lates the osmotic pressure of the cell and thus con- 

 trols its volume. 



Multicellular invertebrate animals can be di- 

 vided into two groups as far as uptake is con- 

 cerned: those with permeable integuments and 

 those without. The majority of marine inverte- 

 brates (colente rates, annelids, mollusks, and 

 echinoderms) have soft bodies with permeable 

 integuments through which ions can diffuse freely. 

 In this situation, the body fluid or blood is quite 

 similar to sea water in composition. The gills of 

 mollusks are coated with a layer of complex carbo- 

 hydrate sulfates which may function as ion ex- 

 changers. The gills of marine Crustacea, which 

 have hard impermeable carapaces, are fully per- 

 meable to water and salts. 



Mode of toxic action. — An element is said to be 

 toxic if it injures the growth or metabolism of an 

 organism when supplied above a certain concen- 

 tration. All elements are toxic at high concentra- 

 tions and some are notorious poisons even at low 

 concentrations. For example, the essential micro- 

 nutrient, copper, which is a necessary constituent 

 of all organisms, is highly toxic at quite small con- 

 centrations. The other essential micronutrients are 

 also toxic when supplied in excess, though not all 

 in such striking fashion. There is an optimum range 

 of concentration, which is sometimes quite narrow, 

 for the supply of each element to each organism. 



When excessive amounts of an element are fed 

 to an organism, they frequently cause death. The 

 usual measure of the amount required to cause 

 death is called the LD50. This is the amount which, 

 when fed to each individual in a population, kills 

 half of the population. The LD^o is an imprecise 

 measure unless it is qualified by specifying: 



( 1 ) The chemical state of the element. 



(2) The means of feeding. 



(3) The age or developmental stage of the 



organism. 



(4) The time elapsed between feeding and 



death. 

 The most important mechanism of toxic action 

 is thought to be the poisoning of enzyme systems. 

 The more electronegative metals, notably copper, 

 mercury, and silver, have a great affinity for amino, 

 imino, and sulfhydryl groups which are doubtless 

 reactive sites on many enzymes. These metals are 

 readily chelated by organic molecules. We thus 

 have discovered attempts to correlate metal toxi- 

 cities with such factors as their electronegativities, 

 the insolubility of their sulfides, or the order of 

 stability of their chelated derivatives: 



(1) Order of electronegativities of some diva- 

 lent metals : Hg > Cu > Sn > Pb > Ni > Co > 

 Cd>Fe>Zn>Mn>Mg>Ca>Sr>Ba 



(2) Order of stabihty products of the sulfides: 

 Hg>Cu>Pb>Cd>Co>Ni>Zn> 



Fe>Mn>Sn>Mg>Ca 



(3) Order of stability of chelates: Hg>Cu> 

 Ni>Pb>Co>Zn>Cd>Fe>Mn>Mg> 

 Ca. 



It appears likely that all the divalent transition 

 metals, as well as the other electronegative metals, 

 that form insoluble sulfides, such as Ag, Mo, Sb, 

 Tl, and W, are poisons by virtue of their reactivity 

 with proteins and especially with enzymes. In view 

 of the large number of enzymes in living cells, the 

 variations in toxicity indicated above are hardly 

 surprising. Studies have shown that metals giving 

 rise to similar toxic effects may be acting on quite 

 unrelated enzymes and also many more atoms of 

 metal are absorbed by an inactivated enzyme than 

 are required to block the reactive sites. Other 

 modes of toxic action are: 



( 1 ) Substances behaving as antimetabolites. 

 This might be arsenate and chlorate occu- 

 pying sites for phosphates and nitrates, 

 respectively. (Fluoride, borate, bromate, 

 permanganate, antimonate, selenate, tellu- 

 rate, tungstate, and beryllium.) 



(2) Substances forming stable precipitates or 

 chelates with essential metabolites. (Al, 

 Be, Sc, Ti, Y, Zr, reacting with phosphate, 

 Ba with sulfate, or Fe with ATP.) 



(3) Substances catalyzing the decomposition of 

 essential metabolites. (La and other lan- 

 thanide cations decompose ATP.) 



(4) Substances combining with the cell mem- 

 brane and affecting its permeability. (Au, 

 Cd, Cu, Hg, Pb, U.) These elements may 

 affect transport of sodium, potassium, 

 chlorine, or organic molecules across mem- 

 branes or even rupture them. 



(5) Substances replacing structurally or elec- 

 trochemically important elements in the 

 cell and then failing to function. (Li replac- 

 ing Na, Cs replacing K, or Br replacing 

 CI.) 



Metal-organic compounds may be either more 

 toxic than the metal ion (ethyl mercuric chloride) 

 or much less so (cupric ion and copper salicyl- 

 aldoxime). 



Silver. — Silver is present in seawater in a con- 

 centration of about 0.0003 mg/1. It is found in 

 marine algae at concentrations up to 0.25 mg/1 

 and in marine mammals in the range of 1 to 3 mg/1 

 (Vinogradov, 1953). It is highly toxic to plants 

 and mammals. 



Arsenic. — Arsenic is found to a small extent in 

 nature in the elemental form. It occurs mostly in 

 the form of arsenites of true metals or as pyrites. 



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