Global consensus estimates of marine isoprene emission (similar to 1 TgC yr(-1)) is significantly smaller than terrestrial isoprene emission (similar to 500 TgC yr(-1)), and the reasons are unclear. With the premise that isoprene emission is metabolically linked to photosynthesis in phytoplankton as in plants, we measured isoprene emission and photosynthesis to changing light levels in high-density cultures of two diatom species, Phaeodactylum tricornutum and Chaetoceros calcitrans. Isoprene emission increased and did not saturate with increasing light levels in C. calcitrans under laboratory conditions, whereas isoprene emission of P. tricornutum decreased at high-light levels with an associated increase in non-photochemical quenching. When tested for CO2 response, emission from P. tricornutum increased under low CO2 (similar to higher plants) but did not cease in absence of dissolved CO2, plausibly due to CO2 concentrating mechanisms. Isoprene emission under full-sun light in outdoor cultures of C. calcitrans (500 +/- 200 fmol mu g Chl(-1) h(-1)) was significantly greater than maximum emission in laboratory cultures (100 +/- 30 fmol mu g Chl(-1) h(-1)). We show that photosynthetic capacity of diatoms can be equal to or greater than that of higher plants. However, the carbon cost of isoprene emission in diatoms (
