We study the effect of ocean biology on tropical surface temperatures using a simplified coupled atmosphere-ocean model. It is shown that moderate phytoplankton blooms, occuring e.g. during La Niña conditions, lead to a vertical redistribution of heat in the surface layers and an associated surface layer warming of about 20 W/m². The positive air-sea coupling in the eastern equatorial Pacific plays an important role in amplyfying this signal, thereby damping La Niña conditions. This temperature-regulating feedback acts as a biological thermostat within the surface ocean and influences also the amplitude and asymmetry of the El Niño-Southern Oscillation.