Inputs of nutrients and dissolved organic matter control phytoplankton- bacteria interactions in a productive coastal ecosystem

2nd Meeting of the Iberian Ecological Society (SIBECOL), XXI conference of the Iberian Association of Limnology (AIL) and 21st National Congress of the Portuguese Ecological Society (SPECO), 3-8 July 2022, AveiroAnthropogenic alterations of global biogeochemical cycles are changing not only the magnitude but also the nature of matter entering into the ocean, thus increasing the C/P and N/P ratios. Allochthonous inputs are particularly significant in coastal systems and even may equal autochthonous sources. The interactions between phytoplankton and bacteria likely play a major role in the response of primary producers to anthropogenic matter inputs. However, the magnitude and nature of such interactions, as well as the factors controlling their intensity and sign are largely unknown in marine planktonic ecosystems. A set of 18 microcosms (2 L) experiments were conducted to test the hypothesis that the nature of the nutrient inputs modulates the strength and sign of the interaction between phytoplankton and bacteria. The experiments consisted in the addition of nutrients (nitrate, ammonium an phosphate), nutrients and dissolved organic matter (DOM) (glucose and amino acids), or riverine water (10% final concentration) containing different DOM:P proportions to intact microbial communities and microbial communities where bacteria were selectively blocked with a previously tested combination of antibiotics. Sampling was conducted in different seasons in the Ría de Vigo, a highly productive coastal ecosystem located NW Spain. An interaction index was calculated as the quotient between the change in chlorophyll-a concentration after nutrient amendment in the intact communities and the corresponding change in the same community but treated with antibiotics. An interaction index >1 thus indicates a positive effect of bacteria on phytoplankton, while an interaction index <1 indicates an overall negative impact. The interaction index ranged from 0.51 to 1.24. The index was significantly higher in autumn (1.01±0.03) than in spring (0.91±0.02), and winter (0.90±0.03). Overall, treatments with a high DOM:P ratio promoted a strong negative impact of bacteria on phytoplankton (0.84±0.06), indicating a predominance of competitive or antagonist interactions. A significant correlation was found between the interaction index and the concentration of nitrite (r=0.44, p