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Ca
Calcium
40.078
Essential: animals and plants
Unknown: Bacteria and Archaea
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(1) Calcium: A Major Cation
Calcium is a major cation and is essential for many eukaryotic organisms, including animals and plants, as a chemical messenger. It regulates a variety of chemical processes such as chemotaxis, segregation of chromosomes, fertilization, ion transport, muscle contraction, and more.
Calcium's role in prokaryotes has been more difficult to demonstrate, but studies suggest that it has a role in several processes, including heat shock, pathogenicity, chemotaxis, differentiation, and the cell cycle (Norris et al., 1996). While both E. coli and B. subtilis can be grown in medium lacking added Ca, no studies to date have rigorously quantified the trace levels that might still be present, and it remains possible that low levels of Ca are required for growth.
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Ca is abundant in both marine and terrestrial environments and tends to form insoluble minerals that have been exploited throughout evolution. The most important Ca-based minerals in biology are CaCO3 (calcium carbonate) and PO4-based minerals (e.g. hydroxyapatite). Hydroxyapatite-type minerals make up half of the mass of bone in vertebrates and occur in a near solid form as tooth enamel (Welborn, 2020). CaCO3 minerals are even more widespread in biology and play a major role in global elemental cycling. For example. Ca interacts with and stabilizes the outer membrane of Gram-negative bacteria by interaction with the lipopolysaccharide and also interacts with anionic surface polymers in Gram-positive organisms. In addition, CaCO3 exoskeletons became widespread around the time of the Cambrian explosion (~540 MYA), and are found in many animal lineages.
(2) Ca Increases Infectivity of MERS-CoV
Studies have found that intracellular Ca ions promote the fusion of MERS-CoV, a major infectious disease, with the host cell, leading to a two-fold increase in infectivity. Ca regulates this interaction by interacting with the negatively charged residues in the fusion protein of the virus. This research suggests that novel drugs against MERS-CoV may be developed that target this calcium interaction (Straus et al., 2020).
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Ca may play other roles in the fight against pathogens. High intracellular Ca ions can be toxic to bacteria, so Ca exporters may be a viable antimicrobial drug target (Rosch et al., 2008).
(3) Sr Substitution for Ca
Studies have shown that in isolated cells, Sr can substitute for Ca, although the response to stimulation is weaker. For example, in isolated pancreatic islets, Sr sustains secretion of insulin in response to glucose, albeit to a lesser extent than Ca does. Another example is Sr's ability to restore insulin-mediated glucose uptake following Ca and Mg starvation. There are also researchers beginning to explore the possibility that Sr may be useful in treatments for osteoporosis because of its similarity to Ca (Nielson 2004).
(4) D. manga Adjusts Carapace under Ca Limitation
Daphnia manga, a small planktonic crustacean, requires Ca for survival, moulting, growth, and egg production. D. manga has a fixed moulting frequency, regardless of Ca concentration in the environment, so the organism is able to grow even under Ca-limited conditions. To compensate for the lack of Ca, D. manga adjusts its carapace calcification, forming a smaller carapace that has less calcification. Although this flexibility in Ca requirement allows the organism to survive at lower Ca concentrations, there are competitive drawbacks to having a smaller, less calcified carapace due to increased vulnerability (Hessen et al., 2000).