Viral Abundance and a High Proportion of Lysogens Suggest That Viruses Are Important Members of the Microbial Community in the Gulf of Trieste D. Stopar 1 , A. C ˇ erne 1 , M. Z ˇ igman 1 , M. Poljs ˇak-Prijatelj 2 and V. Turk 3 (1) Biotechnical Faculty, Department of Food Technology, University of Ljubljana, Vecna pot 111, SI-1000 Ljubljana, Slovenia (2) Institute of Microbiology and Immunology, Zaloska 4, SI-1000 Ljubljana, Slovenia (3) National Institute of Biology, Marine Biological Station, Fornace 41, SI-6330 Piran, Slovenia Received: 23 May 2002 / Accepted: 26 December 2002 / Online publication: 6 June 2003 Abstract Epifluorescence microscopy and transmission electron microscopy were applied to study virioplankton com- munity in the Gulf of Trieste (northern Adriatic Sea). The total viral abundance was in a range between 2.5 · 10 9 /L and 2.9 · 10 10 /L and was positively correlated with trophic status of the environment. Viruslike particles were significantly correlated with bacterial abundance in all samples studied. Correlations with other physico- chemical or biological parameters were not significant. The data suggest that, because of the substantial fraction of tailed viruses present (26%), bacteriophages are an important component of the virioplankton community in the Gulf of Trieste. The abundance of viruslike parti- cles in the seawater changed at hour intervals in a range from 1.3 · 10 9 /L to 5.1 · 10 9 /L. A significant fraction (71%) of the bacterial isolates was inducible in vitro by mitomycin C, and a high occurrence (51%) of lysogenic isolates with more than one phage morphotype present in the lysate was detected. The presence of lysogenic bacteria in the seawater was confirmed in situ with a mitomycin C induction experiment on the natural bacterial popula- tion. Results suggest that virioplankton is an abundant component of the microbial community in the Gulf of Trieste. Introduction Heterotrophic bacteria assimilate a significant fraction of the primary production (approximately 50%) in marine environments [3, 11]. Grazing of the heterotrophic mi- croorganisms by protists transfer bacterial-derived energy to a higher trophic level. In contrast, bacterial lysis by phages may increase the amount of dissolved organic matter, thus uncoupling the microbial loop from the higher trophic levels [9]. It is currently estimated that 10– 20% of the bacterial production in marine waters is converted to dissolved organic matter by phage lysis [35]. Loss of bacterial production through viral lysis is im- portant for our understanding of the role bacteria play in eutrophic environment such as the northern Adriatic Sea. It was demonstrated in different aqueous environ- ments that viruses are the most abundant microorgan- isms, and it was also argued that bacteriophages represent the major virus class [7, 13, 14, 24, 31, 36]. In various surface waters up to 10 9 viruses per mL have been counted [9]. Viral abundance declines from near shore to the open sea, and with increasing depth [5, 7]. Significant seasonal fluctuations in virus densities of approximately two orders of magnitude have been observed [14]. The experimental evidence for diurnal changes is scant, but claims that virus abundance can change in the interval of a couple of hours or in as short a time as 10–20 min have been made [6]. Several field studies suggested the exist- ence of a coupling between viruses and bacteria distri- bution [7]. In earlier studies in the northern Adriatic Sea it was shown that virus activity might play a significant role in food web structure under changing trophic con- ditions [30, 31]. Because virus infection is generally host specific, viruses can shape the microbial community structure [29]. In addition, they mediate horizontal gene transfer by transduction [9, 15, 16]. Bacterial viruses exist in the environment mainly as free virions or as prophages inside a lysogenic host [2]. Prophages contribute to the total virus production in the environment when the host cell is induced, but perhaps even more important, they are capable of altering the Correspondence to: D. Stopar; E-mail: david.stopar@uni-lj.si DOI: 10.1007/s00248-002-3009-5 d Volume 47, 1–8 (2004) d Ó Springer-Verlag New York, Inc. 2003 1