Abstract The shift of mutualistic nutrient relationship among algae, free-living and attached bacteria as well as the effects on algal growth were conducted in experimental mesocosm in which three nutrient addition treatments, including excessive phosphorus (P), excessive carbon (C) and nitrogen (N) to P as 16:1, were designed. The algal community composition did not change greatly in all treatments. The mutualism between algae and its attached bacteria was confirmed in treatments with excessive P and N:P as 16:1, in terms of the significantly higher dissolved organic nutrients derived from algae, extracellular enzyme activities produced by attached bacteria with high abundance and production. Interestingly, compared to treatment with excessive P, this mutualism was even more pronounced in treatment with N:P as 16:1, and resulted in stronger ecological function expressed as higher chlorophyll a content, indicating the importance of nutrient addition with stoichiometric equilibrium. On the contrary, excessive organic C input led to the nutrients competition between algae and free-living bacteria due to the latter’s growth in biomass and luxury uptake of inorganic nutrients, which finally limited algal growth. Our results indicated that mutualistic relationship among algae, free-living and attached bacteria could be shifted by nutrient equilibrium (N:P ratio) and dissolved organic C availability. Graphical Abstract

中文翻译：

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通过不同的营养添加方式，藻类、自由生活和附着细菌之间的共生关系的转变：中观研究

摘要 以过量磷（P）、过量碳（C）和氮三种养分添加处理为研究对象，研究了藻类、自由生活菌和附着菌间共生养分关系的变化及其对藻类生长的影响。 (N) 以 P 为 16:1，分别设计。藻类群落组成在所有处理中都没有发生很大变化。在P和N:P为16:1的处理中证实了藻类与其附着细菌之间的共生关系，从藻类中溶解的有机养分显着增加，高丰度附着细菌产生的胞外酶活性和产量。有趣的是，与过量 P 的治疗相比，这种共生在 N:P 为 16:1 的治疗中更为明显，并导致更强的生态功能，表现为更高的叶绿素 a 含量，表明营养添加与化学计量平衡的重要性。相反，过量的有机碳输入导致藻类和自由生活细菌之间由于生物量的增长和无机养分的过度吸收而导致藻类和自由生活细菌之间的养分竞争，最终限制了藻类的生长。我们的结果表明，藻类、自由生活和附着细菌之间的共生关系可以通过营养平衡（N:P 比）和溶解有机碳的有效性来改变。图形概要 过量的有机碳输入导致藻类和自由生活细菌之间的养分竞争，因为后者生物量的增长和无机养分的过度吸收，最终限制了藻类的生长。我们的结果表明，藻类、自由生活和附着细菌之间的共生关系可以通过营养平衡（N:P 比）和溶解有机碳的有效性来改变。图形概要 过量的有机碳输入导致藻类和自由生活细菌之间的养分竞争，因为后者生物量的增长和无机养分的过度吸收，最终限制了藻类的生长。我们的结果表明，藻类、自由生活和附着细菌之间的共生关系可以通过营养平衡（N:P 比）和溶解有机碳的有效性来改变。图形概要