Phytoplankton as a temperate marine source of brominated methanes

Bromoform (CHBr3) has an important role in transporting bromine from the ocean to the atmosphere, and released bromine catalyses ozone depletion. In temperate ocean waters, a number of studies have observed or presumed CHBr3 production. Here, we studied the ability of marine phytoplankton to produce CHBr3 in cultures of temperate phytoplankton. A temperate marine diatom and a cyanobacterium, Ditylum brightwellii CCMP 358 and Synechococcus sp. CCMP 1334, respectively, were incubated at 24 °C and the concentrations of brominated methanes in the cultured samples were determined using purge and trap gas chromatograph-mass spectrometry. The axenic cultures of the diatom exhibited a remarkable rate of CHBr3 production, ~ 200 nmol (g chlorophyll a)− 1 h− 1, which was several times higher than that for cold water diatoms reported previously. The cyanobacterium also produced CHBr3, with a production rate of ~ 1 nmol (g chlorophyll a)− 1 h− 1, shows that two diverse phytoplankton can produce CHBr3. Both CHBr2Cl and CHBrCl2 were also produced in the D. brightwellii culture, but only CHBr2Cl was produced in the culture of the Synechococcus sp. An incubation experiment with 13CHBr3 revealed that there was no 13CHBr3 degradation (or the formation of 13CHBr2Cl and 13CHBrCl2) in the cultures of these two phytoplankton, and these results suggested that both diatoms and cyanobacteria could produce chlorinated methanes. Our results suggest that brominated methanes such as CHBr3 are produced by temperate phytoplankton and that phytoplankton is a significant source of CHBr3 in the temperate open ocean.