In addition to the major constituents listed in Table 4.1 there are many other elements present in seawater in very small amounts (Table 4.3). The most abundant of the ionized minor constituents are silicate ions at concentrations up to 6 mg/kg, and fluoride ions up to 1.4 mg/kg. The combined weights of all the other minor constituents, numbering nearly fifty, total less than 2 mg/kg, and at this dilution the estimation of many of them is very difficult. Probably all natural elements occur in seawater, though some at infinitesimal concentrations. Several are known to be present mainly because they are concentrated in the bodies of marine organisms. Details of many of the laboratory procedures for determining biologically significant constituents in seawater can be found in references at the end of this chapter (Grasshof, 1976).
Few marine organisms survive for long in an artificial seawater which contains only the major constituents in correct proportion. The minor constituents are evidently of biological importance although in many cases their role is uncertain. Some are known to be essential for the normal growth of plants; for example nitrate, phosphate, iron, manganese, zinc, copper and cobalt. Silicon is an ingredient of diatoms, and some marine algae require molybdenum and vanadium. Many of the minor constituents are also necessary for animal life. Silicon is included in the spicules of most radiolaria and some sponges. Iron is required by all animals. Copper is present in the prosthetic group of the blood pigment haemocyanin which occurs in some molluscs and crustacea. Vanadium and niobium occur in the blood pigment of ascidians. The vertebrate hormone thyroxin is an iodine compound.
Certain organisms concentrate the minor constituents to a remarkable extent. Vanadium in ascidians is an outstanding example, occurring in some species at concentrations approximately a million times greater than in seawater. Iodine, nickel, molybdenum, arsenic, zinc, vanadium, titanium, chromium and strontium are concentrated in the tissues of various marine algae, and some fish concentrate silver, chromium, nickel, tin or zinc. Certain of the heavy metals appear to be essential for normal enzyme activity, notably copper, though toxic at abnormal concentrations.
Whereas the major constituents of seawater, and some of the minor constituents, remain virtually constant in proportion (conservative constituents), certain minor constituents fluctuate in amount due to selective absorption by organisms (non-conservative constituents). The latter include nitrate, phosphate, silicate, iron and manganese, and the list will probably increase as our knowledge of the requirements of marine organisms grows.
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