Release of Reactive Bromine and Iodine from Diatoms and Its Possible Role in Halogen Transfer in Polar and Tropical Oceans

An in situ incubation assay measuring the bromination and iodination of phenol red was developed to detect the release of reactive bromine and iodine (primarily hypobromous acid [HOBr] and hypoiodous acid [HOI], respectively) from a putative extracellular bromoperoxidase of marine diatoms. Six of 11 species snowed significant release compared to controls. Polar species were particularly active, releasing 0.6–180 fmol HOBr cell h⁻¹ (0.04–1.8 µmol HOBr [mg total chlorophyll]⁻¹ hr⁻¹ at the seawater bromide concentration, 840 µmol L⁻¹) and 1.9–271 fmol HOI cell⁻¹ h⁻¹ (0.02– 2.7 µmol HOI [mg total chlorophyll]⁻¹ hr⁻¹, at 100 µmol L⁻¹ iodide). Porosira glacialis consistently showed the highest rates of release. Several temperate diatoms, including Achnanthes cf longipes, known to have a bromide-sensitive peroxidase involved in stalk formation, and warmwater species also showed the ability to release reactive bromine and iodine. This release was influenced by light, temperature, bromide (and iodide) concentration, H₂O₂ concentration, and pH. The rate of HOBr release by polar diatoms was much greater than bromoform emissions measured by others from laboratory cultures and seaice algae in the field. This indicates that most of the HOBr released may react with dissolved organic matter (DOM) to form nonvolatile bromine organics. Some fraction of diatom-produced HOBr and HOI may also form volatile Br₂ and I₂, which could transfer to the polar troposphere. The reaction of diatom-released reactive bromine and iodine with seawater DOM may represent the major mechanism in the formation of oceanic polybromo- and polyiodo-methanes.