Antarctic lakes are an example of a microbially dominated ecosystem; they support a low number of metazoans. In truncated food webs such as these viruses are expected to have an increased role in the microbial loop.
Because of the low complexity of Antarctic lake systems it is possible to do whole community-based molecular analyses, with the idea that the role of viruses in microbial dynamics can be more understood, it is this that is the paper’s aim.
This paper reports the analysis of surface water from Organic Lake, highlighting the presence of a previously described virophage, a small eukaryotic virus that requires a helper Acanthamoeba polyphaga mimivirus (APMV) to replicate.
A complete genome was constructed for the Organic Lake virophage (OLV) (only the second described after Sputnik), and near complete genomes of its probable helper Organic Lake pychodnaviruses (OLPVs).
Water and microbial biomass samples were collected from Organic Lake, Antarctica, in December 2006. Organic Lake is hyper-saline, shallow (7m) and meromictic (stratified), with varying surface temperatures (-14 to +15 C). Viral abundance has been reported to increase with trophic status and with salinity in Antarctic lakes.
Using PCR and sequencing it is known that the virophage genome is circular (like the genome of Sputnik). With the use of TEM virus-like particles were found that were similar in size and morphology to Sputnik. The large number of homologs indicates that Sputnik and the OLV also have physiological similarities. However, the OLV is different from Sputnik because proteins share 27-42% amino acid identity.
There is evidence for gene exchange and possible coevolution between virophages and pychodnaviruses (PVs). From comparison between OLV and OLPV a 7,408-bp region was identified in OLV encoding six proteins with identity (32-65%) to sequences in OLPV. Genes from this region of OLV can be mapped to OLPV.
To test how OLV affects OLPV and host population dynamics, OLV was modelled as an extra predator in a Lotka-Volterra simulation, with the model revealing that the virophage stimulates the flux of secondary production through the microbial loop by reducing overall mortality of the host algal cell after a bloom and also by increasing the frequency of blooms during the summer light periods.
Two years after the initial samples were taken, in November and December 2008, it was found that sequences with 99% amino acid identity to OLV persisted. Also, sequences with lower identity (25-90%) were detected, signifying that OLVs are diverse.
Surface samples from a nearby lake (Ace Lake), had a large number of sequences matching OLV and PVs. In further water samples including tropical lakes, an estuary and an ocean upwelling site virophage signatures were also discovered.
This paper specifies that many virophages exist, and that they play an important, previously unrecognised role in regulating the host-virus interactions, and therefore influencing ecosystem function in aquatic environments.
Although the paper mainly focuses on Antarctic lakes, virophages have also been found in marine environments. It is possible that with further research more virophages will be found in more marine environments. I personally found this paper a hard read, but with perseverance I found it an interesting read none-the-less. I believe this paper will be the first of many more discovering virophages.
A review of;
Yau S, Lauro FM, DeMaere MZ, Brown MV, Thomas T, Raftery MJ, Andrews-Pfannkock C, Lewis M, Hoffman JM, Gibson JA and Cavicchioli R (2011) ‘Virophage control of Antarctic algal host-virus dynamics' PNAS. 108(15), 6163-6168.
1 comment:
This is a really hot area of research and we will be exploring in the seminar. Sputnik was the first such virophage, infecting Mimivirus. A virophage has recently been found associated with the Cafeteria roenbergensis virus. since C. roenbergensis is one of the most abundant grazing protists, the concept discovered in Organic Lake shows that there is another layer of complexity controlling host populations and influencing nutrient flow.
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