Collective Microbubble Interactions Drive Sulfur Transformation in Seawater

Breaking waves, marine biological activity, and submarine geological processes continuously generate vast microbubble populations in seawater, yet the chemical consequences of bubble-bubble interactions remain poorly understood. Here, we address an apparent contradiction in saline interfacial chemistry: although high ionic strength suppresses oxidative responses at isolated bubble interfaces, dense microbubble swarms sustain transient photon emission and reactive oxygen species formation during bubbling. This contrast suggests that salinity weakens single-bubble interfacial reactivity, whereas collective bubble-bubble interactions create localized redox microenvironments through interfacial screening, conductivity effects, and repeated close interbubble encounters. The localized photon bursts observed during close bubble-bubble encounters, together with concurrent radical formation, are consistent with interbubble discharge-like events termed microlightning. Using dimethyl sulfide (DMS) as a representative marine organic sulfur compound, we further demonstrate that microbubble-mediated transformation redirects DMS toward inorganic sulfur species, including sulfate (SO42-) and reduced inorganic sulfur species. Electron spin resonance, oxidative fluorescence, spin-trapping mass spectrometry, and ion chromatography support a proposed branched radical-mediated network involving sulfur-centered activation, C-S fragmentation, oxygenation, and aqueous sulfur redox conversion. These findings reconcile salt-suppressed isolated-bubble reactivity with sustained swarm-level redox chemistry, identifying dense microbubble swarms as collective interfacial microreactors for aqueous sulfur transformation in seawater.