How hot is too hot for life to survive? Ever since microbes were discovered squirming around in hydrothermal springs several decades ago, the limit of heat-loving (thermophilic) organisms has been a moving target. The current record-holder is “strain 121,” an archaeon isolated from the Mothra hydrothermal vent deep beneath the surface of the northeast Pacific Ocean; suitably enough, it can grow at a blistering temperature of 121 °C.

There are some physical limitations of biological activi