SAR11 viruses in the ocean- could these affect ecology?

Abundant SAR11 viruses in the ocean

SAR11 bacteria thrive in the oceans despite the high abundance of viruses. These bacteria are the most abundant microbes in all of the world's oceans, and as such are an important factor in carbon cycling.  Several reports have proposed that the dominance of the SAR11 clade in the oceans is due to resistance to Bacteriophages. But alternatively, the evolution of high surface-to-volume ratios coupled with minimal genomes with high-affinity transporters enables an unusually efficient metabolism for oxidizing dissolved organic matter in the world’s oceans that could support vast population sizes despite being vulnerable to phages.

The ‘Kill-the-Winner’ hypothesis is a theory for understanding the impact of top-down control in microbial communities. The basis is that rising population densities and cell metabolic activity expose cells to more viral predation by increasing host–phage encounter rates. Evidence supporting this hypothesis has already been demonstrated. The global dominance of SAR11 in marine communities has led to speculative theories that SAR11 avoids phage predation either by having a small cell size or by growing so slowly as to make infection inefficient. But the surface area and volume of SAR11 cells are comparable to those of Prochlorococcus spp, where multiple phages have been isolated. The contribution of SAR11 to bacterial heterotrophic production in the open ocean is greater than their relative abundance which suggests that SAR11 does not avoid viral predation by growing slowly.

Zhao et al (2013) reported the isolation of diverse SAR11 viruses belonging to two virus families in culture. They proposed the name ‘pelagiphage’. The pelagiphage genomes were highly represented in marine viral metagenomes, which indicated their importance in nature. One of the new phages, “HTVC010P”, represents a new virus subfamily more abundant than any seen previously and may be the most abundant virus subfamily in the whole biosphere. This discovery disproves the theory that SAR11 cells are immune to viral predation and is consistent with the alternative explanation that the success of this highly abundant microbial clade is the result of successfully evolved adaptation to resource competition.

Zhao et al (2013) propose that the dominance of SAR11 in marine bacterial communities is a result of their superior competitiveness for nutrient uptake. High host abundance can provide protection against population decimation by viral predation.  For example high population densities increase encounter rates with infective phage particles, but they also increase recombination. Recombination enables genomic elements to pass in populations more rapidly than is possible by a sexual reproduction and thus offers an advantage to the spread of immunity. Furthermore, recombination rates in SAR11 populations were reported previously to be among the highest on record. This is what led Zhao and co to propose that high recombination rates in SAR11 populations allow for rapid adaptation to novel phage phenotypes. This mechanism for co-evolution and co-existence of pelagiphage and SAR11 is archetypal of the Red Queen Hypothesis,  an evolutionary theory which where organisms must constantly adapt, not just to gain reproductive advantage, but also simply to survive while pitted against ever-evolving opposing organisms in an ever-changing environment

I think this study is important for understanding plankton ecology because the authors emphasise the potentially important role of top-down mechanisms in predation, thus determining the size of SAR11 populations and their concomitant role in biogeochemical cycling. In summary this study describes the isolation and subsequent culture of several viruses that infect SAR11 from water samples taken on the Oregon coast and off Bermuda. Metagenomic analysis reveals that these 'pelagiphages' are abundant in the Pacific Ocean. These findings argue against a recent hypothesis developed to explain the success of SAR11 — that they might be immune to viral predation. Rather, the authors suggest, SAR11's dominance may reflect successfully evolved adaptation to resource competition.

http://www.nature.com/nature/journal/v494/n7437/abs/nature11921.html