Emilio Fernández

Human activities are altering nutrient inputs into coastal waters, impacting the C:P and N:P ratios of dissolved matter. These changes may modify present nutrient limitations, especially P limitation, in microbial communities. The response of microbial communities from the Ría de Vigo (NW Iberian Peninsula) to both the type (inorganic versus inorganic and organic mixture) and the dissolved inorganic N (DIN) to dissolved inorganic P (DIP) ratio (above and below Redfield) of nutrient inputs was assessed in 8 microcosm experiments. Concomitantly, the response of phytoplankton to nutrient treatments in the presence or absence of bacterial activity was explored. The DIN:DIP ratio and dissolved organic C availability modulated the response of microbial communities to matter inputs and also the bacterial impact on phytoplankton growth, potentially playing a major role in regulating biotic interactions between these 2 microbial compartments. Our study strongly suggests that when labile C is available, bacteria seem to outcompete phytoplankton under P scarcity conditions, while when both C and P are available, the impact of bacteria on phytoplankton tends to be positive. In a future global change scenario, with increased organic C and N:P ratios of allochthonous matter inputs to coastal waters, our data suggest that bacteria could outcompete phytoplankton for inorganic nutrients, reducing coastal primary production and affecting higher trophic levels.

Unveiling the interactions among phytoplankton and bacteria at the level of species requires axenic isolates to experimentally demonstrate their mutual effects. In this study, we describe the interactions among the diatoms Pseudo‐nitzschia granii and Chaetoceros tenuissimus and their associated bacterial species, isolated from surface water of a coastal upwelling system using coculture experiments. Microalgae growth was assessed in axenic monocultures or in coculture with each of their co‐isolated bacteria in the presence or absence of B vitamins. Pseudo‐nitzschia granii growth was limited by B‐vitamin supply, except when cultured with the bacteria Jannaschia cystaugens, which seemed to provide adequate levels of B vitamins to the diatom. Chaetoceros tenuissimus growth was reduced in the absence of B vitamins. Moreover, the growth of C. tenuissimus was stimulated by Alteromonas sp. and Celeribacter baekdonensis during the exponential growth. These results show a diversity of specific interactions between the diatoms and co‐isolated bacteria, ranging from allelopathy to commensalism. Understanding how interactions between phytoplankton and bacteria modulate the structure and function of marine microbial plankton communities will contribute to a greater knowledge of plankton ecology and improve our ability to predict nutrient fluxes in marine ecosystems or the formation of blooms in a context of global change.