Higher contribution of globally rare bacterial taxa reflects environmental transitions across the surface ocean

16 pages, 6 figures, 1 table, supporting information https://dx.doi.org/10.1111/mec.15026Microbial taxa range from being ubiquitous and abundant across space to extremely rare and endemic, depending on their ecophysiology and on different processes acting locally or regionally. However, little is known about how cosmopolitan or rare taxa combine to constitute communities and whether environmental variations promote changes in their relative abundances. Here we identified the Spatial Abundance Distribution (SpAD) of individual prokaryotic taxa (16S rDNA-defined Operational Taxonomic Units, OTUs) across 108 globally-distributed surface ocean stations. We grouped taxa based on their SpAD shape (“normal-like”- abundant and ubiquitous; “logistic”- globally rare, present in few sites; and “bimodal”- abundant only in certain oceanic regions), and investigated how the abundance of these three categories relates to environmental gradients. Most surface assemblages were numerically dominated by a few cosmopolitan “normal-like” OTUs, yet there was a gradual shift towards assemblages dominated by “logistic” taxa in specific areas with productivity and temperature differing the most from the average conditions in the sampled stations. When we performed the SpAD categorization including additional habitats (deeper layers and particles of varying sizes), the SpAD of many OTUs changed towards fewer “normal-like” shapes, and OTUs categorized as globally rare in the surface ocean became abundant. This suggests that understanding the mechanisms behind microbial rarity and dominance requires expanding the context of study beyond local communities and single habitats. We show that marine bacterial communities comprise taxa displaying a continuum of SpADs, and that variations in their abundances can be linked to habitat transitions or barriers that delimit the distribution of community membersThis work was funded by the Spanish Ministry of Economy and Competitiveness (MINECO) through the Consolider‐Ingenio program (Malaspina 2010 Expedition, ref. CSD2008‐00077), with contributions from grant CTM2015‐70340R, CTM2015‐65720‐R, CTM2015‐69936‐P and King Abdullah University of Science and Technology (KAUST). [...]. This is a contribution of Grup Consolidat de Recerca of the Catalan Government 2014SGR/1179. CRG was supported by a Juan de la Cierva (IJCI‐2015‐23505, MINECO, Spain) fellowship and RL by a Ramón y Cajal fellowship (RYC‐2013‐12554, MINECO, Spain). MS was supported by grant EcoRARE (CTM2014‐60467‐JIN), funded by the Spanish Government and the European Regional Development Fund (ERDF), REMEI (CTM2015‐70340‐Rf) funded by the Spanish Government, and a Viera y Clavijo contract funded by the ACIISI and the ULPGC. MM was supported by CONICYT (FONDAP‐IDEAL 15150003). SGA was funded by BIOSENSOMICS through the ‘Convocatoria 2015 de ayudas Fundación BBVA a investigadores y creadores culturales'. RRM is supported by CONICYT FONDECYT 11170748Peer Reviewe