8 7 RESEARCH ARTICLE Phytoplankton^bacterioplankton interactions and carbon £uxes through microbial communities in a microtidal lagoon (Lagoon of Venice, Northern Italy) Q1 Alessandra Pugnetti 1 , Paola Del Negro 2 , Michele Giani 3 , Francesco Acri 1 , Fabrizio Bernardi Aubry 1 , Franco Bianchi 1 , Daniela Berto 3 & Annalisa Valeri 2 1 Istituto di Scienze Marine (ISMAR), Consiglio Nazionale delle Ricerche, Castello, Venezia, Italy; 2 Dipartimento di Oceanografia Biologica (BiO), Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy; and 3 Istituto Superiore per la Protezione e la Ricerca Ambientale (ISPRA), Chioggia, Italy Correspondence: Alessandra Pugnetti, Istituto di Scienze Marine (ISMAR), Consiglio Nazionale delle Ricerche, Castello 1364/A, I-30122 Venezia, Italy. Tel.: 139 041 240 4711; fax: 139 041 520 4126; e-mail: alessandra.pugnetti@ismar.cnr.it Present address: Michele Giani, Dipartimento di Oceanografia Biologica (BiO), Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Via A. Piccard 54, I-34051 Trieste, Italy. Received 16 March 2009; revised 11 December 2009; accepted 28 December 2009. DOI:10.1111/j.1574-6941.2010.00839.x Editor: Riks Laanbroek Keywords phytoplankton productivity; bacterial production; organic carbon; exoenzimatic activity; Venice lagoon; carbon fluxes. Abstract The strength of the bacteria–phytoplankton coupling and the importance of the microbially mediated carbon fluxes have been investigated in a microtidal lagoon (Lagoon of Venice), with emphasis on the trophic variations, in a within-system perspective. The large trophic heterogeneity of the three stations considered corresponded to an elevated variability of phytoplankton biomass and production (from 0.1 up to 300 mgCL 1 h 1 ), while bacteria standing stock and production (from 2 to 8 mgCL 1 h 1 ) appeared, in comparison, to be much more constant. The relationships between bacteria and the phytoplankton community could not be related to the trophic state in a straightforward way; rather, some patterns common to the three stations could be evidenced. In particular, the two communities appeared to be clearly uncoupled, bacterial carbon demand (BCD) always exceeding dissolved primary production (DPP) and, mostly, also total primary production, independent of the station and the season considered. The occurrence of situations in which bacterial production was larger than primary production and the continuous prevalence of BCD over DPP implied that, quite independent of the trophic variability, sources of organic carbon other than phytoplankton production were necessary and available to sustain the bacterial metabolism in the Lagoon of Venice. Introduction Across-ecosystem analyses of the relationships between bacterial carbon production (BCP) or bacterial carbon de- mand (BCD) and primary production (PP) have supported the general conceptual model that the bacterioplankton–phy- toplankton coupling varies as a function of ecosystem char- acteristics. In particular, a trophic dependence of bacteria on algae, through the photosynthetic production of dissolved organic carbon [dissolved primary production (DPP)], would be characteristics of aquatic ecosystems where bacteria are carbon limited. There is a general trend towards a tight linkage in open oceans and clear water lakes and of loose coupling in coastal or eutrophic sites. In open seas and clear water lakes, the general picture is one of a nutrient-controlled energy flow, where phytoplankton particulate organic matter is the main energy source for grazers and dissolved organic carbon (DOC) derived from phytoplankton (e.g. direct exudation through DPP, lyses) represents the main energy source for bacteria, supporting all the BCD, so that the bacteria–phytoplankton coupling is tight. In contrast, in estuaries and coastal lagoons, the plankton system might show a net heterotrophic metabo- lism (respiration 4 gross primary production) or a seasonal shift from net autotrophy to net heterotrophy that may be due to turbidity and nutrient variations and due to inputs of organic matter allochthonous to the planktonic system 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 FEMS Microbiol Ecol ]] (2010) 1–12 c  2010 Federation of European Microbiological Societies Published by Blackwell Publishing Ltd. All rights reserved FEMSEC 839 B Dispatch: 2.2.10 Journal: FEMSEC CE: Mahendra Journal Name Manuscript No. Author Received: No. of pages: 12 TE: Deepa/Mini FEMSEC 839 (BWUK FEMSEC 839 Webpdf:=02/02/2010 09:53:21 451272 Bytes 12 PAGES n operator=M.Chackalayil) 2/2/2010 9:53:29 PM