Investigating Volatile Organic Compound Emissions from Ozonolysis of Phytoplankton Cultures

The ocean’s surface is covered by the sea-surface microlayer (SML), a distinct boundary layer that plays a critical role in mediating the air-sea exchange of atmospheric trace gases. The oxidation of unsaturated organic material enriched in the SML by ozone is a significant but poorly quantified abiotic mechanism leading to the emission of volatile organic compounds (VOCs) into the marine boundary layer. The properties of these VOCs make them efficient precursors for secondary organic aerosol formation which can alter the oxidative capacity of the atmosphere. 

In this laboratory study, axenic cultures of the model marine diatom Phaeodactylum tricornutum and its coculture with Yoonia bacteria were selected as biologically and chemically relevant proxies for the SML. Ozone-enriched air was passed over the culture medium in a heterogenous flow reactor, and the emitted gas-phase VOCs were monitored using high resolution proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS). Experiments were conducted on the cultures in both their exponential and stationary growth phases with nonanal, the C₅H₈H⁺ peak, and the C₆H₁₀H⁺ peak being identified as major product ions. Ozonolysis-mediated abiotic VOC emissions were greater from cultures in exponential phase compared to stationary phase. Additionally, emissions from the P. tricornutum axenic monoculture were higher than from the P. tricornutum-Yoonia coculture indicating consumption of precursor compounds by the bacteria. The addition of iodide, a well-known reactant with ozone, to axenic P. tricornutum cultures in the exponential phase was associated with a reduction in the VOC emissions. This research provides a deeper insight into the interactions between iodide and organics during ozone uptake to the SML, and the impact of these competing processes on marine atmospheric chemistry.