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It has been well established that a number of trace metals are essential for various biological functions and are critical in many of the enzymatic and metabolic reactions occurring within an organism. The essentiality of nickel is now generally accepted, based on the numerous symptoms caused by nickel deficiency (mainly in terrestrial vertebrates) and its essential role in various enzymes in bacteria and plants. The information on optimal and deficient concentrations of nickel, however, is limited and the essentiality of nickel to aquatic animals is not established. The purpose of this review is to synthesize the available information on nickel essentiality and homeostasis in aquatic organisms. There is less information on these topics compared to that for other essential metals. Nickel essentiality to aquatic organisms can only be confirmed for plants and (cyano)bacteria due to the documented role of nickel in the urease and hydrogenase metabolism. Deficiency levels ranged from 10-12 M to 2 × 10-6 M Ni in different species. No studies were identified that had the explicit objective of evaluating homeostatic mechanisms for nickel in aquatic life. However, inferences could be made through the evaluation of nickel bioconcentration and tissue distribution data and a comparison to other metals that have been more thoroughly studied. Data suggest active regulation and therefore nickel essentiality, since there are no known examples of active regulation of non-essential metals in invertebrates. [ABSTRACT FROM AUTHOR]

Nickel (Ni) is a naturally occurring element with many industrial uses, including in stainless steel, electroplating, pigments, and ceramics. Consequently, Ni may enter the environment from anthropogenic sources, resulting in locally elevated concentrations in soils. However, Ni is a minor essential element, and, therefore, biota have established systems that maintain Ni homeostasis. This paper discusses the role of Ni as an essential element and reviews storage, uptake, and transport systems used to maintain homeostasis within terrestrial biota. The bioaccumulation and distribution of metals in these organisms are also addressed. In all cases, information on Ni essentiality is very limited compared to other essential metals. However, the available data indicate that Ni behaves in a similar manner to other metals. Therefore, inferences specific to Ni may be made from an understanding of metal homeostasis in general. Nevertheless, it is evident that tissue and organ Ni concentrations and requirements vary considerably within and between species, and metal accumulation in various tissues within a single organism differs as well. High rates of Ni deposition around smelters indicate that Ni in acidic soils may reach concentrations that are toxic to plants and soil decomposers. However, with the exception of hyperaccumulator plants, Ni does not biomagnify in the terrestrial food web, suggesting that toxicity to higher trophic levels is unlikely. [ABSTRACT FROM AUTHOR]