Virus genes in the Arctic: the sleeping cousins of Antarctic viruses?

With an estimated abundance of 10³⁰, viruses are by far the most abundant biological entity in the oceans. The rapid turnover of bacteria and archaea by viral lysis indicates the importances of viruses in geochemical cycling (e.g. see Sean’s blog “Viruses, driving primary production?”). However, our knowledge on how viral occurrence and impact vary between regions is still patchy. Cottrell and Kirchman (2012) specifically wanted to explore the prevalence of virus genes in the genomes of bacteria from the western Arctic Ocean in comparison to samples from Monterey Bay and Antarctic waters. In fact, previous studies had shown that the relationship between viruses and prokaryotes in the Arctic appeared to differ from that in lower latitudes in terms of both lower bacterial and viral abundances and lower viral lysis. So far, no study had systematically surveyed virus DNA in genomes of uncultivated bacteria, although prophages (remember these are the latent forms of bacteriophages in which the viral genes are incorporated into the bacterial genome without causing disruption of the bacterial cell) have been found in 60 to 70 % of all sequenced genomes of cultivated bacteria and about half of all cultivated marine bacteria contain inducible prophages.

In order to be able to study uncultivated bacteria, metagenomic analyses were used: the bacterial genomes were sampled by cloning environmental DNA into a fosmid vector and were examined for phage genes using basic local alignment search tool (BLAST). 

The BLAST analyses using viral metagenome queries revealed that the number of phage genes in genomes of uncultivated bacteria was significantly higher in Arctic samples: 2-fold more viral genes than in the Monterey Bay bacterial DNA and 10-fold more than in the Antarctic bacterial DNA. However, although the environmental conditions vary a lot between Arctic, Pacific oceanic and coastal Antarctic waters, the metabolic pathways in the microbial communities themselves did not differ significantly.

The authors point out that these results represent a paradox, considering that the rates of viral mortality in Arctic waters are among the lowest determined for any aquatic system. So the high observed frequency of virus DNA in the Arctic bacterial metagenome suggests a substantial viral impact with poor conditions for actual viral lysis. Possibly the frequency of temperate phages, capable of entering the lysogenic cycle, is increased in Arctic waters, which has been supported by previous studies that found a higher abundance of the integrase genes, indicative of temperate phages. In this case, the viral impact would be majorly influenced by the factors which trigger the prophage induction leading to lysis. This could be linked to the hypothesis suggested by Weinbauer et al. (2003) suggesting that lysogeny is an adaptation to low host abundance and activity, although the authors did not make this link. 

It is interesting how much the ecology of viruses between Arctic and Antarctic waters differ, the former one with the lowest, the latter one with the highest rates of bacterial mortality by viral lysis. Although the underlying mechanisms for this discrepancy are still unclear, evidence points to there being more differences than similarities. Also how do these results compare with De Corte et al.’s (2012) findings, suggesting that host availability and temperature account for 69% of the variation in viral abundance in Arctic waters? (see Georgia’s blog “Factors influencing viral distribution and abundance along a latitudinal transect of the North Atlantic Ocean at different depths.”)

Cottrell, M. & Kirchman, D., 2012. Virus genes in Arctic marine bacteria identified by metagenomic analysis. Aquatic Microbial Ecology, 66(2), pp.107–116. Available at: http://www.int-res.com/abstracts/ame/v66/n2/p107-116/ 

Weinbauer, M., Brettar, I. & Höfle, M., 2003. Lysogeny and virus-induced mortality of bacterioplankton in surface, deep, and anoxic marine waters. Limnology and Oceanography, 48(4), pp.1457–1465. Available at: http://www.jstor.org/stable/10.2307/3597469