CH3I Production Research Articles

Influence Factors on Photochemical Production of Methyl Iodide in Seawater

Methyl iodide (CH3I) is an important trace greenhouse gas in the atmosphere and an ozone-depleting substance. The influence of different environmental factors, such as the duration of illumination; the strength of illumination; the concentrations of humic acid, ferric ion (Fe3+), and iodide ion (I−); and pH, on the photochemical production of CH3I in artificial seawater (ASW) were tested by simulated solar irradiation. In addition, Yangtze River Estuary waters from inshore to offshore were used to explore the relationship between the photochemical production of CH3I and different sources of dissolved organic matter (DOM) in natural sea-water (NSW). The results revealed that higher concentrations of humic acid and I−, as well as higher strengths of illumination and longer illumination durations, promoted the photochemical formation of CH3I in ASW. The addition of Fe3+ accelerated the photochemical production of CH3I, but high concentrations of Fe3+ inhibited the formation of CH3I. Experiments on NSW obtained from the Yangtze River estuary spiked when concentrations of DOM were high, confirming that DOM plays an important role in facilitating the photochemical production of CH3I within the Yangtze River Estuary. The photochemical production of CH3I in the seawater was significantly higher under light conditions relative to dark conditions, indicating that illumination accelerated the production of CH3I.

A simple decontamination procedure for unintended iodide impurity during [11C]methionine production

A solid-phase extraction cartridge method is a widely used carbon-11 labeling method for obtaining [11C]methionine. Although the process is convenient and efficient, unintended impurity sometimes observed in the final product owing to contamination by volatile iodide generated during [11C]CH3I production. Herein, we suggest a simple method to eliminate impurities contained in [11C]methionine solution using an anion-exchange resin cartridge.

Methyl halide production by cultures of marine thraustochytrids, Aurantiochytrium sp., Botryochytrium radiatum, and Schizochytrium sp.

The oceans are an important source of the methyl halides released into the atmosphere and impacting the stratospheric ozone layer. Marine bacteria such as Erythrobacter and Pseudomonas are known to be sources of methyl halides in marine environments, but the production of methyl halides by thraustochytrids, the other decomposers in the marine ecosystem, have not been studied. Here, we report the productions of three methyl halides, CH3Cl, CH3Br, and CH3I, in cultures of thraustochytrids. Thraustochytrids were incubated at 20 °C, 25 °C, and 30 °C, and the concentrations of methyl halides in the gas phase above these cultures were determined using dynamic headspace gas chromatography–mass spectrometry. Growth of thraustochytrids was monitored by measuring optical density at 600 nm during the culture period. Cell density was also measured for some samples of thraustochytrids by counting with a plankton-counting chamber during the culture period. Three of the four tested strains of marine thraustochytrids were found to produce methyl halides in culture: Botryochytrium radiatum produced CH3Cl and CH3I while Aurantiochytrium sp. and Schizochytrium sp. produced CH3Br. Increases of CH3Cl, CH3Br, and CH3I concentrations were observed during the exponential-growth phase in all cultures of the three thraustochytrids, except for the 30 °C culture of B. radiatum. The maximum concentration for CH3Cl production by a thraustochytrid (B. radiatum) was increased as the culture temperature increased from 20 °C to 30 °C; the maximum concentrations of CH3Cl in the cultures incubated at 20 °C, 25 °C, and 30 °C were about 6000, 9600 and 14,000 pmol L−1, respectively. However, the maximum concentrations for the CH3I production of thraustochytrid (B. radiatum) decreased when the temperature was increased from 20 °C to 30 °C. To the best of our knowledge, this is the first report of methyl halide production by heterotrophic thraustochytrids. Our results suggest that thraustochytrids may be a source of methyl halides in marine environments.

