Abstract
Abstract. Above the Kerguelen Plateau in the Southern Ocean natural iron fertilization sustains a large phytoplankton bloom over 3 months during austral summer. During the KEOPS1 project (KErguelen Ocean and Plateau compared Study1) we sampled this phytoplankton bloom during its declining phase along with the surrounding high-nutrient–low-chlorophyll (HNLC) waters to study the effect of natural iron fertilization on the role of viruses in the microbial food web. Bacterial and viral abundances were 1.7 and 2.1 times, respectively, higher within the bloom than in HNLC waters. Viral production and virus-mediated mortality of bacterioplankton were 4.1 and 4.9 times, respectively, higher in the bloom, while the fraction of infected cells (FIC) and the fraction of lysogenic cells (FLC) showed no significant differences between environments. The present study suggests viruses to be more important for bacterial mortality within the bloom and dominate over grazing of heterotrophic nanoflagellates (HNFs) during the late bloom phase. As a consequence, at least at a late bloom stage, viral lysis shunts part of the photosynthetically fixed carbon in iron-fertilized regions into the dissolved organic matter (DOM) pool with potentially less particulate organic carbon transferred to larger members of the food web or exported.
Highlights
A quarter of a century ago the importance of viruses as the most abundant biological entity in the oceans (Bergh et al, 1989) and their role in the material and energy cycles were recognized (Proctor and Fuhrman, 1990; Suttle et al, 1990)
viral abundance (VA) ranged from 3.1 to 14.2 × 106 mL−1, with the highest values found at the main bloom station A3 and the lowest value detected in the deep layer of the HNLC station B11
At the Fe-fertilized stations, on average 29.4 ± 11.1 viruses contacted a bacterial cell per day, while in the HNLC waters contact rates were 14.2 ± 4.4 viruses cell−1 d−1, with the highest values at the bloom station A3 and the lowest at the HNLC station B11 in accordance to the highest and lowest viral abundances, respectively
Summary
A quarter of a century ago the importance of viruses as the most abundant biological entity in the oceans (Bergh et al, 1989) and their role in the material and energy cycles were recognized (Proctor and Fuhrman, 1990; Suttle et al, 1990). Studies on viral abundance and production or infectivity in the cold, high-latitude marine environments remained limited or have only recently been accumulating (Bird et al, 1993; Brussaard et al, 2008b; Evans and Brussaard, 2012; Evans et al, 2009; Guixa-Boixereu et al, 2002; Higgins et al, 2009; Manganelli et al, 2009; Marchant et al, 2000; Payet and Suttle, 2008, 2013; Smith et al, 1992; Steward et al, 1996; Strzepek et al, 2005; Weinbauer et al, 2009) These observations demonstrate that viruses are ecologically as important in these cold environments as in the world’s other oceans. By this process, called the “viral shunt” (Wilhelm and Suttle, 1999), heterotrophic bacteria are Published by Copernicus Publications on behalf of the European Geosciences Union
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