Bacterioplankton are responsible for a large fraction of respiration and dissolved organic matter (DOM) remineralisation in the ocean; therefore, changes in their taxonomic composition due to changes in nutrient inputs may have important biogeochemical implications. The impact of nutrient loading will depend on the type of input and the initial environmental conditions. We conducted a set of 5 microcosm experiments along a broad latitudinal transect in the Atlantic Ocean (26°N to 29°S) in order to assess the effects of inorganic (nitrate, ammonium, phosphate, silica) and organic (glucose, amino acids) inputs, added separately as well as jointly, on the bacterioplankton community composition at the level of major groups. We followed changes in the relative abundance of important bacterial phylogenetic (Roseobacter spp., SAR11, Gammaproteobacteria, Bacteroidetes) and cytometric (low, high and very high nucleic acid content) groups. We observed that distinct groups responded differently to nutrient additions: SAR11 and Bacteroidetes responded negatively to organic and mixed additions, while Roseobacter spp. and Gammaproteobacteria responded positively. Only the group Roseobacter spp. responded positively to inorganic additions. The abundance of very high nucleic acid content bacteria was low in the control and the inorganic treatment; however, they reached very high numbers in the organic and mixed treatments. There was a clear increasing north to south gradient in the magnitude of response of Gammaproteobacteria to organic inputs (containing carbon and nitrogen), likely related to an increasing north to south gradient in phosphate concentration. The magnitude of response of Gammaproteobacteria explained 89% of the observed variability in the magnitude of bacterial production response, which suggests a strong link between bacterial community composition and carbon cycling in the oligotrophic ocean.

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