Marine Viruses 2016

The research effort, publication rate and scientific community within the field of marine viruses have been growing rapidly over the past decade and viruses are now known to play key roles in microbial population dynamics, diversity and evolution as well as biogeochemical cycling. The compilation of papers included in the current Special Issue highlights the exploration of eukaryotic and prokaryotic viruses, from discovery to complex interplays between virus and host and virus–host interactions with ecologically relevant environmental variables. The discovery of novel viruses and new mechanisms underlying virus distribution and diversity exemplify the fascinating world of marine viruses. The oceans greatly shape Earth’s climate, hold 1.37 billion km3 of seawater, produce half of the oxygen in the atmosphere, and are integral to all known life. In a time where life in the oceans is under increasing threat (global warming, pollution, economic use) it is pressing to understand how viruses affect host population dynamics, biodiversity, biogeochemical cycling and ecosystem efficiency

Viral abundance and genome size distribution in the sediment and water column of marine and freshwater ecosystems

The size distribution of viral DNA in natural samples was investigated in a number of marine, brackish and freshwater environments by means of pulsed field gel electrophoresis (PFGE). The method was modified to work with both water and sediment samples, with an estimated detection limit for individual virus genome size groups of 1-2 × 104 virus-like particles (VLP) mL−1 water and 2-4 × 105 VLP cm−3 sediment in the original samples. Variations in the composition and distribution of dominant virus genome sizes were analyzed within and between different habitats that covered a range in viral density from 0.4 × 107 VLP mL−1 (sea water) to 300 × 107 VLP cm−3 (lake sediment). The PFGE community fingerprints showed a number of cross-system similarities in the genome size distribution with a general dominance of genomes in the 30-48, 50-70 and 145-200 kb size fractions, and with many of the specific genome sizes detected in all the investigated habitats. However, large differences in community fingerprints were also observed between the investigated sites, and some virus genome sizes were found only in specific biotopes (e.g. lake water), in specific ecosystems (e.g. a particular lake) or even in specific microhabitats (e.g. a particular sediment stratum

Marine viruses:key players in marine ecosystems

Viruses were recognized as the causative agents of fish diseases, such as infectious pancreatic necrosis and Oregon sockeye disease, in the early 1960s [1], and have since been shown to be responsible for diseases in all marine life from bacteria to protists, mollusks, crustaceans, fish and mammals [2].[...

Quorum sensing determines the choice of antiphage defense strategy in <i>Vibrio anguillarum</i>

Selection for phage resistance is a key driver of bacterial diversity and evolution, and phage-host interactions may therefore have strong influence on the genetic and functional dynamics of bacterial communities. In this study, we found that an important, but so far largely overlooked, determinant of the outcome of phage-bacterial encounters in the fish pathogen Vibrio anguillarum is bacterial cell-cell communication, known as quorum sensing. Specifically, V. anguillarum PF430-3 cells locked in the low-cell-density state (ΔvanT mutant) express high levels of the phage receptor OmpK, resulting in a high susceptibility to phage KVP40, but achieve protection from infection by enhanced biofilm formation. By contrast, cells locked in the high-cell-density state (ΔvanΟ mutant) are almost completely unsusceptible due to quorum-sensing-mediated downregulation of OmpK expression. The phenotypes of the two quorum-sensing mutant strains are accurately reflected in the behavior of wild-type V. anguillarum, which (i) displays increased OmpK expression in aggregated cells compared to free-living variants in the same culture, (ii) displays a clear inverse correlation between ompK mRNA levels and the concentration of N-acylhomoserine lactone quorum-sensing signals in the culture medium, and (iii) survives mainly by one of these two defense mechanisms, rather than by genetic mutation to phage resistance. Taken together, our results demonstrate that V. anguillarum employs quorum-sensing information to choose between two complementary antiphage defense strategies. Further, the prevalence of nonmutational defense mechanisms in strain PF430-3 suggests highly flexible adaptations to KVP40 phage infection pressure, possibly allowing the long-term coexistence of phage and host