Carbon costs of isoprene emission in diatoms (0.0005%) is significantly small compared to that in higher plants (0.9%) Non-photochemical quenching can influence isoprene emission from phytoplankton Emissions increase under low CO2 exposure but do not cease in absence of CO2 Diatoms emit ~ 4.8 TgC yr-1 as isoprene to global atmosphere, far greater than current estimates for entire marine ecosystem