Distribution of Virus-Infected Bacteria in the Western Equatorial Pacific

Viruses are generally considered an important agent of bacterial loss in diverse marine environments. However, the impact of viruses on bacteria is unknown in the western equatorial Pacific, where surface waters are warm and phytoplankton biomass is low (i.e., oligotrophic). Further, little is known about their importance in the mesopelagial, where bacteria and heterotrophic nanoflagellates are known to be metabolically active. To elucidate the ecological characteristics of viruses in the western equatorial Pacific, abundances of bacteria and viruses were measured, along with frequencies of visibly infected cells (FVIC) and frequencies of dividing cells (FDC) in epipelagic and mesopelagic samples at three stations near the equator from August to September 2002. Measurements of Secchi depth (20 m) and chlorophyll a concentrations (0.07– 0.4 mg chl a liter ) indicated that the study area was oligotrophic during the investigation. FVIC ranged from 0.4% to 1.8% and 0.5% to 1.8% in the epipelagic and mesopelagic zones, respectively. Virally induced bacterial mortality was inferred to range from 4.5% to 20.8% in the epipelagic zone, suggesting that viruses contribute substantially to bacterial mortality in oligotrophic seawaters. In addition, these values were similar to those estimated for the mesopelagic zone (5.0%–21.2%). Overall, viruses appear to be an important factor in the loss of bacterial production in both oligotrophic epipelagic and mesopelagic zones in the study area. Viruses are known to be an important agent of bacterial mortality in diverse environments (Fuhrman 1999, Wommack and Colwell 2000). However, limited studies are available on the viral mortality of bacteria in oligotrophic environments. In oligotrophic seawaters of the East Sea of Korea and the North Pacific Subtropical Gyre, the impact of viruses on bacteria has been suggested to be important (Hwang and Cho 2002b, Brum 2005), but the viral mortality of bacteria was reported to be very low in the oligotrophic western Mediterranean (Guixa-Boixareu et al. 1999). In the open ocean, many studies have focused on viral ecology in the epipelagial. Only a few studies have examined mesopelagic viruses (Steward et al. 1996, Weinbauer et al. 2003). In the mesopelagic zone, it has been recognized that coupling between sinking particulate organic carbon and free-living bacteria production is tight (Cho and Azam 1988, Simon et al. 1992). In addition, a year-long study in the North Pacific showed that @76%–90% of all cells at a depth of 200–1,000 m contained significant amounts of rRNA, indicating that most mesopelagic microorganisms are metabolically active (Karner et al. 2001). Furthermore, heterotrophic nanoflagellates, a link between bacteria and microzooplankton, have been suggested to be active and survive well in the Pacific Science (2010), vol. 64, no. 2:177–186 doi: 10.2984/64.2.177 : 2010 by University of Hawai‘i Press All rights reserved 1 This work was partly supported by ‘‘Multidisciplinary Investigation of the Western Pacific’’ of the Korea Ocean Research and Development Institute project (PE82400) and by the BK21 project of the Korean Government. Manuscript accepted 25 May 2009. 2Molecular and Microbial Ecology Laboratory, School of Earth and Environmental Sciences and Research Institute of Oceanography, Seoul National University, Seoul 151-742, Korea. 3 Corresponding author (phone: 82-2-880-8171; fax: 82-2-877-9774; e-mail: [email protected]). mesopelagial (Cho et al. 2000, Tanaka and Rassolzadegan 2002). Limited data on the frequency of visibly infected cells (ranging from 0.5% to 0.7%) in the mesopelagial of the North Pacific Ocean, the Mediterranean Sea, and the Chukchi Sea suggest that viruses have an important impact on bacterial losses (Proctor and Fuhrman 1991, Steward et al. 1996, Weinbauer et al. 2003). The primary goals of this study were to estimate the importance of virally induced bacterial mortality in the epipelagial and mesopelagial of the oligotrophic western equatorial Pacific, where marine viruses have not yet been studied. materials and methods