Abstract:

Volatile aldehydes are highly reactive compounds at the ocean–atmosphere interface, where they influence radical cycling, tropospheric ozone production, and secondary organic aerosol formation. However, their biological sources in the ocean remain poorly constrained, and photochemical production alone cannot explain persistent mismatches between field observations and model estimates. Marine picocyanobacteria, including Prochlorococcus and Synechococcus, are plausible sources of volatile aldehydes because some of their conserved metabolic pathways involve aldehydes as intermediates. Yet whether they actively release aldehydes into seawater, which compounds they produce, and which metabolic pathways generate the dominant products remain unresolved. In this study, we combined culture experiments, transcriptomics, and stable-isotope validation to investigate volatile aldehyde production by Prochlorococcus MED4 and Synechococcus WH8102. These cyanobacteria released a broad range of C1–C11 aldehydes, with formaldehyde and acetaldehyde predominating. Coculture experiments further showed that heterotrophic bacteria are important regulators of net aldehyde flux, consuming ~74% of labile aldehydes while also enhancing aldehyde production in specific pairings. Pathway analysis suggests that sarcosine oxidase and the 7-cyano-7-deazaguanine (preQ0) biosynthesis pathway are key sources of formaldehyde and acetaldehyde, respectively. Extrapolation based on experimentally constrained rates suggests that picocyanobacteria may account for 0.01–9.7% of observed seawater acetaldehyde across latitudes and could sustain a net sea-to-air aldehyde flux of 28–48 Tg C yr⁻¹. Together, these results identify marine picocyanobacteria as a previously overlooked biological source of volatile aldehydes and suggest that biological production should be considered in marine aldehyde budgets and ocean–atmosphere models.