Research progress on applications of calcium derived from marine organisms

Calcium is an important mineral that plays an integral role in human health, especially bone health. Marine biological calcium is an abundant resource that is generally accepted and has a complex active structure. This review evaluates research progress on marine biological calcium with regards to its sources, use of calcium supplements, calcium bioavailability, and novel applications of marine calcium. The potential for future development and the use of products incorporating marine biological calcium in biomedical research and the pharmaceutical, health care, and food industries are also reviewed. The goal of this review is to provide a comprehensive documentation on resource utilization and product development from marine organisms.

Calcium from seaweed. Seaweed from the ocean, especially green algae, is rich in minerals such as calcium 35 . For example, Aquamin, a typical calcium-rich supplement derived from the calcified skeletal remains of the red seaweed species Lithothamnion, has calcium concentrations of up to 31%/weight 36 . A previous study has indicated that concerning calcium sources for horses, marine algae is better than calcium carbonate supplements 37 . Calcium extracted from marine algae was also found to show a beneficial anabolic effect on bone skeletal calcification in animal models of osteoporosis 38 . Algal calcium prepared from oyster shell powder and seaweed has a higher bioavailability than calcium carbonate 39 .

Calcium supplements and bioavailability
Direct ingestion of marine-derived calcium. The most common direct calcium supplements are small dried shrimp, shell powder, and small fishes. Several marine calcium supplements, such as oyster shells and coral calcium, have been commercialized in different countries. However, the main components derived from these marine sources are calcium carbonate and calcium polyhydroxy phosphate, which are difficult to absorb and increase gastric burden 40 . To improve the calcium absorption rate, marine sources are typically crushed or vacuum heated first 41,42 . Studies have found that marine-derived calcium has certain advantages over calcium carbonate supplements or other calcium-rich food. For example, Aquamin has better bioavailability and potential to slow down bone loss compared to calcium carbonate 36 . Hake fishbone (HBF) was a good source of calcium, with comparable efficacy to Lithotame (L), a calcium supplement derived from Lithothamnion calcareum 17 . A fishbone powder (Phoscalim) and a ray cartilage hydrolysate (Glycollagene) were comparable to milk for both short-term calcium absorption and bone resorption 16 . Tablets made with calcium from haddock bones were adequate for calcium supplementation and osteoporosis prevention 43 . Currently, the international recommended daily intake of calcium for general population is 700-1200 mg per day. However, teenagers (9-18 years old) need approximately 1300 mg calcium per day, and pregnant women with low dietary calcium intake need 1500-2000 mg calcium per day 44,45 . Studies have shown that more than 50% of the calcium deficient population include men and women older than 70 years, women aged 51-70 years, boys and girls aged 9-13 years, and girls aged 14-18 years 46 . Taking a conscious supplement of marine calcium is very effective in preventing calcium deficiency. Direct calcium ingestion from marine organisms is very suitable for daily calcium supplementation; however, it is insufficient for treating calcium deficiency diseases. In the treatment of diseases such as calcium deficiency, there is also a need to choose higher doses of calcium supplements or drugs 5.
Organic acid calcium. Organic acid calcium, such as calcium citrate, l-calcium lactate, calcium gluconate, calcium acetate, calcium formate, and calcium propionate, have higher bioavailability, solubility, and absorption rates, regardless of gastric contents, because they are less sensitive to gastric pH than calcium carbonate 11,40,47 . It is mainly prepared by neutralization or fermentation of calcium compounds (Fig. 2). As a dietary calcium supplement, calcium formate has been found to exhibit significant advantages over both calcium carbonate and calcium citrate 48 . Calcium glucoheptonate has exhibited a high relative bioavailability of calcium and is welltolerated in humans than calcium carbonate 49 . However, calcium gluconate and calcium lactate are less concentrated forms of calcium, making them impractical oral supplements. Calcium acetate and calcium propionate are not widely used either 50 . Calcium organic acids alone are not good for absorption because they can bind to oxalic acid or phytic acid present in food. Calcium combined two or more organic acids, such as calcium citrate malate www.nature.com/scientificreports/ (CCM) 10 , which combines bovine collagen peptides with calcium citrate 51,52 . The combined use of polycan and calcium lactate-gluconate 53,54 was found to have beneficial synergistic effects compared with the use of calcium organic acids alone. Marine sources of calcium organic acids are primarily fishbones, shrimp, crab shells, and other shells 3 . To facilitate easy calcium absorption, appropriate processes such as calcination, enzymatic hydrolysis and fermentation methods should be selected according to the nutritional composition and associated processing properties 55,56 . Subsequently, citric acid, gluconic acid, lactic acid, acetic acid, and/or propionic acid are added to prepare calcium organic acids. The solubility and bioavailability of calcium from natural sources of shellfish calcium with citrate and lactate were increased after decompression treatment 26 . Fishbones can be fermented with Leuconostoc mesenteroides to obtain high amounts of soluble calcium with free calcium, calcium amino acids, calcium acetate, small peptide calcium, and calcium lactate. The fermentation of grass fishbones can increase calcium bioavailability and also help avoid wastage of fishbone calcium and aquatic proteins 57 .
Calcium chelate. Calcium chelate refers to the metal complex formed by stable bonds between amino acids or peptides and metal calcium ions and includes two main products, calcium amino acid and calcium peptide chelate [58][59][60] . It is mainly prepared by chelating polypeptides or oligopeptides with calcium ions or when a single or complex amino acid chelates with calcium ions (Fig. 3). Amino acid chelated calcium is not dependent on vitamin D3 and can be absorbed by the human body through amino acid metabolism. For example, calcium lysinate, a new form of calcium preparation, may have better absorption, making it a better calcium supplement than calcium carbonate and CCM 61 . However, peptide chelated calcium has advantages over other calcium supplements [62][63][64] . A growing number of chelating peptides have been identified and have been shown to promote and improve mineral bioavailability 65,66 . The calcium peptide chelate produced by combining fishbone calcium and calcium-binding bone collagen peptide through enzymatic hydrolysis demonstrated improvement in calcium bioavailability [67][68][69] . The algae peptide-based calcium-chelating complex and calcium alginate nanoparticles have the potential to be utilized as a calcium supplement to improve bone health [70][71][72] . However, the production cost of peptide chelated calcium is high, and the yield is low. With the development of new preparation technology, peptide chelated calcium will likely become a good calcium supplement.

