This paper examines the effects of heavy metals on microorganisms in the aqueous environment; the mechanisms by which metals may exert toxic effects on microbes and the factors affecting microbial response to metals; the ways in which microbial activity may alter the metal balance of an environment and the modifications produced in microbes by heavy metal ions; the effects of the toxic copper ion on the growth, respiration, magnesium content, cytochrome synthesis and osmotic sensitivity of some organisms studied in the laboratory; and the feasibility of the participation of microbes in geochemical processes considering the demonstrable resistance to toxic metals by some bacteria and the fact that natural environments may contain high levels of metals rendered less toxic by binding to natural chelating compounds. IndroductionIn any discussion of the possible role of microorganisms in effecting geochemical transformations, one important aspect to be considered is the response of such organisms to the high concentration of heavy metals occurring in many geological situations. It has long been considered that all forms of life, including microorganisms, are highly sensitive to the presence of heavy metals. In fact some of the earliest attempts to control microorganisms by chemical means were based on this idea --for example, the use of copper sulphate as a plant fungicide and the use of mercury saks in the treatment of certain infectious diseases. The recent successes in the biological leaching of heavy metal sulphides and other minerals highlight the fact that this early concept of metal toxictiy is oversimplified. Indeed most organisms exhibit a biphasic response to a number of heavy metals --that is at low concentrations of the metal there is a stimulation of growth but as the metal concentration is increased growth becomes progressively inhibited and finally ceases. Chromium, copper, gold,

Our present knowledge of the physiology and biochemistry of green sulfur bacteria (genus Chlorobium) is largely based on the studies of van Niel' and of Larsen.2 Their work showed that the members of this genus are strictly anaerobic, obligate photolithotrophs, which perform a typical bacterial photosynthesis using reduced inorganic sulfur compounds as electron donors. The physiology of these photosynthetic bacteria is accordingly similar to that of the purple sulfur bacteria, with one significant exception: all members of the latter group so far studied in pure culture can also use a variety of simple organic substrates for photosynthesis in the absence of a reduced inorganic sulfur compound. The possible participation of organic compounds in the photometabolism of green bacteria was examined in detail by Larsen,2 using Chlorobium thiosulfatophilum. He found that none of the simple organic compounds commonly used as substrates for photosynthesis by purple sulfur bacteria could support the photosynthetic growth of C. thiosulfatophilum. However, manometric experiments suggested that resting cells could bring about a slow and limited transformation of organic acids anaerobically in the light. Only with propionate was a rapid metabolism observed. More detailed study showed that C. thiosulfatophilum performs a unique photosynthetic reaction with this substrate, grossly expressed by the equation:CH3CH2COOH + CO2 light HOOCCH2CH9COOH.(1)A large part of the propionic acid metabolized is excreted into the medium as succinic acid. The physiological function of this reaction is obscure. Larsen2 commented: "it can be safely concluded that the formation of succinic acid can hardly be considered as an important stage in the normal assimilatory process." However, the existence of the reaction revealed one very important fact: the failure of green bacteria to utilize organic compounds for growth cannot be attributed to the failure of such compounds to enter the cell.Recently, Mechsner3 has reported that a new species of Chlorobium, C. chiorochromatii, can grow well photosynthetically at the expense of peptone and malate. He confirmed, however, that the two species studied by Larsen, viz. C. limicola and C. thiosulfatophilum, are unable to utilize organic compounds for growth. This paper reports certain new observations on the utilization of organic substrates by C. limicola.Materials and Methods-Organisms and culture conditions: Several strains of C. limicola were isolated from local mud samples using the procedures described by Larsen2 for enrichment and purification. Larsen's mineral medium,"z which contains 0.024 M NaHCO3 and 0.0042 M Na2S, was used routinely for cultivation. In this medium, sulfide is the growth-limiting nutrient, as shown by preliminary experiments in which the sulfide content was varied, other components being kept constant. There is strict proportionality between cell yield and sulfide concentration 1328