贡献了超过三分之一的铵、尿素、氨基酸和无机碳群落吸收/固定，但在浮游植物生物量较高的 8 月份，贡献了< 10%。总体而言，与菌落微微蓝藻相比，单细胞的铵和尿素摄取率显着更高。在波罗的海中部近海的 6 年时间序列（2015-2020）中，夏季群落微微蓝藻丰度达到 105 个细胞 mL−1，占平均浮游植物生物量的 5% 以上，表明它们对生态系统具有周期性重要作用。使用 16S rRNA 基因扩增子数据无法区分菌落菌株鉴定，这突出表明需要改进的工具来鉴定菌落形式。 这项研究表明了单细胞微咸味微微蓝藻对营养循环的重要性，以及在评估微微蓝藻在水生生态系统中的作用时考虑吸收和生活方式策略的重要性。