Subsurface Superoxide Spans the Baltic Sea

• Published in: Journal of geophysical research. Oceans 2024-10, Vol.129 (10), p.n/a
• Main Authors: Taenzer, Lina, Pardis, William, Wankel, Scott D., Kolbe, Martin, Voss, Maren, Schulz‐Vogt, Heide, Burmeister, Christian, Hardisty, Dalton S., Hansel, Colleen M.
• Format: Article
• Language: English
• Subjects: Anthropogenic factors; Baltic Sea; Biogeochemical cycle; Biogeochemical cycles; biogeochemistry; Biological activity; Carbon cycle; Chemical activity; Chemistry; Dissolved oxygen; geophysics; Human influences; Inflow; Metal concentrations; Mixed layer; Oceans; Organic matter; Oxygen; Reactive oxygen species; Seawater; solar radiation; Sunlight; Superoxide; Trace metals; Water circulation; Water column
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2024JC021438

Superoxide is a reactive oxygen species that is influential in the redox chemistry of a wide range of biological processes and environmental cycles. Using a novel in situ sensor we report the first water column profiles of superoxide in the Baltic Sea, at concentrations higher than previously observed in other oceans. Our data revealed consistent peaks of superoxide (2.0–15.1 nM) in dark waters just below the mixed layer. The oxic waters, low metal concentrations, and lack of sunlight imply that the peak is likely of biological origin. Several profiles displayed a concomitant dip in dissolved oxygen mirroring this superoxide peak, strongly suggesting a link between the two features. The magnitude and distribution of superoxide observed warrants re‐evaluation of the most relevant sources and controls of superoxide in seawater. Locally, these high concentrations of superoxide may create environments conducive to reactions with trace metals and organic matter and present an overlooked sink of oxygen in the Baltic Sea. Plain Language Summary The combination of unusual inflow activity and topographic conditions with strong anthropogenic influence in the Baltic Sea, offer a unique environment to study redox chemistry. To gain further insight into the biogeochemical cycles in this area we made the first measurements of superoxide, a reactive oxygen species that plays significant roles in organismal health, as well as the chemistry of metals and carbon in seawater. We made direct measurements of superoxide in seawater, leading to continuous profiles between 2 and 80 m at 7 different locations in the Baltic Sea. Our results revealed the presence of widespread superoxide maxima that consistently appeared in dark waters at depths between 23 and 35 m. In some instances, the superoxide peaks are mirrored by dips in oxygen concentrations. This work demonstrates that light‐independent processes were the dominant source of superoxide in the Baltic Sea in September, and point to an association between heterotrophic activity, oxygen loss, and superoxide production. The high levels of superoxide observed in this study suggest that a reevaluation of the abundances of this compound in seawater is necessary, and underline the advantages of in situ techniques for capturing total superoxide concentrations. Key Points Superoxide maxima consistently appear in dark waters below the mixed layer Corresponding dips in oxygen at depths of superoxide maxima suggest link between heterotro