What really causes the termination of Emiliania huxleyi blooms?

The article I chose to blog about this week follows on from Professor Willie Wilson’s lecture, where we learnt about Coccolithovirus’:

Emiliania huxleyi has been recognised as a globally important coccilithophore due to its huge contributions to the Carbon cycle, through its ability to calcify. The double stranded DNA virus family Phycodnaviridae (classified into the genus Coccolithovirus) have been seen to act as mortality agents for E. huxleyi, which forms blooms covering large expanses of water (Highfield et al., 2014).

E. huxleyi-infecting viruses (EhVs) have been implicated in the demise of these E. huxleyi blooms - several of these Coccolithovirus’ have been seen to have similar propagation strategies, host ranges and genomic sequences. Few EhVs, including EhV86, have unique properties – for example, EhV86 is surrounded by a lipid envelope and enters host cells via an endocytotic or lipid fusion mechanism (Mackinder et al., 2009).

The authors state that one viral genotype is found to dominate, and this is typically the genotype which goes on to cause the termination of the bloom. They based their paper on a previous study done by Martinez Martinez et al, 2007, where two separate mesoscosm experiments were conducted in a Norwegian Fjord in 2000 and 2003, during E. huxleyi blooms. In this study Denaturing Gradient Gel Electrophoresis (DGGE) showed that the dominant EhVs during these blooms were the same in both years.

Highfield et al., 2014, looked at both the horizontal and vertical EhV population within a bloom of E. huxleyi in the Western English Channel during July 2006. They reported, for the first time, that there was no dominant EhV genotype detected and so the termination of the bloom may have been due to a combination of different EhVs.  During this study, two stations were used for sampling. Station 2 had a more defined thermocline, as well as a nutricline and halocline present; whilst station 1 had more constant environmental gradients. Flow cytometry was used to examine the E. huxleyi populations, and it was seen that station 1 (at the edge of the bloom) had much lower coccolithophore concentrations than station 2 (in the centre of the bloom). Coccolithovirus concentrations reflected those of the coccolithophores. The two stations were sampled roughly 10 hours apart, so temporal variations may have had an effect on the varying EhV populations observed.

The authors conclude that the external environment may be an important factor in the infectivity/infection cycle and persistence of EhVs. This may ultimately influence whether the bloom is terminated by viruses or not – however, as a higher number of EhV genotypes were found in this study (64 genotypes) it is likely that there were a number of different virus host interactions at play. I think further studies are definitely needed to look at how environmental variations can effect these Coccolithovirus’, and whether this truly does affect the cause of termination of E. huxleyi blooms.

Studied paper

Highfield, A., Evans, C., Walne, A., Miller, P., Schroeder, D. (2014). How many Coccolithovirus genotypes does it take to terminate an Emiliania huxleyi bloom? Virology (466), 138-145.

References

Martinez Martinez, J., Schroeder, D., Larsen, A., Bratbak, G., Wilson., W. (2007). Molecular dynamics of Emiliania huxleyi and Cooccurring Viruses during Two Separate Mesocosm Studies. Applied and Environmental Microbiology (73), 554-562.

Mackinder, L., Worthy, C., Biggi, G., Hall, M., Ryan, K., Varsani, A., Harper, G., Wilson, W., Brownlee, C., Schroeder, D. (2009). A unicellular algal virus, Emiliania huxleyi virus 86, exploits an animal-like infection strategy. Journal of General Virology (90), 2306–2316.