Production of methyl halides by Prochlorococcus and Synechococcus

[1] The ocean is the dominant source of atmospheric methyl iodide (CH3I). The mechanisms of CH3I 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 CH3I. In this study, CH3I 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 CH3I cell−1 d−1; these rates were 1000-fold lower than production rates reported for the previous study, which reported comparable concentrations of CH3I in cultures but cell concentrations about 1000-fold lower than measured in this study. Extrapolating CH3I 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 CH3I production, suggesting P. marinus is not a globally significant source of CH3I.