Biogeochemical Transformations of Selenium in Anoxic Environments

Ronald S. Oremland, U.S. Geological Survey

Introduction

	Selenium is a micronutrient involved in diverse biochemical 
reactions and has long been known to be required in the diet of animals and 
for the growth of plants. Indeed, a deficiency of dietary selenium in cattle 
results in deleterious effects, including anemia and death. Selenium is 
required for the growth of several species of phytoplankton and bacteria. In 
contrast with its widespread use as a cofactor in biochemical systems, when 
absorbed at high concentrations selenium poses a threat to the health of 
animals, and selenium oxyanions (e.g., selenate and selenite) inhibit the 
growth of a variety of microorganisms.

	Anthropogenic sources of selenium to the environment are diverse 
and include fly ash from fossil fuel combustion, petroleum refining, mine 
drainage, and domestic household sources such as dandruff shampoo. Ironically, 
it is selenium that is already abundant in the environment in the form of 
naturally occurring seleniferous salts that have caused the most problems. 
Thus, the leaching of selenium oxyanions caused by agricultural activity on 
irrigated seleniferous soils, such as those of the western San Joaquin 
Valley of California, have commanded the most attention and have become the 
focus of debate in the scientific community, among water management 
professionals, in the agricultural industry, and at public forums. 
Consequently, attention was drawn to the fact that although much was known 
concerning the toxicity of selenium to individual species, there was little 
information available with regard to the geochemical properties of selenium, 
its biologically mediated geochemical reactions (biogeochemistry), or the 
bioavailability of its various chemical forms. For the past decade, therefore, 
research has been focused on the transport of selenium in the environment 
and how these properties can be exploited to devise treatment technologies and 
enlightened management practices.

	Microorganisms are known to have biochemical interactions with 
selenium that can affect its chemical speciation and complexation and hence 
may be of significance in affecting its mobility in nature. Indeed, a 
biogeochemical redox selenium "cycle" analogous to that of sulfur or 
nitrogen was first proposed three decades ago. Because many of the 
environments that receive seleniferous waste can be characterized as anoxic-
for example, subsurface saturated soils or organic-rich marsh sediment-
biochemically mediated transformations of selenium carried out by anaerobic 
bacteria have been a logical area of investigation. This chapter summarizes 
the work conducted in this area, taking special note of the efforts made 
over the past decade.