Production of methyl halides by Prochlorococcus and Synechococcus

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.