Sources and sinks of methylgermanium in natural waters

Abstract

New data have extended our understanding of the distribution and behavior of methylgermanium in the environment. Laboratory attempts to induce aerobic methylation with known biological and abiotic methylating agents were unsuccessful; this confirmed previous field observations of methylgermanium's unreactive behavior. However, biomethylation of inorganic germanium was observed in the anaerobic digestor of a sewage treatment plant, which suggested a terrestrial methanogenic source. Attempts to locate such a source in methanogenic swamps and their drainages reveal very low methylgermanium concentrations typical of other remote, pristine rivers. Polluted rivers have monomethylgermanium (MMGe) and dimethylgermanium (DMGe) concentrations 3–100 times higher than those of pristine rivers, which suggests an anthropogenic source of methylgermanium as a result of the synergistic effects of sewage treatment and coal-ash derived inorganic germanium contamination. A new high-precision profile of MMGe and DMGe in the Sargasso Sea shows conservative behavior with no vertical gradients. However, marine anoxic basins have both inorganic germanium enrichment and methylgermanium depletion, which suggests that of marine anaerobic processes are responsible for demethylating marine organogermanium. These results all suggest that methylgermanium is produced on the continents, is unreactive in the open ocean, and is destroyed in marine anoxic environments. The residence time of organogermanium in the sea, based on a continental source (pristine rivers), is at least 1 Ma, consistent with its unreactive nature, its observed distribution in the ocean, and rates of destruction in anoxic basins.