Viral Infection? No Hux Given.

Bloom forming algae are important in global carbon and sulphur cycles due to the production of dimethylated sulphur species (DSS) such as dimethylsulphoniopropionate (DMSP) and dimethylsulphide (DMS). These compounds eventually form cloud condensation nuclei which, as the name suggests, provide the basis for cloud formation, and can significantly alter the earth’s radiation budget. Emiliania huxleyi forms large, dense blooms annually and these blooms produce a huge amount of DSS. Generally, the blooms end as a result of host-specific viral infection, in this case by E. huxleyi Virus (EhV).

The Intergovernmental Panel on Climate Change predicts that by 2100 sea surface temperature will have increased by 1.5-3_­^oC, yet the effect of this increase in temperature on host-virus interactions had been unexplored until Kendrick et al remedied this in 2014. In the study they inoculated bottles of growth media with axenic cultures of E. huxleyi CCMP 374, and kept half at 18^oC and half at 21^oC, to represent the predicted increased in sea surface temperature. Cultures were infected with EhV86 when they had attained a cell density comparable to that of a typical bloom, and were thereafter sampled daily to measure various parameters including culture density, viral abundance, burst size, and total DMSP (DMSP_t). A series of experiments was also carried out using six different E. huxleyi strains to see if the temperature effect was shared.

Kendrick et al ensured that the increase in temperature was affecting the host’s susceptibility to infection, rather than directly affecting the infectivity of the virus by incubating EhV86 isolates at 18^oC and 21^oC and then using these to infect E. huxleyi 374 growing at 18^oC. Burst size (number of viral progeny per lysed host cell) was significantly higher in both treatments than in the controls, indicating that exposing EhVs to increased temperature alone does not affect infectivity.

In the 18^oC treatment, successful viral adsorption is seen – where over 24 hours virus density initially decreases as the virions contact and cross the host plasma membrane, then increases at around 4 hours post-infection as viral progeny are released. In the 21^oC treatment this pattern in virus density is not seen, and this is explained due to changes in the cell membrane which prevent the virions from binding or crossing the membrane. The authors hypothesise that temperature induced changes in cell membrane lipids may be responsible for the viral resistance. Temperature induced resistance did not affect DSS production, but if the resistance lengthens the bloom duration this could possibly affect the total amount of DSS produced which could in turn impact global sulphur cycles.

This is a fascinating study which makes it clear that temperature plays an important role in host-virus interactions and shows how they may change as our global climate changes. As in any lab based experiment, the results may not accurately reflect real life as it lacks microbial ecosystem complexity. Further research into the changes in lipids at different temperatures, and the difference in lipid content or concentration between strains of E. huxleyi, may confirm the mechanism of the viral resistance.

Kendrick B.J., DiTullio G.R., Cyronak T.J., Fulton J.M., Van Mooy B.A.S., Bidle K.D.(2014) Temperature-Induced Viral Resistance in Emiliania huxleyi (Prymnesiophyceae). PLoS ONE 9(11) e112134

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0112134