A 3-year time series of volatile organic iodocarbons in Bedford Basin, Nova Scotia: a northwestern Atlantic fjord

Abstract. We report weekly observations of volatile organic iodocarbons (CH3I, CH2ClI and CH2I2) over the time period May 2015 to December 2017 from four depths in Bedford Basin, a coastal fjord (70 m deep) on the Atlantic coast of Canada. The fjord is subject to wintertime mixing, seasonal stratification and bloom dynamics, subsurface oxygen depletion, local input of freshwater, and occasional intrusions of higher-density water from the adjacent continental shelf. Near-surface concentrations showed strong seasonal and sub-seasonal variability, which is compared with other coastal time series. The vertical variation of CH2I2 and CH2ClI within the upper 10 m is consistent with rapid photolysis of CH2I2. Average annual sea-to-air fluxes (46.7 nmol m−2 day−1) of total volatile organic iodine were similar to those observed in other coastal and shelf time series, and polyiodinated compounds contributed 80 % of the total flux. Fluxes were subject to strong interannual variability (a factor of 2) mainly due to wind speed variability. Near-surface net production of CH3I averaged 1 pmol L−1 day−1 and was similar to rates in the English Channel but an order of magnitude higher than in shallow waters of the Kiel Fjord, Germany, possibly due to higher microbial degradation in the latter. The near-bottom (60 m) time series showed evidence of CH3I production associated with organic matter degradation and a possible “switch” from the production of CH3I via an alkylation pathway to the production of CH2I2 by a haloform-type reaction. Near-bottom CH3I production varied strongly between years but was generally ca. 20 times lower than near-surface production.

A time series of incubation experiments to examine the production and loss of CH3I in surface seawater

AbstractIn order to investigate production pathways of methyl iodide and controls on emissions from the surface ocean, a set of repeated in vitro incubation experiments were performed over an annual cycle in the context of a time series of in situ measurements in Kiel Fjord (54.3°N, 10.1°E). The incubation experiments revealed a diurnal variation of methyl iodide in samples exposed to natural light, with maxima during day time and losses during night hours. The amplitude of the daily accumulation varied seasonally and was not affected by filtration (0.2 µm), consistent with a photochemical pathway for CH3I production. The methyl iodide loss rate at nighttime correlates with the concentration accumulated during daytime suggesting a first‐order loss mechanism (R2 = 0.29, p << 0.01). Daily (24 h) net production (Pnet) was similar in magnitude between in vitro and in situ mass balances. However, the estimated gross production (Pgross) of methyl iodide ranged from −0.07 to 2.24 pmol L−1 d−1 and was up to 5 times higher in summer than Pnet calculated from the in situ study. The large excess of Pgross over Pnet in summer revealed by the incubation experiments is a consequence of large losses of CH3I by as‐yet uncharacterized processes (e.g., biological degradation or chemical pathways other than Cl− substitution).

Methyl iodide production in the open ocean

Abstract. Production pathways of the prominent volatile organic halogen compound methyl iodide (CH3I) are not fully understood. Based on observations, production of CH3I via photochemical degradation of organic material or via phytoplankton production has been proposed. Additional insights could not be gained from correlations between observed biological and environmental variables or from biogeochemical modeling to identify unambiguously the source of methyl iodide. In this study, we aim to address this question of source mechanisms with a three-dimensional global ocean general circulation model including biogeochemistry (MPIOM–HAMOCC (MPIOM – Max Planck Institute Ocean Model HAMOCC – HAMburg Ocean Carbon Cycle model)) by carrying out a series of sensitivity experiments. The simulated fields are compared with a newly available global data set. Simulated distribution patterns and emissions of CH3I differ largely for the two different production pathways. The evaluation of our model results with observations shows that, on the global scale, observed surface concentrations of CH3I can be best explained by the photochemical production pathway. Our results further emphasize that correlations between CH3I and abiotic or biotic factors do not necessarily provide meaningful insights concerning the source of origin. Overall, we find a net global annual CH3I air–sea flux that ranges between 70 and 260 Gg yr−1. On the global scale, the ocean acts as a net source of methyl iodide for the atmosphere, though in some regions in boreal winter, fluxes are of the opposite direction (from the atmosphere to the ocean).

