Prof La Scola and his colleagues were surprised to spot a smaller type of virus attached to the virus-making factory inside infected cells. The new virus - Sputnik - was unable to infect cells by itself but seemed to hijack the larger to achieve its infectious aims.

By regulating the growth and death of plankton, giant viruses - and satellite viruses such as Sputnik - could be a major influence on ocean nutrient cycles and climate.

"These viruses could be major players in global systems," Nature is told by Prof Curtis Suttle, an expert in marine viruses at the University of British Columbia in Vancouver.

*cough*