Production of methyl halides by Prochlorococcus and Synechococcus

The ocean is the dominant source of atmospheric methyl iodide (CH(3)I). The mechanisms of CH(3)I production in the marine environment are poorly understood. A previous laboratory and field study suggested Prochlorococcus marinus, a ubiquitous marine cyanobacterium, is a globally significant biological producer of CH(3)I. In this study, CH(3)I concentrations were measured in cultures of P. marinus (high-light-adapted MED4 and low-light-adapted CCMP 1427) and the marine cyanobacterium Synechococcus. Cell-normalized production rates from P. marinus cultures (MED4) averaged 1.6 (+/-0.9) molecules of CH(3)I cell(-1) d(-1); these rates were 1000-fold lower than production rates reported for the previous study, which reported comparable concentrations of CH(3)I in cultures but cell concentrations about 1000-fold lower than measured in this study. Extrapolating CH(3)I production rates from the current study yields a global production rate of 0.6 Mmol yr(-1) which accounts for 0.03% of the estimated global CH(3)I production, suggesting P. marinus is not a globally significant source of CH(3)I.