Production of volatile organic iodine compounds by the marine cyanobacterium Calothrix parasitica under different light intensities

Abstract

Volatile organic iodine compounds (VOICs) that exist in the marine environment, such as diiodomethane (CH2I2), chloroiodomethane (CH2ClI), and bromoiodomethane (CH2BrI), are known to transport iodine from the ocean to the atmosphere. These VOICs play important roles in various atmospheric chemistry processes, such as ozone decomposition. So far, marine phytoplankton such as diatoms and chlorarachniophytes have been reported to produce these VOICs in the culture experiments. In this study, we examined the production of VOICs by the marine cyanobacterium Calothrix parasitica NIES-267 (axenic) cultured at 24 °C for several weeks. The effect of light intensity on the production of VOICs was also studied by irradiating the culture medium of C. parasitica with light at intensities of 60, 90, and 120 μmol photons m−2 s−1. In addition, incubation experiments with 13CH2I2 added to the culture medium were performed to evaluate the decrease of 13CH2I2 due to photolysis during incubation. The concentrations of VOICs in the culture medium were measured using a purge-and-trap gas chromatograph-mass spectrometer. The concentrations of VOICs in the culture medium increased during the exponential or stationary phase at each light intensity. During the exponential phase, the chlorophyll a-normalized production rates of CH2ClI and CH2BrI increased with increasing light intensity from 60 to 120 μmol photons m−2 s−1, while the chlorophyll a-normalized maximum production rate of CH2I2 was found to be 28 pmol (μg chlorophyll a) −1 d−1 at 90 μmol photons m−2 s−1. During the incubation period, 13CH2I2 was photodegraded by about 17% at 120 μmol photons m−2 s−1. The relative production rates were CH2I2 > CH2ClI > CH2BrI for all light conditions, and the relative amount of CH2I2 tended to decrease under high light intensity conditions (120 μmol photons m−2 s−1). These results suggest that the cyanobacterium C. parasitica could be one of the significant sources of VOICs such as CH2I2 in the marine environment, and that light intensity would be an important factor in estimating VOICs emissions from marine cyanobacteria.