Heavy Metals: Heavy metal salts in solution 

 may constitute a very serious form of pollution 

 because they are stable compounds, not readily 

 removed by oxidation, precipitation, or any other 

 natural process. A characteristic feature of heavy 

 metal pollution is its persistence in time as well as 

 in space for years after the pollutional operations 

 have ceased. 



The number of substances that may be described 

 as "poisonous" is very large and they vary enor- 

 mously in the degree of their effect. For man and 

 other air-breathing animals, the threshold dose of 

 a toxic material generally means the maximum 

 quantity that can be taken without causing death. 

 For aquatic animals living in a water environment 

 containing a toxic substance, the situation is some- 

 what different. Instead of receiving an absolute 

 quantity at one time, they are being continually 

 exposed to a given concentration of the toxic mate- 

 rial. This is similar to a man regularly drinking 

 water containing lead or breathing air containing a 

 noxious gas or vapor. It is not surprising, therefore, 

 that the student of pollution problems turns his at- 

 tention toward the concentration of the poison he 

 is investigating and the manner in which the effect 

 is related to this, rather than to the absolute 

 amount required to harm or kill. Animals have the 

 ability to eliminate poisons at least to some degree 

 or even to destroy them. Their ability to do this at 

 a rate permitting survival depends on the concen- 

 tration of the toxic material to which they are 

 exposed. 



One of the characteristics of living cells is their 

 abihty to take up elements from a solution against 

 a concentration gradient. This is perhaps most ob- 

 vious for marine organisms, especially for auto- 

 trophic algae which obtain all their nutrients di- 

 rectly from seawater. The ability of marine 

 organisms to concentrate elements above that level 

 found in their environment has been recognized 

 for some time. The following points should be 

 noted in relation to their concentrating ability. 



( 1 ) All elements are concentrated to a degree 

 with the exception of chlorine, which is rejected, 

 and sodium, which is weakly rejected. The concen- 

 tration factors are of the order of one for bromine, 

 fluorine, magnesium, sodium, and sulfur, and 

 higher for all other elements. 



(2) Among cations (including metallic ele- 

 ments such as iron, which may exist as colloids in 

 the sea), the order of affinity for living matter is, 

 generally: tetravalent and trivalent elements > di- 

 valent transition elements > divalent group II-A 

 metals > univalent group I metals. The tetravalent 

 and trivalent subgroup have rather different afiini- 

 •ties for plankton and brown algae. 



84 



plankton: Fe>Al>Tl>Cr, Si>Ga 



brown algae: Fe>La>Cr>Ga>Li>Al>Si 



Similar differences are found between these orga- 

 nisms in their affinities for the divalent transition 

 metals. 



plankton: Zn>Pb>Cu>Mn>Co>Ni>Cd 

 brown algae: Pb>Mn>Zn>Cu, Cd>Co>Ni 



Of interest is the affinity of both organisms for lead, 

 which has no known biological function. 



It is clear that the heavier elements in these 

 groups tend to be more readily taken up than the 

 lighter ones, which may be connected with their 

 greater, ease of polarization. 



(3) The order of affinity of living matter for 

 anions is: 



nitrate > trivalent anions > divalent anions > 



univalent anions 



It is probable that most polyvalent metallic ele- 

 ments are more or less chelated by organic matter. 



The main features of the uptake of ions by cells 

 can be accounted for by assuming that another 

 process operates apart from simple diffusion. This 

 process is called active uptake and is closely linked 

 with metabolic activities within the cell. The meta- 

 bolic processes provide the energy necessary for 

 the uptake against a concentration gradient. Active 

 uptake has a larger temperature coefficient than 

 does uptake by diffusion. In long-term experi- 

 ments, the effect of temperature is probably com- 

 plicated by increased rates of growth, cell division, 

 and so on. Active uptake requires oxygen and oc- 

 curs only in cells which are respiring freely. Sub- 

 stances which inhibit respiration also inhibit up- 

 take of ions. The rate of uptake of ions may be 

 limited either by the rate of exchange at the cell 

 membrane or by bulk phase diffusion inside the 

 cell. The former is usually limiting for ions present 

 at low external concentration and the latter for 

 ions at high external concentrations. It has been 

 suggested that bulk phase diffusion limits the rates 

 of uptake of most cations. There appears to be at 

 least two active transport systems in addition to 

 the diffusion processes. A large number of theories 

 have been advanced to explain active transport. 

 One of the most popular is the carrier hypothesis. 

 Accordingly, the ions are transported across mem- 

 branes as chelates with metabolically produced or- 

 ganic molecules. 



Uptake by invertebrate animals. — The most 

 primitive animals, the unicellular protozoa, take 

 up ions from solution by diffusion in the same 

 ways as do algae. Many marine species have 

 vacuoles and these are able to open at intervals 

 and extrude fluid from the cell. The vacuole regu- 



