Warming-induced changes in denitrifier community structure modulate the ability of phototrophic river biofilms to denitrify Stéphanie Boulêtreau a,b, ⁎, Emilie Lyautey a,b,1 , Sophie Dubois c , Arthur Compin a,b , Cécile Delattre d , Aurélie Touron-Bodilis d , Sylvain Mastrorillo a,b , Frédéric Garabetian c a Université de Toulouse, UPS, INP, EcoLab (Laboratoire Ecologie Fonctionnelle et Environnement), 118 route de Narbonne, F-31062 Toulouse, France b CNRS, EcoLab, F-31062 Toulouse, France c Université de Bordeaux, EPOC - OASU, UMR 5805, Station Marine d'Arcachon, 2 rue du Professeur Jolyet, 33120 Arcachon, France d EDF Recherche et Développement, LNHE (Laboratoire National d'Hydraulique et Environnement), 6 quai Watier, F-78401 Chatou, France HIGHLIGHTS • We produced river biofilms in 2 mean temperature conditions: 17 vs 19.5 °C. • We compared their denitrifiers' structuring and functioning in 6d- and 21d-old biofilms. • A difference of 2.5 °C produced contrasted denitrifier communities. • The indirect temperature effect on denitrification activity shifted between biofilm age. • Warming impact strongly depends on the bacterial successional trajectory. abstract article info Article history: Received 7 March 2013 Received in revised form 16 July 2013 Accepted 31 July 2013 Available online xxxx Editor: Christian EW Steinberg Keywords: Global change Potential denitrification mRNA Temperature sensitivity Epilithon Periphyton Microbial denitrification is the main nitrogen removing process in freshwater ecosystems. The aim of this study was to show whether and how water warming (+2.5 °C) drives bacterial diversity and structuring and how bacterial diversity affects denitrification enzymatic activity in phototrophic river biofilms (PRB). We used water warming associated to the immediate thermal release of a nuclear power plant cooling circuit to produce natural PRB assemblages on glass slides while testing 2 temperatures (mean temperature of 17 °C versus 19.5 °C). PRB were sampled at 2 sampling times during PRB accretion (6 and 21 days) in both temperatures. Bacterial community composition was assessed using ARISA. Denitrifier community abundance and denitrification gene mRNA levels were estimated by q-PCR and qRT-PCR, respectively, of 5 genes encoding catalytic subunits of the denitrification key enzymes. Denitrification enzyme activity (DEA) was measured by the acetylene-block assay at 20 °C. A mean water warming of 2.5 °C was sufficient to produce contrasted total bacterial and denitrifier communities and, therefore, to affect DEA. Indirect temperature effect on DEA may have varied between sampling time, increasing by up to 10 the denitrification rate of 6-day-old PRB and decreasing by up to 5 the denitrification rate of 21-day-old PRB. The present results suggest that indirect effects of warming through changes in bacterial community composition, coupled to the strong direct effect of temperature on DEA already demonstrated in PRB, could modulate dissolved nitrogen removal by denitrification in rivers and streams. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The debate continues as to determine the precise magnitude increases, mean global surface temperature is projected to increase by ca 3 °C over the next century. Many aquatic ecosystems will become warmer, and warming will occur with unprecedented rapidity (IPCC, 2007). Streams and rivers, whose water temperature tracks air temper- ature (Webb and Nobilis, 1994; Langan et al., 2001; Mouthon and Daufresne, 2006), are particularly vulnerable to climate change because they are relatively isolated, physically fragmented, and already the most heavily human-impacted of all natural ecosystems (Malmqvist and Rundle, 2002), by anthropogenic loading of dissolved inorganic N particularly (Vitousek et al., 1997). Within a century, human production of reactive nitrogen including nitrogen oxide, nitrous oxide, ammonia and nitrate had more than doubled the global rates of nitrogen fixation, threatening the quality of air, soil and water (Sutton et al., 2011). Science of the Total Environment 466–467 (2014) 856–863 ⁎ Corresponding author at: Université de Toulouse, UPS, INP, EcoLab (Laboratoire Ecologie Fonctionnelle et Environnement), 118 route de Narbonne, F-31062 Toulouse, France. Tel.: +33 5 61557348; fax: +33 5 61556096. E-mail address: stephanie.bouletreau@univ-tlse3.fr (S. Boulêtreau). 1 Present address: Université de Savoie, UMR 42 CARRTEL, F-73376 Le Bourget du Lac, France. 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.07.121 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv