Nickel is the sixth most abundant element on Earth. This statement might sound surprising, since Ni is considered to be a trace element on the Earth’s surface, but most of this element resides in the core of our planet.¹  In the anoxic Archean eon, nickel containing lavas erupted from the ocean’s crust, supplying Ni to seawater (Chapter 2 reveals fascinating details of nickel biogeochemistry). Nowadays, nickel remains an evolutionary relict of those times, being an essential catalytic cofactor of enzymes found in lower organisms, such as eubacteria, archaebacteria, fungi, and also plants.²  So far, no enzymes or co-factors that include nickel have been identified in higher organisms. For humans, Ni is considered to be carcinogenic. It accumulates in kidneys and pulmonary absorption is the major route of concern (Chapter 3 discusses details of nickel toxicity and carcinogenesis).