Iodinated C₁-C₄ hydrocarbons released from ice algae in Antarctica

Abstract

As part of an extensive investigation of halocarbons (low molecular weight halogenated hydrocarbons) in the waters of the Weddell Sea, Antarctica, in January-February 1993, a set of four brominated and eight iodinated hydrocarbons were measured in sea ice containing algal colonies, and the results were compared with those of surrounding surface seawater. Samples were collected in two ways. On one occasion, brownish ice was taken underneath an ice floe, and the ice pore water analysed. On another occasion, samples were collected from colonies living in the interface between the snow and the sea-ice. Some of the latter samples were also analysed for their content of chlorophyll a.

The brominated compounds measured were dibromomethane (CH₂Br₂), bromodichloromethane (CHBrCh₂), dibromochloromethane (CHBr₂Cl) and bromoform (CHBr₃). Iodinated halocarbons measured were methyl iodide (CH₃I), chloroiodomethane (CH₂CII), diiodomethane (CH₂I₂), ethyl iodide (CH₃CH₂I), n-propyl iodide (CH₃CH₂CH₂I), isopropyl iodide (CH₃CHICH₃), n-butyl iodide (CH₃CH₂CH₂CH₂I) and sec-butyl iodide (CH₃CHICH₂CH₃).

In none of the ice algae samples were brominated halocarbons found at concentrations significantly higher than in the surface water, in contrast to the published observations of other workers. On the contrary, the levels of bromocarbon concentrations in ice pore water (from the underside of an ice floe) were only 40–60% of the concentrations in surrounding water collected just under the floe.

On the other hand, elevated concentrations of four iodinated compounds were observed in the ice pore water. These were methyl iodide, ethyl iodide, n- and isopropyl iodide . The concentration of methyl iodide, 5.4 ng 1_-1 , was about 50% higher than the seawater level 3.5 ng 1_-1 . Ethyl iodide, 4.3 ng 1_-1, was about 10 times higher in the ice pore water. n-propyl iodide was not detected at all in the seawater, while the ice pore water contained 40 ng 1_-1. Also, isopropyl iodide was detected in the ice pore water only, and at comparatively low concentrations, about 1 ng 1_-1.