Relationship between isoprene emission and photosynthesis in diatoms, and its implications for global marine isoprene estimates

Global consensus estimates of marine isoprene emission (~ 1 TgC yr^− 1) is significantly smaller than terrestrial isoprene emission (~ 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 μg Chl^− 1 h^− 1) was significantly greater than maximum emission in laboratory cultures (100 ± 30 fmol μg 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 (≤ 0.0005% of photosynthesis) is significantly smaller than that in plants (0.9%). Based on these findings, we calculate that isoprene emission from diatoms alone can contribute ~ 4.8 TgC yr^− 1 to the atmosphere, which is much greater than current modelled projections for annual isoprene emission from the entire marine ecosystem.