Response of rare, common and abundant bacterioplankton to anthropogenic perturbations in a Mediterranean coastal site

12 pages, 4 figures, 3 tables, supplementary data http://femsec.oxfordjournals.org/content/suppl/2015/05/31/fiv058.DC1Bacterioplankton communities are made up of a small set of abundant taxa and a large number of low-abundant organisms (i.e. 'rare biosphere'). Despite the critical role played by bacteria in marine ecosystems, it remains unknown how this large diversity of organisms are affected by human-induced perturbations, or what controls the responsiveness of rare compared to abundant bacteria. We studied the response of a Mediterranean bacterioplankton community to two anthropogenic perturbations (i.e. nutrient enrichment and/or acidification) in two mesocosm experiments (in winter and summer). Nutrient enrichment increased the relative abundance of some operational taxonomic units (OTUs), e.g. Polaribacter, Tenacibaculum, Rhodobacteraceae and caused a relative decrease in others (e.g. Croceibacter). Interestingly, a synergistic effect of acidification and nutrient enrichment was observed on specific OTUs (e.g. SAR86). We analyzed the OTUs that became abundant at the end of the experiments and whether they belonged to the rare (1% relative abundance) fractions. Most of the abundant OTUs at the end of the experiments were abundant, or at least common, in the original community of both experiments, suggesting that ecosystem alterations do not necessarily call for rare members to growThe experiments were funded by projects ACDC (CTM2009-08849, to EC and CP), STORM (CTM2009-09352/MAR, to CM and JMG) and ECOBAF (CTM2010-10462-E/MAR, to JMG). FB was supported by an University of Otago Research Grant (UORG). The diversity work was supported by projects from the European Science Foundation (EuroEEFG project MOCA) and the Swedish Research Council to JPPeer Reviewe