Methyl halide production from cyanobacteria cultures: Prochlorococcus marinus as a source of marine methyl iodide

The ocean is the dominant source of methyl iodide (CH3I) to the atmosphere which plays an important role in stratospheric ozone destruction. Marine CH3I production mechanisms are not well explained. A previous laboratory and field study suggested Prochlorococcus marinus, an ubiquitous marine cyanobacterium, is a globally significant biological producer of CH3I (Smythe-Wright et al., 2006, Global Biogeochem. Cycles, 20). In my study, methyl halide (CH 3I, CH3Br, CH3C1) concentrations were measured from cultures of P. marinus (MED4) and Synechococcus. Cell-normalized production rates from P. marinus cultures ranged from 2 to 5 molecules of CH3I cell-1d -1; these rates were 1000 fold lower than production rates reported for the previous study. Extrapolating CH3I production rates from the current study yields a global production rate of 0.7 Gg yr-1 which accounts for 0.6% of the estimated global CH3I production, suggesting P. marinus is not a globally significant source of CH3I.