Nonbiogenic Toxic Substances Metals

As a result of industrialization and changes in the environment, heavy metal pollution has become a serious problem for humans and other organisms.45

Metals are a unique class of toxicants that occur and persist in nature, but their chemical form may be changed by physical-chemical, biological, or anthropogenic activities.46 They are neither created nor destroyed by organisms (plants, animals, or humans) because as chemical elements they cannot be degraded beyond their elemental state.47

Metals are naturally redistributed in the environment by both geologic and biological cycles, the latter of which include bioconcentration by plants and animals and incorporation into food cycles. Human industrial activity may shorten the time of metals in ore, form new compounds, and greatly enhance worldwide distribution by disposal into land, water, and the atmosphere.48

It is a well-known fact that virtually all microorganisms are capable of accumulating heavy metals at concentrations that are several orders of magnitude higher than those present in the surrounding media. Their accumulation in algae is a relatively rapid process.35 In Spirulina, special attention has been paid to the concentration of heavy metals (lead, cadmium, mercury, and arsenic).11

On the other hand, various algae strains have shown appropriate properties for heavy-metal removal from wastewater.49 This biosorption property of Spirulina has been used for removing metals such as lead,50,51 cadmium,52 and chromium.53

Some of the toxic effects of these metals involve hematopathies, neuropathies, nephropathies, and carcinogenesis attributed to lead; pulmonary edema, nausea, vomiting, abdominal pain, prostate, and lung cancer attributed to cadmium; neuropathies, teratogenesis, and mutagenesis attributed to mercury; and both central nervous system (CNS) and peripheral nervouos system (PNS) pathologies, including muscle weakness and loss of sensory perception, attributed to arsenic.47

To illustrate the extent of heavy metals found in algal samples, selected analytical data found in the literature are summarized in Table 2.1 and are compared to recommendations on upper limits of heavy-metal contents in different food goods.54 Estimates differ widely, depending on the origin of the algal samples as well as on the laboratories doing the analyses.11

Several concentrations detected in the algal samples can be seen to exceed the recommended limit. The main causes for these high levels are environmental pollutants, owing to unfavorable locations of algal plants as well as to contamination introduced through water and fertilizers. Moreover, under alkaline conditions and in the presence of phosphate and sulfate ions from fertilizers, dissolved cadmium and lead ions in the medium produce slightly soluble compounds that precipitate or can

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