The impact of viruses and protists on bacterioplankton mortality was examined monthly during 2 years (May 2005-April 2007) in an oligotrophic coastal environment (NW Mediterranean Sea). We expected that in such type of system, (i) bacterial losses would be caused mainly by protists, and (ii) lysogeny would be an important type of virus-host interaction. During the study period, viruses and grazers together were responsible for 50.6 +/- 40.1% day(-1) of bacterial standing stock losses (BSS) and 59.7 +/- 44.0% day(-1) of bacterial production losses (BP). Over the first year (May 2005-April 2006), protists were the principal cause of bacterial mortality, removing 29.9 +/- 20.4% day(-1) of BSS and 33.9 +/- 24.3% day(-1) of BP, whereas viral lysis removed 13.5 +/- 17.0% day(-1) of BSS and 12.3 +/- 12.3% day(-1) of BP. During the second year (May 2006-April 2007), viruses caused comparable bacterial losses (29.2 +/- 14.8% day(-1) of BSS and 40.9 +/- 20.7% day(-1) of BP) to protists (28.6 +/- 25.5% day(-1) of BSS and 32.4 +/- 20.0% day(-1) of BP). In 37% of cases higher losses of BP due to viruses than due to protists were found. Lysogenic infection was detected in 11 of 24 samplings. Contrary to our expectations, lytic infections dominated over the two years, and viruses resulted to be a significant source of bacterial mortality in this oligotrophic site.

Viral activity exerts a particularly important role in the dark ocean across the global tropical and subtropical oceans.

We investigated the effects of an increase in dissolved CO2 on the microbial communities of the Mediterranean Sea during two mesocosm experiments in two contrasting seasons: winter, at the peak of the annual phytoplankton bloom, and summer, under low nutrient conditions. The experiments included treatments with acidification and nutrient addition, and combinations of the two. We followed the effects of ocean acidification (OA) on the abundance of the main groups of microorganisms (diatoms, dinoflagellates, nanoeukaryotes, picoeukaryotes, cyanobacteria, and heterotrophic bacteria) and on bacterial activity, leucine incorporation, and extracellular enzyme activity. Our results showed a clear stimulation effect of OA on the abundance of small phytoplankton (pico- and nanoeukaryotes), independently of the season and nutrient availability. A large number of the measured variables showed significant positive effects of acidification in summer compared with winter, when the effects were sometimes negative. Effects of OA were more conspicuous when nutrient concentrations were low. Our results therefore suggest that microbial communities in oligotrophic waters are considerably affected by OA, whereas microbes in more productive waters are less affected. The overall enhancing effect of acidification on eukaryotic pico- and nanophytoplankton, in comparison with the non-significant or even negative response to nutrient-rich conditions of larger groups and autotrophic prokaryotes, suggests a shift towards medium-sized producers in a future acidified ocean.