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.