Seasonal variability of methyl iodide in the Kiel Fjord

From October 2008 to November 2010, CH3I concentrations were measured in the Kiel Fjord together with potentially related biogeochemical and physical parameters. A repeating seasonal cycle of CH3I was observed with highest concentrations in summer (ca. 8.3 pmol L−1; June and July) and lowest concentrations in winter (ca. 1.5 pmol L−1; December to February). A strong positive correlation at zero lag between [CH3I] and solar radiation (R2 = 0.93) was observed, whereas correlations with other variables (SST, Chlorophyll a) were weaker, and they lagged CH3I by ca. 1 month. These results appear consistent with the hypothesis that SSR is the primary forcing of CH3I production in surface seawater, possibly through a photochemical pathway. A mass balance of the monthly averaged data was used to infer mean rates of daily net production (Pnet) and losses for CH3I over the year. The sea-to-air flux of CH3I in the Kiel Fjord averaged 3.1 nmol m−2 d−1, the mean chemical loss rate was 0.047 pmol L−1 d−1, and Pnet varied systematically from winter to summer (from 0 to 0.6 pmol L−1 d−1). Pnet was correlated at zero lag with SST, SSR, and Chla (R2 = 0.55, 0.67, and 0.73, respectively, p << 0.01). The lagged cross-correlation analysis indicated that SSR led Pnet by 1 month, whereas the strongest cross correlations with Chla were at lags of 0 to +1 month, and SST lagged Pnet by 1 month. The broad seasonal peak of Pnet makes it difficult to determine the key factor controlling CH3I net production using in situ concentration data alone.

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

Iodomethane (CH 3I) 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 CH 3I. The rate of CH 3I production by P. marinus was found to vary depending on the physiological state of the cells as defined by photosynthetic health ( F v/ F m) 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 CH 3I cell Total day −1) were observed when F v/ F m was ≤ 0.3 and ≥ 75% of the cells were SYTOX Green labelled. Relatively lower rates of CH 3I production (146 [103–267] molecules CH 3I cell Total day − 1 ) were observed when F v/ F m was ≥ 0.5 and ≤ 20% of the P. marinus cells were SYTOX Green labelled. Our results suggest that discrepancies in the CH 3I 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 CH 3I 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.

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.

Methyl iodide in the NW Atlantic: Spatial and seasonal variation

While the global ocean is an important source of atmospheric methyl iodide (CH3I), the major producers of CH3I within the ocean remain unclear. During a seasonal study in the NW Atlantic, the relationship between CH3I and some characteristic phytoplankton pigments was examined in order to identify possible phytoplankton producers of CH3I. Although no characteristic pigments exhibited a strong positive correlation with CH3I, in the surface mixed layer, there was a weak correlation (R = 0.35, n = 70, p = 0.003) between the concentrations of CH3I and zeaxanthin, a pigment characteristic of cyanobacteria in the open ocean. In this study, a moderate correlation was observed between the surface mixed layer CH3I concentration and depth‐averaged daily radiant exposure (R = 0.61, n = 15, p = 0.02), which indicates a positive influence of solar radiation on CH3I production. However, the results from this study do not conclusively show whether the influence was exerted through photochemistry or other pathways. A positive correlation between the CH3I concentration and sea surface temperature was also observed (R = 0.61, n = 79, p ≪ 0.001).

Synthesis of Methyl Halides from Biomass Using Engineered Microbes

Methyl halides are used as agricultural fumigants and are precursor molecules that can be catalytically converted to chemicals and fuels. Plants and microorganisms naturally produce methyl halides, but these organisms produce very low yields or are not amenable to industrial production. A single methyl halide transferase (MHT) enzyme transfers the methyl group from the ubiquitous metabolite S-adenoyl methionine (SAM) to a halide ion. Using a synthetic metagenomic approach, we chemically synthesized all 89 putative MHT genes from plants, fungi, bacteria, and unidentified organisms present in the NCBI sequence database. The set was screened in Escherichia coli to identify the rates of CH(3)Cl, CH(3)Br, and CH(3)I production, with 56% of the library active on chloride, 85% on bromide, and 69% on iodide. Expression of the highest activity MHT and subsequent engineering in Saccharomyces cerevisiae results in productivity of 190 mg/L-h from glucose and sucrose. Using a symbiotic co-culture of the engineered yeast and the cellulolytic bacterium Actinotalea fermentans, we are able to achieve methyl halide production from unprocessed switchgrass (Panicum virgatum), corn stover, sugar cane bagasse, and poplar (Populus sp.). These results demonstrate the potential of producing methyl halides from non-food agricultural resources.