Other functions of marine source calcium
Biological activity. Marine biological calcium has biological functions other than improving calcium homeostasis and bone health. For example, coral calcium was shown to regulate blood pressure and prevent the metastasis of colon cancer 30,73,74 . Calcium spirulan, derived from Spirulina platensis (Arthrospira platensis), a filamentous blue-green microalga from rivers and lakes, was shown to inhibit herpes simplex virus 1 actively, and possibly, infections caused by other herpesviruses 75 . Coral calcium hydroxide can act as an antioxidant, slowing senescence in mice and preventing hepatic steatosis [76][77][78] . The calcium oxide made from scallop shells was shown to inhibit Pseudomonas aeruginosa, a spoilage bacterium for eggs with a strong resistance to chemical agents such as sanitizers and disinfectants 79 . Calcium derived from oysters exhibited good efficacy in suppressing the formation and proliferation of oral squamous cell carcinoma 80 . www.nature.com/scientificreports/ New materials. Calcium from marine sources can serve as a raw material for the production of highvalue-added compounds that can be used in biomedical research and pharmaceutical, healthcare, and food industries 81 . Previous studies have found a huge potential for producing porous scaffolds from oyster shells, clamshells, cuttlefish bones, and salmon bones [82][83][84][85] . The structural features of these scaffolds were found to be conducive to improve biological activities, including mechanical properties, and bone tissue growth and vascularization 86 . The production of natural hydroxyapatite (nHAP) from salmon bones and rainbow trout has a great potential as bone implant material substitutes in bone tissue engineering 87 . Marine biological calcium can also be used to prepare adsorption materials, demonstrating its potentially wide applications in water treatment. For example, calcium-rich biochar prepared from crab shells can be used to remove dyes and phosphorus from wastewater 88,89 . The acid-insoluble calcium silicate hydrates synthesized from oyster shells were also applicable in removing organic pollutants and heavy metal ions 90 . Single-phase hydroxyapatite (HA) and biphasic calcium phosphate (HA/β-TCP), which are derived from Atlantic cod bones, have no known cytotoxic effects and have demonstrated good bioactivity in simulated body fluid 91 . Consequently, calcium phosphate derived from marine organisms has a promising future in fabricating bacterial infection-resistant bone substitutes or bone defect healing. HA (Ca 10 (PO 4 ) 6 (OH) 2 , HAp) derived from codfish bones is a calcium phosphate, which is a safer option for sunscreen formulation, indicating its potential across a wide range of applications in health care products and cosmetics 92 .
Food additives. Biological calcium from marine processing waste can still be used in food processing. For example, fish bones can be added to fish surimi to improve the gel performance of the product 93 . Oyster shell calcium powder can improve the chewiness and springiness of restructured ham 94 . Calcium-rich shrimp and crab shells can also be used to prepare food flocculants 95 . There are many food additives containing calcium, such as calcium carbonate, calcium silicate, calcium sulphate, and calcium lactate. The calcium additives from marine organisms may be safer because they have a natural origin.

Conclusions and future perspectives
Marine processing waste is often considered useless; however, it is an abundant and low-cost source of calcium.
A study found that 55 brands of calcium supplements can be classified into seven categories based on the major ingredient in them and three or more categories were found to be derived from marine organisms mainly oyster/ clamshells, algae, shark cartilage, and chelated calcium products (Table 1) 10 . In addition, calcium from marine organisms has good bioavailability and biological function. Reusing by-products from marine organisms can increase the added calcium value and reduce the risk of environmental pollution. For the development of calcium supplements, future work should focus on the comprehensive utilization of proteins, collagen, chitin, calcium, and other nutrients in marine organisms and the use of specific active ingredients to increase the bioavailability of calcium. In other applications, research must likely focus on the transformation of marine calcium into health foods, new materials, or food additives to expand to a commercial scale.