Calcium is an element whose careful regulation within every living organism is critical to its survival. Eukaryotic cells use calcium ions as intracellular messengers, signaling environmental stresses and inducing changes in gene expression. Calcium is also an important structural component of cells, present in cell walls and membranes, and is a counterion for anions in cell vacuoles. The presence of too much calcium is, however, problematic and organisms have evolved ways to manage excess calcium. For example, earthworms contain calciferous glands that excrete calcium carbonate when too much calcium has been ingested and certain tree species (e.g., Norway spruce (Picea abies (L.) Karst.)) are thought to incorporate excess calcium into extracellular calcium oxalate crystals in their foliage.
Calcium is categorized as a major mineral nutrient for plants, and deficiencies of calcium affect plant health. Studies have demonstrated correlations between calcium availability and susceptibility of trees to insect, drought, frost damage, and disease. Tree species, such as sugar maples (Acer saccharum Marsh.), with greater requirements for calcium are more readily damaged due to low calcium availability. Accordingly, Ca deficiency in ecosystems can lead to shifts in plant species composition that may in turn have effects on the entire food web. Generally speaking, while both monocots (e.g., grasses, corn, and other grains) and dicots require calcium, monocots need less of it than dicots. Legumes, on the other hand, need roughly twice as much calcium as grasses. Plants can be classified into two groups, calcifuges (e.g., rhododendrons, heaths, and azaleas), which grow in acid soils with low calcium, and calcicoles (e.g., the Brassicaceae family including cabbage, broccoli, and kale), which require calcium-rich soils.
Invertebrates such as snails and mollusks use calcium to build their shells. Wood lice and millipedes prefer Ca-rich soils and serve as sources of calcium for their predators. Freshwater crayfish require at least a month of exposure to calcium-rich water after molting or their exoskeletons and claws fail to harden. Predatory birds can be affected if their prey and main calcium source becomes scarce due to inadequate levels of calcium; specifically, studies have demonstrated that egg shells become more fragile as prey (caterpillars, snails, arthropods) populations decline in areas of calcium depletion. Higher organisms, such as birds and humans, require calcium for more cellular structures and biochemical processes than the aforementioned; most of their calcium is contained within bones, but it is also used in nerve impulses, muscle contractions (e.g., heart contractions), DNA transcription, and blood clotting.
The calcium needs of organisms within lakes are the same as any other organism. The most basic difference between land and water terrestrial ecosystems is that organisms in lakes are submerged in water and the calcium content is linked with lakewater pH and, therefore, the survivability of an organism. Bodies of water with low relative concentrations of calcium are usually oligotrophic and can be dystrophic. Dystrophic lakes often have high concentrations of decaying organic matter, high concentrations of organic acids, and a low pH (e.g., bog lakes). The low abundance of calcium arises because the lakes are in an area with Ca-poor rocks or the dissolved organic matter has reacted with all of the available calcium (or both).
In addition to playing critical roles in the biochemistry of living cells, the major role calcium plays within marine ecosystems is as a major component of biominerals such as calcium carbonate (CaCO3). Corals, mollusks, including the pelagic pteropods, and the green alga, Halimeda, produce aragonite, the more soluble polymorph of calcium carbonate. Coccolithophores, most foraminiferans, coralline algae, many crustaceans, and echinoderms produce the more stable polymorph, calcite.
Calcium-biomineralizing organisms play no small role in ocean ecosystems. Coccolithophores are one of the main types of phytoplankton in the ocean and their production of calcium carbonate significantly diminishes the effectiveness of the biological pump for sequestering carbon dioxide (CO2) in the deep ocean. Pteropods are a major source of food for carnivorous zooplankton, fishes such as cod, salmon, and herring, and baleen whales. Corals and Halimeda and other calcareous algae together form tropical reefs which serve as habitat and food for a diverse community of microbes, invertebrates, and fish. Echinoderms such as sea urchins, brittle stars, and sea stars are important predators, grazers, and scavengers in benthic ecosystems from tropics to poles, shallow waters to deep.
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