Methyl iodide production from marine phytoplankton cultures

Unialgal cultures of 15 species of marine phytoplankton were examined for the production of methyliodide (CH3I). CH3I production was quantified during log and stationary phases of phytoplankton growth in f/2 medium and compared to CH3I levels in control vessels. Significant increases in CH3I production were seen in five cultures. Phytoplankton CH3I production varied from a low of 10−4 amol CH3I cell−1 d−1 to a high of 8 × 10 3 amol CH3I cell−1 d−1. In three diatom cultures, the increase in total CH3I and cell‐based CH3I production rates were much higher during the log phase than during the stationary phase, even though the bacterial numbers continued to increase, supporting the premise of phytoplankton CH3I production. In two cultures, it was more difficult to distinguish between phytoplankton and bacterial production. Additional biotic and abiotic sources and sinks of CH3I were considered in the analysis, including bacterial consumption, chemical removal, and photochemical production. The only significant CH3I sink during the course of the experiment was its nucleophilic substitution with Cl−.

Biosynthesis of halomethanes and methanethiol by higher plants via a novel methyltransferase reaction

ABSTRACTBiogenic emissions of halomethanes (CH3CI, CH3Br and CH3I) and methanethiol (CH3SH) are of major significance to atmospheric chemistry, but there is little information on such emissions from higher plants. We present evidence that plants can produce all these gases through an identical methyltransferase reaction. A survey of 118 herbaceous species, based on CH3I production by leaf discs supplied with Kl, detected the presence of in vivo halide methyltransferase activity in 87 species. The activities ranged over nearly 4 orders of magnitude. Plants generally considered salt tolerant had relatively low activities, and salinization of three such species did not increase the activity. The highest activities were found in the family Brassicaceae. Leaf extracts of Brassica oleracea catalysed the S‐adenosyl‐L‐methioninc‐dependenl niethylalion of the halides I−, Br− and CI− to the respective halomethanes. In addition, the extract similarly methylated HS− (bisulphide) to CH3SH. These two types of enzyme activity (halide and bisulphide methyltransferase) were also present in all of the 20 species comprising a subsample that represented the range of CH3I emissions observed in the initial survey of in vivo CH3I production ability, and in a marine red alga Endocladia muricata. Moreover, the two activities occurred in approximately the same ratio in all the higher plants tested. These findings highlight the potential of higher plants to contribute to the atmosphericbudget of halomethanes and melhanethiol. The halide and bisulphide methyltransferase activities may also provide a mechanism for the elimination of halide and HS− ions, both of which are known to be phytotoxic.

The possible involvement of methylcobalamin in the production of methyl iodide in the marine environment

Methyl iodide (CH3I) is a biogenic iodocarbon of marine and estuarine waters, and is an important component in the biogeochemical cycling of iodine. A possible significant abiotic marine source of CH3I is the reaction of methylcobalamin (CH3B12) with either iodide or molecular iodine (I2) in seawater. These reactions were shown to occur in seawater and buffered water. The reaction between iodide and CH3B12 was apparently second order. The pH had a slight but significant effect on the reaction rate at pH between 7 and 8, and a more pronounced effect at a pH < 7. Chloride, bromide, iodate and bicarbonate had no significant effects on CH3I production at the standard reaction pH of 8.5. Iodate in the reaction mixture did not affect the reaction rate at pH > 7.5, however, at pH < 7.5 the presence of iodate increased the rate of CH3I production (9 fold at pH 5) due to the probable formation of I2. The addition of aquocobalamin to the reaction mixture (CH3B12 + iodide) enhanced CH3I production (63% at pH 5; 38% at pH 8.5) presumably by catalyzing the oxidation of iodide to I2. Molecular iodine also reacted with CH3B12 to form CH3I. The separate addition of CH3B12, I− or I2 to coastal seawater, for a final concentration of 0.22 mM, 4.8 mM and 25μM, respectively, resulted in the detectable formation of CH3I. The significance of these reactions to the formation CH3I in the oceans is discussed.