The importance of being temperate when it comes to extremes: marine viruses in the Southern ocean

Viruses have a substantial role in the ocean as they shape the microbial ecosystem and have the potential to affect the flow of energy and matter in the marine environment. Viral infections resulting in cell lysis lead to a “short circuit” in the microbial loop, as bacterial production is being fueled by recycling and respiration and the amount of organic matter available to macroorganisms is being reduced (Fuhrman 2000). Viruses use different replication strategies and grasping the “why” and the “how” behind those is critical to understanding viral ecology. Let’s start with the “how”.

Viral infections can be lytic: the viral DNA replicates independently from the host, which results in lysis of the cell (Miller and Day, 2008). Phages relying solely on this strategy are called virulent. In the lysogenic virus infections the viral DNA is maintained within the host as a prophage, where it’s being copied alongside the host’s DNA when the infected cell replicates, until induced to replicate lytically (Miller and Day, 2008). Viruses using both the lysogenic and the lytic cycle are called temperate. A great amount of studies examining the correlation between virus replication strategy and bacterial abundance and production have been rather inconclusive. Here Brum et al. 2015 demonstrate that temperate viruses dominate the dsDNA viral assemblage of the western Antarctic Peninsula (WAP) in the Southern ocean, and manage to gain insight into the strategy behind it.

According to the study temperate viruses dominate the WAP dsDNA viral assemblage primarily using lysogeny when bacterial production is low (in the spring) and switching to lytic replication in response to increased bacterial abundance and productivity (in the summer). Moreover, comparative analysis of viromes revealed that the taxonomic composition of the sequences unique to temperate and lytic viruses in the WAP was different. Namely, myoviruses were more abundant in the lytic-enriched virome and podoviruses were more abundant in temperate-enriched viromes. However, we can’t really talk about dominant sequences, as 93–95% of the reads remained taxonomically unassigned. Comparing this research with other studies of viral communities from lower-latitude marine ecosystems exhibits a difference in the trend in lysogeny. Higher-latitude environments display strong seasonality in percent lysogeny, with the highest values in winter and spring, in contrast to lower-latitude environments where lysogeny is sporadically detected. These trends with latitude might be explained by the relative abundance of temperate viruses, which dominate polar aquatic systems, probably because the intense seasonal variations in productivity in these regions favor viruses capable of switching replication strategies in response to changes in host productivity. However, it is important to say that because of the usage of quantitative dsDNA virome construction and comparison as a main research method this paper does not provide any data on single-stranded DNA or RNA viruses. Even though this study is a stepping stone in the understanding of polar viral-ecology, it is still a baby step in this very wide and unexplored field.

Reviewed paper:

Brum, J., Hurwitz, B., Schofield, O., Ducklow, H., Sullivan, M. (2015). Seasonal time bombs: dominant temperate viruses affect Southern Ocean microbial dynamics. In: The ISME Journal (2015), 1–13. doi: 10.1038/ismej.2015.125 

References:

Fuhrman, J. (2000). Impact of viruses on bacterial processes. In: Kirchman DL (ed) Microbial Ecology of the Oceans. Wiley-Liss, Inc.: New York, NY, USA, pp 327–350.

Miller RV, Day MJ. (2008). Contribution of lysogeny, pseudolysogeny, and starvation to phage ecology. In: Abedon ST (ed) Bacteriophage Ecology: Population Growth, Evolution, and Impact of Bacterial Viruses. Cambridge University Press: Cambridge, UK, pp 114–143.