Abstract
Many marine algae are strong accumulators of halogens. Commercial iodine production started by burning seaweeds in the 19th century. The high iodine content of certain seaweeds has potential pharmaceutical and nutritional applications. While the metabolism of iodine in brown algae is linked to oxidative metabolism, with iodide serving the function of an inorganic antioxidant protecting the cell and thallus surface against reactive oxygen species with implications for atmospheric and marine chemistry, rather little is known about the regulation and homoeostasis of other halogens in seaweeds in general and the ecological and biological role of marine algal halogenated metabolites (except for organohalogen secondary metabolites). The present review covers these areas, including the significance of seaweed-derived halogens and of halogens in general in the context of human diet and physiology. Furthermore, the understanding of interactions between halogenated compound production by algae and the environment, including anthropogenic impacts, effects on the ozone layer and global climate change, is reviewed together with the production of halogenated natural products by seaweeds and the potential of seaweeds as bioindicators for halogen radionuclides.
Highlights
The oceans are the largest reservoirs of bioavailable chlorine, bromine and iodine on the planet [50] and, from there, the elements are transferred to the atmosphere
A carboxylic acid is needed, that is proposed to react with the serine to form an ester, which is attacked by H2 O2 to form a peracid, so that the enzyme could be called a perhydrolase [88]; the peracid reacts with the halide to produce the halogenating intermediate, which is not known with certainty, but must be electrophilic
Halogens interact with the Earth’s geo, bio, hydro- and atmosphere through their biogeochemical cycle, in which marine algae have a key role at the interface between the Ocean, the atmosphere and land
Summary
The halogens are a family of nonmetal elements in group 17 (VIIA) of the periodic table, consisting of six elements: fluorine, chlorine, bromine, iodine, astatine, and tennessine. Halogens are found in the environment mostly in the form of ions or compounds; because of their great reactivity their elemental forms tend to be short-lived. In their elemental forms, the halogens are very reactive diatomic molecules, which exist at room temperature and atmospheric pressure in different physical states: fluorine and chlorine are gases, bromine is a liquid, while iodine is solid. The solar system contains 0.4 ppm F, 4.8 ppm Cl, 23 ppb Br, and 3 ppb I, making the halogens the 25th (F), 18th (Cl), 34th (Br), and 50th (I) most abundant elements in the solar system [36]
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