Iodomethane production by two important marine cyanobacteria: Prochlorococcus marinus (CCMP 2389) and Synechococcus sp. (CCMP 2370)

Iodomethane (CH3I) production by Prochlorococcus marinus (CCMP 2389) and Synechococcus sp. (CCMP 2370) was investigated using laboratory cultures. In contrast to Synechococcus, where no trace gas production was observed, P. marinus cells produced CH3I. The rate of CH3I production by P. marinus was found to vary depending on the physiological state of the cells as defined by photosynthetic health (Fv/Fm) measurements, SYTOX Green staining (which evaluates membrane permeability) and calculated in vivo fluorescence per cell (a proxy of pigment content per cell). The highest total cell normalised production rates (7189 [range 5225–11,517] molecules CH3I cellTotal day−1) were observed when Fv/Fm was ≤ 0.3 and ≥ 75% of the cells were SYTOX Green labelled. Relatively lower rates of CH3I production (146 [103–267] molecules CH3I cellTotal day− 1) were observed when Fv/Fm was ≥ 0.5 and ≤ 20% of the P. marinus cells were SYTOX Green labelled. Our results suggest that discrepancies in the CH3I production rates by Prochlorococcus observed between previous studies could be due to differing cell physiology. Calculations are presented which suggest that Prochlorococcus could contribute significantly to CH3I production in the eastern tropical Atlantic Ocean. This study highlights the importance of considering cell physiological state when determining trace gas production rates in laboratory culture and extrapolating them to the natural environment.

Research highlights
► Prochlorococcus was found to produce CH3I but no other halocarbons or alkyl nitrates.
► CH3I production by Prochlorococcus was found to increase as cell physiology declines.
► Calculations suggest that Prochlorococcus could be a major CH3I source in the tropical Atlantic.