The ver­min only teaze and pinch
Their foes supe­ri­or by an inch.
So, nat­u­ral­ists observe, a flea
Has small­er fleas that on him prey;
And these have small­er still to bite ’em,
And so pro­ceed ad infinitum.

Jonathan Swift, “On Poet­ry: A Rhapsody”

There’s so much to love about this sto­ry from Nature News. While sur­vey­ing genet­ic snip­pets from a hyper­saline lake in Antarc­ti­ca, an Aus­tralian research team found a virus that preys on oth­er virus­es. And they sort­ed out the ecol­o­gy of the lake well enough to fig­ure out that the virus infect­ed by the virus-infect­ing virus is a major preda­tor on the lake’s algae. With­out a par­a­site to con­trol the algal par­a­site, algal pop­u­la­tions would decline and the lake’s abil­i­ty to sus­tain life at all would decline.

An Aus­tralian research team found the virophage while sur­vey­ing the extreme­ly salty Organ­ic Lake in east­ern Antarc­ti­ca. While sequenc­ing the col­lec­tive genome of microbes liv­ing in the sur­face waters, they dis­cov­ered the virus, which they dubbed the Organ­ic Lake Virophage (OLV).

The OLV genome was iden­ti­fied nestling with­in the sequences of phy­cod­navirus­es — a group of giant virus­es that attack algae. Evi­dence of gene exchange, and pos­si­ble co-evo­lu­tion, between the two sug­gests that OLV preys on the phy­cod­navirus. Although OLV is the dom­i­nant virophage in the lake, the work sug­gests oth­ers might be present.

By killing phy­cod­navirus­es, the OLV might allow algae to thrive. Ricar­do Cav­ic­chi­oli, a micro­bi­ol­o­gist at the Uni­ver­si­ty of New South Wales in Syd­ney, Aus­tralia, and his col­leagues found that math­e­mat­i­cal mod­els of the Organ­ic Lake sys­tem that took account of the virophage’s toll on its host showed low­er algal mor­tal­i­ty and more blooms dur­ing the lake’s two ice-free sum­mer months.

Virophage means virus-eater, just as bac­te­rio­phage (a term you may recall from high school biol­o­gy class­es) means bac­teri­um-eater. This is the first virophage detect­ed in the wild, but oth­er virophages have been shown to reg­u­late algal pop­u­la­tions as well. Anoth­er paper com­ing out this week finds that a wide­spread zoo­plank­ton called Cafe­te­ria roen­ber­gen­sis are attacked by a virus, and that infec­tions of that virus are also reg­u­lat­ed by a virophage called Mavirus:

“The Mavirus is able to res­cue the infect­ed zoo­plank­ton — which, in a way, con­fers immu­ni­ty from infec­tion,” says Cur­tis Sut­tle, a marine micro­bi­ol­o­gist at the Uni­ver­si­ty of British Colum­bia in Van­cou­ver, Cana­da, and leader of the team that dis­cov­ered the Mavirus.

“We unknow­ing­ly had Mavirus in cul­ture with our Cafe­te­ria sys­tem since the ear­ly 1990s,” says Sut­tle. But the virophage was not iden­ti­fied until the Cafe­te­ria genome was sequenced. 

These virus­es could be detect­ed because their genes inte­grat­ed into the plank­ton or algal genome, and were detect­ed as researchers tried to match chunks of those genomes against oth­er known genes. Both new virophages matched sequences from the first virophage ever detect­ed, a virus called Sput­nik detect­ed in a French water-cool­ing tow­er. Sim­i­lar gene snip­pets have been found else­where but not traced to a func­tion­al virus, and the researchers think there are lots of virophages wait­ing to be dis­cov­ered. It’ll be inter­est­ing to see how strong an effect they have on oth­er host pop­u­la­tions, and to explore whether virophagy evolved more than once. The three virus-infect­ing virus­es known so far are all relat­ed, but the lat­est two were iden­ti­fied because we had Sput­nik’s gene sequences to com­pare new sam­ples against. 

Oth­er lin­eages of virophages will have to be detect­ed the way Sput­nik was, by observ­ing tiny virus­es being spun off from a virus fac­to­ry inside a host cell, and then fil­ter­ing out the virus­es and sequenc­ing their genes.

Image: From the first paper describ­ing a virophage, small par­ti­cles of the virophage Sput­nik inside the cap­sule of a mamavirus. Scale bar = 200 nm, Sput­nik par­ti­cles are rough­ly 50 nm across. From fig­ure 1 of La Sco­la, et. al (2008) “The virophage as a unique par­a­site of the giant mimivirus,” Nature 455:100–104.