One of the initial challenges of the earliest cells would have been the ability to take up, utilize, and/or detoxify transition metal ions and metalloids. This article will focus on geological and biological factors and phenomena that cause redox-active metals and metalloids to cycle through the environment, focusing on the role of and responses by bacteria, archaea, and eukaryotic microbes. Two transition metals, copper and manganese, and two metalloids, arsenic and selenium, will be reviewed. Each except arsenic is required for microbial metabolism, and each is toxic in excess. Many good and recent reviews have recently been written on enzymes involved in iron and mercury cycling. Here, the emphasis is on recent findings on enzymes responsible for redox cycling of metal(loid)s less studied. Finally, this article will summarize identification and characterization, especially at the structural level, of transport proteins, transcriptional regulators, and enzymes for oxidation, reduction, and methylation of metal(loid)s.
