Viral production in the Gulf of Trieste (Northern Adriatic Sea): Preliminary results using different methodological approaches A. Karuza a, ⁎, P. Del Negro a , E. Crevatin a , S. Fonda Umani b a Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Dipartimento di Oceanografia Biologica, v. A. Piccard, 54 - 34014 Trieste, Italy b Department of Life Sciences, University of Trieste - v. Valerio 28/1 34127 Trieste, Italy abstract article info Article history: Received 6 February 2009 Received in revised form 1 December 2009 Accepted 2 December 2009 Keywords: Adriatic Sea Bacteria Gulf of Trieste Marine viruses Virus production Although the temporal and spatial variability of virioplankton in the northernmost part of the Adriatic Sea has been repeatedly explored suggesting that viruses constitute an extremely dynamic component of the plankton community and hypothesizing their importance in marine food webs and mucilage events, there is still no information about viral replication rates. Hence, the contribution of viruses to bacterial mortality and the cycling of organic matter in this part of the Adriatic basin are still not fully comprehended. Assessment of the role of viral lysis requires a robust means of estimating viral production. Since, up to now, none of the available methods evolved to a state of a standard yet, in this preliminary study 3 different experimental approaches were simultaneously assayed (viral production estimated by radiotracer incorporation method [TdR], dilution technique for the estimate of viral production in already infected bacteria [DIL] and serial dilution method in manipulated phage-host assemblage [SER]). The present study provided the first evidence of viral production rates in this study area, that resulted in comprising between ∼ 3.5–15×10 8 viruses L -1 h -1 and critically faced up the results obtained by different techniques with the consideration that they suffer from different biases. Based on TdR and DIL viral proliferation estimates, viral lysis was responsible for the loss of 54 to 95% of the bacterial standing stock, while the viral-induced mortality by SER (325% d -1 ) was likely consistently overestimated. These results indicate that viral lysis is a significant factor for prokaryotic mortality suggesting its implication as an important pathway for the cycling of dissolved organic matter in the Gulf of Trieste. © 2009 Elsevier B.V. All rights reserved. 1. Introduction It is generally recognized that marine viruses are consistently the most abundant biological entities in the sea (Fuhrman, 1999; Wommack and Colwell, 2000; Weinbauer, 2004), which affect numerous processes and characteristics (Bergh et al., 1989; Proctor and Fuhrman, 1990) including primary production, nutrient cycling, food web dynamics, bacterial and algal species diversity, and carbon biogeochemistry (Proctor, 1997; Fuhrman, 1999; Wommack and Colwell, 2000; Weinbauer, 2004). Viruses account for a significant percentage of the prokaryotic mortality influencing microbial food web dynamics (Fuhrman and Noble, 1995; Steward et al., 1996; McManus and Fuhrman, 1988; Pace, 1988). Viral lysis of bacteria releases dissolved and colloidal organic carbon that may represent a major source of DOM in the sea (Agustí et al., 1998). The resulting DOM may be consumed by bacteria enhancing planktonic respiration and nutrient regeneration (Middelboe et al., 1996; Noble et al., 1999). Early studies have already shown that virus populations are extremely dynamic, and can change quickly over short timescales (Bratbak et al., 1990, 1996; Heldal and Bratbak, 1991; Steward et al., 1992a). Viral production involves the lysis of host cells and the release of cell contents and biomass into organic detritus (both dissolved and particulate), which can then be used again by non-infected prokaryotes (this cycle of energy and matter is named viral shunt). This process supports heterotrophic production, but it also decreases the efficiency of carbon transfer to higher trophic levels (Fuhrman, 1999; Wilhelm and Suttle, 1999; Suttle, 2005). Therefore the measurement of viral replication rates is of primary importance for determining the viral-induced alterations of organic matter flows within microbial food webs (Danovaro et al., 2008) and for an improved understanding of the function of the world's oceans (Suttle, 2007). The dependence of virus particles on the hosts' metabolism determines their very short time persistence in the extracellular environment. Since the estimate of viral abundance in seawater obtained by direct counts (TEM-transmission electronic microscopy and EM-epifluorescence microscopy) or flow cytometry provides only Journal of Experimental Marine Biology and Ecology 383 (2010) 96–104 ⁎ Corresponding author. Via A. Piccard, 54, 34010 Trieste, Italy. Tel.: + 40 2249740; fax: +40 2249770. E-mail address: akaruza@ogs.trieste.it (A. Karuza). URLS: http://www.ogs.trieste.it (A. Karuza), http://www.ogs.trieste.it (P. Del Negro), http://www.ogs.trieste.it (E. Crevatin), http://www.units.it (S. Fonda Umani). 0022-0981/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2009.12.003 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe