Abstract
During the Austral summer 2009 we studied three areas surrounding the Antarctic Peninsula: the Bellingshausen Sea, the Bransfield Strait and the Weddell Sea. We aimed to investigate, whether viruses or protists were the main agents inducing prokaryotic mortality rates, and the sensitivity to temperature of prokaryotic heterotrophic production and mortality based on the activation energy (Ea) for each process. Seawater samples were taken at seven depths (0.1–100 m) to quantify viruses, prokaryotes and protists abundances, and heterotrophic prokaryotic production (PHP). Viral lytic production, lysogeny, and mortality rates of prokaryotes due to viruses and protists were estimated at surface (0.1–1 m) and at the Deep Fluorescence Maximum (DFM, 12–55 m) at eight representative stations of the three areas. The average viral lytic production ranged from 1.0 ± 0.3 × 107 viruses ml−1 d−1 in the Bellingshausen Sea to1.3 ± 0.7 × 107 viruses ml−1 d−1 in the Bransfield Strait, while lysogeny, when detectable, recorded the lowest value in the Bellingshausen Sea (0.05 ± 0.05 × 107 viruses ml−1 d−1) and the highest in the Weddell Sea (4.3 ± 3.5 × 107 viruses ml−1 d−1). Average mortality rates due to viruses ranged from 9.7 ± 6.1 × 104 cells ml−1 d−1 in the Weddell Sea to 14.3 ± 4.0 × 104 cells ml−1 d−1 in the Bellingshausen Sea, and were higher than averaged grazing rates in the Weddell Sea (5.9 ± 1.1 × 104 cells ml−1 d−1) and in the Bellingshausen Sea (6.8 ± 0.9 × 104 cells ml−1 d−1). The highest impact on prokaryotes by viruses and main differences between viral and protists activities were observed in surface samples: 17.8 ± 6.8 × 104 cells ml−1 d−1 and 6.5 ± 3.9 × 104 cells ml−1 d−1 in the Weddell Sea; 22.1 ± 9.6 × 104 cells ml−1 d−1 and 11.6 ± 1.4 × 104 cells ml−1 d−1 in the Bransfield Strait; and 16.1 ± 5.7 × 104 cells ml−1 d−1 and 7.9 ± 2.6 × 104 cells ml−1 d−1 in the Bellingshausen Sea, respectively. Furthermore, the rate of lysed cells and PHP showed higher sensitivity to temperature than grazing rates by protists. We conclude that viruses were more important mortality agents than protists mainly in surface waters and that viral activity has a higher sensitivity to temperature than grazing rates. This suggests a reduction of the carbon transferred through the microbial food-web that could have implications in the biogeochemical cycles in a future warmer ocean scenario.
Highlights
To understand the functioning of the biogeochemical carbon cycles in extreme marine systems, such as the Antarctic Ocean, it is essential to elucidate the relation between the prokaryotic members of the community and their predators, protists and viruses, since their impact in the carbon cycles are different
In both cases they reported that mortality rates caused by viruses were higher than those caused by protists, suggesting that the viral shunt would drive the destiny of the prokaryotic carbon in those regions
We aimed to test the following working hypotheses: (a) since high viral production rates were detected previously in polar systems, the mortality caused by viruses should be at least as important or higher than the protistan impact in Antarctic waters; (b) taking into account that DOC is not a limiting factor for prokaryotic production during the Austral summer, we expected that prokaryotes are top-down controlled at this time of the year; (c) prokaryotic production, viral infection and grazing by protists would show different sensitivity to temperature changes as suggested by study in the Arctic Ocean
Summary
To understand the functioning of the biogeochemical carbon cycles in extreme marine systems, such as the Antarctic Ocean, it is essential to elucidate the relation between the prokaryotic members of the community and their predators, protists and viruses, since their impact in the carbon cycles are different. The few studies focused on viral infection of prokaryotes were carried out mainly in waters adjacent to the Antarctic Peninsula (Guixa-Boixereu et al, 2002), in coastal regions of Antarctica (Pearce et al, 2007), the Weddell Sea and Polar Frontal zones, and in the Southern Ocean including Sub-Antarctic areas (Bonilla-Findji et al, 2008; Evans et al, 2009; Evans and Brussaard, 2012; Malits et al, 2014) All those studies point toward a high viral lytic production and suggest a major role of viruses in prokaryotic mortality. In both cases they reported that mortality rates caused by viruses were higher (at least in some periodes of the year) than those caused by protists, suggesting that the viral shunt would drive the destiny of the prokaryotic carbon in those regions
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