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Chromium in its toxic Cr(VI) valence state is a common contaminant particularly associated with alkaline environments. A well-publicized case of this occurred in Glasgow, United Kingdom, where poorly controlled disposal of a cementitious industrial by-product, chromite ore processing residue (COPR), has resulted in extensive contamination by Cr(VI)-contaminated alkaline leachates. In the search for viable bioremediation treatments for Cr(VI), a variety of bacteria that are capable of reduction of the toxic and highly soluble Cr(VI) to the relatively nontoxic and less mobile Cr(III) oxidation state, predominantly under circumneutral pH conditions, have been isolated. Recently, however, alkaliphilic bacteria that have the potential to reduce Cr(VI) under alkaline conditions have been identified. This study focuses on the application of a metal-reducing bacterium to the remediation of alkaline Cr(VI)-contaminated leachates from COPR. This bacterium, belonging to the Halomonas genus, was found to exhibit growth concomitant to Cr(VI) reduction under alkaline conditions (pH 10). Bacterial cells were able to rapidly remove high concentrations of aqueous Cr(VI) (2.5 mM) under anaerobic conditions, up to a starting pH of 11. Cr(VI) reduction rates were controlled by pH, with slower removal observed at pH 11, compared to pH 10, while no removal was observed at pH 12. The reduction of aqueous Cr(VI) resulted in the precipitation of Cr(III) biominerals, which were characterized using transmission electron microscopy and energy-dispersive X-ray analysis (TEM-EDX) and X-ray photoelectron spectroscopy (XPS). The effectiveness of this haloalkaliphilic bacterium for Cr(VI) reduction at high pH suggests potential for its use as an in situ treatment of COPR and other alkaline Cr(VI)-contaminated environments.

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The zone of continental margins is most important for the ocean’s productivity and nutrient budget and connects the flow of material from terrestrial environments to the deep-sea. Microbial processes are an important “filter” in this exchange between sediments and ocean interior. As a consequence of the variety of habitats and special environmental conditions at continental margins an enormous diversity of microbial processes and microbial life forms is found. The only definite limit to microbial life in sedimentary systems of continental margins appears to be high temperatures in the interior earth or in fluids rising from the interior. Many of the catalytic capabilities which microorganisms possess are still only incompletely explored and appear to continuously expand as new organisms are discovered. Recent discoveries at continental margins such as the microbial life in the deep sub-seafloor, microbial utilization of hydrate deposits, highly specialized microbial symbioses and the involvement of microbial processes in the formation of carbonate mounds have extended our understanding of the Earth’s bio- and geosphere dramatically. The aim of this paper is to identify important scientific issues for future research on microbial life in sedimentary environments of continental margins.

The zone of continental margins is most important for the ocean’s productivity and nutrient budget and connects the flow of material from terrestrial environments to the deep-sea. Microbial processes are an important “filter” in this exchange between sediments and ocean interior. As a consequence of the variety of habitats and special environmental conditions at continental margins an enormous diversity of microbial processes and microbial life forms is found. The only definite limit to microbial life in sedimentary systems of continental margins appears to be high temperatures in the interior earth or in fluids rising from the interior. Many of the catalytic capabilities which microorganisms possess are still only incompletely explored and appear to continuously expand as new organisms are discovered. Recent discoveries at continental margins such as the microbial life in the deep sub-seafloor, microbial utilization of hydrate deposits, highly specialized microbial symbioses and the involvement of microbial processes in the formation of carbonate mounds have extended our understanding of the Earth’s bio- and geosphere dramatically. The aim of this paper is to identify important scientific issues for future research on microbial life in sedimentary environments of continental margins.