Results and Discussions

A graph of the vertical time-series cross section in Funka-Bay is shown in Figure 2. In both years, high chlorophyll concentrations were observed in March (Fig. 2a) and diatom blooms were identified. Immediately after that, the concentration of ethyl iodide (C2H5I) in the sediment became high (Figure 2b), and the following month, the concentration in the bottom water became high (top of Figure 2b). Methyl iodide (CH3I) shows a similar trend. Results for the Chukchi Sea, Bering Sea, and Funka-Bay for the ratio of each compound to total organic iodine compounds in water and sediment are shown in Figure 3. In all areas, a high percentage of ethyl iodide was found on the sediment surface. Assuming that these compounds are generated from diatom aggregates sedimented on the seafloor, we conducted a dark culture experiment in which diatom aggregates collected during the bloom in Funka-Bay were sealed and stored in glass bottles (Figure 4). When diatom aggregates were placed in the dark, ethyl iodide and methyl iodide were observed to be generated after a few days.

https://www.nature.com/articles/s43247-022-00513-7/figures/2
Figure 2 Vertical time series in the Funka-Bay (a) chlorophyll, (b) ethyl iodide (C₂H₅I), (c) methyl iodide (CH₃I)

https://www.nature.com/articles/s43247-022-00513-7/figures/
Figure 3 Composition ratios of organic iodine in sediments and water in the Chukchi Sea, Bering Sea, and Funka-Bay

Figure 4　Diatom aggregate dark experiment (Left) ethyl iodide (C₂H₅I), (Right) methyl iodide (CH₃I)