An annual cycle of dimethylsulfoniopropionate-sulfur and leucine assimilating bacterioplankton in the coastal NW Mediterranean Maria Vila-Costa, 1 Jarone Pinhassi, 1,2 Cecilia Alonso, 3† Jakob Pernthaler 3‡ and Rafel Simó 1 * 1 Institut de Ciències del Mar, CSIC. Pg. Marítim de la Barceloneta 37-49, 08003 Barcelona, Catalonia, Spain. 2 Marine Microbiology, Department of Biology and Environmental Sciences, University of Kalmar, SE-39182 Kalmar, Sweden. 3 Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany. Summary The contribution of major phylogenetic groups to het- erotrophic bacteria assimilating sulfur from dissolved dimethylsulfoniopropionate (DMSP) and assimilating leucine was analysed in surface seawaters from Blanes Bay (NW Mediterranean) over an annual study between March 2003 and April 2004. The percentage of bacteria assimilating DMSP-S showed a strong seasonal pattern, with a steady increase from winter (8  5%) to summer (23  3%). The same seasonal pattern was observed for the rate of DMSP-S assimilation. The annual average percentage of DMSP-S-assimilating bacteria (16  8%) was lower than the corresponding percentage of leucine- assimilating cells (35  16%), suggesting that not all bacteria synthesizing protein incorporated DMSP-S. Smaller differences between both percentages were recorded in summer. Members of the Alphaproteo- bacteria (Roseobacter and SAR11) and Gammapro- teobacteria groups accounted for most of bacterial DMSP-S-assimilating cells over the year. All major bacterial groups showed an increase of the percent- age of cells assimilating DMSP-S during summer, and contributed to the increase of the DMSP-S assimila- tion rate in this period. In these primarily P-limited waters, enrichment with P + DMSP resulted in a stimulation of bacterial heterotrophic production comparable to, or higher than, that with P + glucose in summer, while during the rest of the year P + glucose induced a stronger response. This suggested that DMSP was more important a S and C source for bacteria in the warm stratified season. Overall, our results suggest that DMSP-S assimilation is con- trolled by the contribution of DMSP to S (and C) sources rather than by the phylogenetic composition of the bacterioplankton. Introduction Microorganisms control biogeochemical cycles in the sea. One of the current challenges in marine microbial ecology consists of identifying what phylogenetic groups mediate particular key biogeochemical processes in the ocean, to increase our understanding of how these processes are regulated over various temporal and spatial scales. For instance, if relatively few studies have addressed the sea- sonal variability of bacterial diversity in marine waters (Pinhassi and Hagström, 2000; Eilers et al., 2001; Schauer et al., 2003; Brown et al., 2005; Ghiglione et al., 2005; Morris et al., 2005; Mary et al., 2006; Alonso-Sáez et al., 2007), to our knowledge only one has analysed the seasonality in the processing of different components of the dissolved organic matter (DOM) pool (Alonso-Sáez and Gasol, 2007). Novel methods to link bacterial diversity and bio- geochemical activity include the search and affiliation of specific genes in metagenome analyses (e.g. Béjà et al., 2002) and the single cell biogeochemistry analysis techniques. Among the latter, the techniques that have been applied with greatest success involve the use of radioisotope substrates: MARFISH, a combination of microautoradiography (MAR) and fluorescence in situ hybridization of RNA (FISH) (Ouverney and Furhman, 1999; Lee et al., 1999; Cottrell and Kirchman, 2000), and flow cytometry cell sorting (e.g. Servais et al., 2003). A critical issue of these techniques is the choice of model molecules from the complex mixture of the DOM pool, which are representative of major matter fluxes in the sea. Dimethylsulfoniopropionate (DMSP) is an ubiquitous compound in the upper ocean, where it represents a Received 27 January, 2007; accepted 13 May, 2007. *For correspondence. E-mail rsimo@icm.csic.es; Tel. (+34) 932309590; Fax (+34) 932309555. Present addresses: † Instituto de Investiga- ciones Biológicas Clemente Estable, Av. Italia 3318, CP11600, Montevideo, Uruguay; ‡ Limnological Station, Institute of Plant Biology, Seestr. 187, CH-8802 Kilchberg, Switzerland. Environmental Microbiology (2007) 9(10), 2451–2463 doi:10.1111/j.1462-2920.2007.01363.x © 2007 The Authors Journal compilation © 2007 Society for Applied Microbiology and Blackwell Publishing Ltd