The shift of mutualistic relationships among algae, free-living and attached bacteria through different nutrient addition mode: a mesocosm study

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.