Covering about 70% of the earth’s surface, the ocean is home to a diverse array of organisms that significantly affect our planet’s ecosystem. These include microscopic phytoplankton, which play a major role as a carbon sink. This means marine phytoplankton are a crucial factor influencing climate. And it is virtually certain that giant viruses are one of the factors regulating phytoplankton. Unraveling the relationship between these giant viruses and phytoplankton is likely to make a substantial contribution to future measures to combat climate change.