Heavy metals and other trace elements
Marine algae concentrate inorganic elements from seawater and may contain heavy metals and other mineral contaminants. For that reason, seaweeds for food and feed are subject to national and international regulations concerning their content in trace elements. In the European Union, calcareous marine algae must contain less than 10 mg/kg, 15 mg/kg of lead and 1000 mg/kg of fluorine; seaweed meals and seaweed-based feed materials should contain less than 40 mg/kg of arsenic, and less than 2 mg/kg if requested by the competent authorities (all values relative to a feed with moisture content of 12%) (European Commission, 2011). The brown algae Sargassum fusiforme (Harvey) Setchell (synonym Hizikia fusiformis (Harvey) Okamura) is known for its high concentration of arsenic and is singled out in the latter Commission Directive (Moreda-Piñeiro et al., 2012).
The Orkney sheep of the North Ronaldsay Island, who feed almost exclusively on seaweeds, absorb large amounts of arsenic, up to 100 times higher than the levels of unexposed sheep, which are excreted via the kidneys or stored in tissue samples or wool (Raab et al., 2002; Feldmann et al., 2000; Martin et al., 2005).
Seaweeds, and particularly brown algae, contain high amounts of iodine and may cause iodine poisoning when fed for prolonged periods. In the European Union, the maximum iodine concentrations found in complete feeds are 2 mg/kg (dairy ruminants), 3 mg/kg (laying hens) and 3 mg/kg (horses) (EFSA, 2013).
Mineral overload
The high mineral content of algae has deleterious effects on the health of Orkney sheep: they suffer from severe dental disease precipitated by heavy deposits of tartar, and from extensive mineralization of the kidney medulla (Britt et al., 1990).