Of all the halides, chlorine (Cl) is perhaps the most biologically relevant to the broadest range of organisms, particularly to animals and plants. Cl is an essential element for all animals. The Cl ion is found in animal cell fluid and extracellular fluid as a counter ion to the positively charged cations like K and Na. Cl is also an essential part of both HCl, which is found in the stomach, as well as hypochlorite, which has a crucial role in the immune system and is critical to protecting the body from pathogens. 

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Cl has been considered an essential micronutrient for plants for decades (Broyer et al., 1954). Chlorine is important for plant photosynthesis because it is needed for the opening and closing of the stomata to control cell turgor and volume, as well as a major ion in action potentials and currents (Raven, 2017). Chlorine deficiency can be observed through the wilting of leaves at their edges, as well as eventual curling, bronzing, chlorosis, and necrosis (Ozanne et al., 1957). 

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Unlike plants and animals, Cl is not a required element for microbes. However, like many halides, it is used in some microbial secondary metabolites. Secondary metabolites are not required for the survival nor reproduction of organisms, but they can be used to provide some advantage to the organism and are thus beneficial but not essential. Cl-containing secondary metabolites are produced by some species of marine and soil bacteria, fungi, and phototrophic eukaryotes (Atashgahi et al., 2018).