The production of extracellular membrane vesicles (hereafter called MVs) is a universal cellular feature, common to the three domains of life (Deatherage and Cookson, 2012). In particular, it has been known for decades that bacteria, including marine species, produce MVs in the laboratory but also in biofilms or during infections (Schooling and Beveridge, 2006; Deatherage and Cookson, 2012). However, until recently, the presence of MVs in natural environments has been largely overlooked by molecular ecologists. This attitude is probably going to change with a recent report in Science in which Biller et al. (2014) demonstrate the abundance of bacterial vesicles in marine ecosystems. They first show that several strains from the numerically dominant marine phytoplankter Prochlorococcus produce large amounts of MVs in the laboratory, suggesting that marine phototrophic bacteria also produce MVs in their natural environment. This hypothesis has been directly tested by examining two distinct ocean water samples for the presence of MVs. As expected, the authors succeeded to isolate abundant MVs from these two samples, with concentrations ranging from 105 to 106 vesicles ml−1 of sea water. This is similar to the lower range of concentrations reported for viral particles in the oceans (105 to 109 virus-like particles/ml; Suttle, 2007). It is therefore surprising that Biller et al. (2014) observed only negligible number of apparent tailed phages (or gene transfer agents (GTA)) in their vesicle-rich ocean samples even though the methods they used to isolate MVs were similar to those traditionally employed for the isolation of viral particles. Indeed, the dimensions, morphology and molecular composition of MVs are very similar to those of some virions (Forterre et al., 2013). This observation suggests that MVs could outnumber true viral particles in some marine environments.

The presence of bacterial DNA in MVs isolated from ocean samples, together with the fact that MVs appear much more abundant than bona fide viral particles in these samples, validates our prediction that MVs present in various environments could lead to an overestimation of viral titers by EFM and even suggest the presence of viruses when there are none (Forterre et al., 2013). EFM has been widely used for the last two decades by microbial ecologists to quantify viral particles (Patel et al., 2007), leading to the conclusion that viruses are usually 10 times more abundant than bacteria in environmental samples and have major impact on global biogeochemical cycles (Danovaro et al., 2011). If MVs are indeed more abundant than true viral particles in natural environments, these conclusions should be reconsidered.

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