Copper complexation by siderophores from filamentous blue-green algael

From our experimental evidence that iron lirnitation greatly increases the extracellular concentration of strong copper-complexing agents in cultures of Anabaena flos-aquae and Anabaena cyhdrica, and that the iron-algal exudate complex is much more stable than the copper complex, we conclude that strong copper-complexing agents released by filamentous blue-green algae are siderophores. Further experiments demonstrate that siderophore excretion is not a mechanism by which blue-green algae overcome the toxic effects of micromolar copper additions. From estimates of concentrations of iron, copper, and siderophores in ironlimited blue-green algal blooms, it is predicted that the copper-siderophore complex is the major copper species. The copper-siderophore complex is probably not toxic and its presence in freshwaters may be advantageous to cyanophyte populations. It has been shown in laboratory and field experiments that under iron limitation blue-green algae excrete strong ironcomplexing agents-hydroxamate siderophores (Simpson and Neilands 1976; Murphy et al. 1976). (The term siderophore, formerly siderochrome, emphasizes the role of these compounds in microbial iron transport). Murphy et al. (1976) suggested that eucaryotic algae cannot assimilate iron from the iron-hydroxamate siderophore complex and that bluegreen algae monopolize low iron concentrations by excretion of hydroxamate siderophores. In previous work we found that four blue-green algal species cxcreted strong copper-complexing agents (“K > 108) and that eucaryotic algae only released weak copper-complexing agents (“K < lo75 McKnight and Morel 1979). We also detected bound hydroxamate in the blue-green algal culture media and found the conditional formation constant for a copper-siderophore complex ( 108e4) to be similar to constants for strong copper-cyanophycean exudate complex (lo*1012). These results led to the hypothesis l This work was supported in part by International Copper Research Association Project No. 252, by Office of Sea Grant (NOAA), 04-7158-44079, and by National Science Foundation grant OCE-7709000. 2 Present address: U.S. Geological Survey, Water Resources Division, Box 25046, Mail Stop 413, Denver Federal Center, Denver, Colorado 80225. that the strong copper-complexing agents produced by cyanophytes are hydroxamate siderophores. The work presented here, which tests this hypothesis, relies on what is already known about iron metabolism and siderophore production in microorganisms (Neilands 1973; E mery 1974). Besides cyanophytes, bacteria, fungi, and yeast have been found to produce either hydroxamate or phenolate siderophores (Neilands 1973). Th e role of siderophores in the uptake of iron by microorganisms was demonstrated in studies with iron mutants of Salmonella typhimuriu,m (Luckey et al. 1972) and Bacillus megate&m (Davis and Byers 1971). Simpson and Neilands (1976) first identified a trihydroxamate siderophore, called schizokinen, in media from iron-starved cultures of Anabaena sp. Bound hydroxamate, indicative of hydroxamate siderophores, was detected in laboratory cultures of Anabaena flos-aquae and field samples from an Anabaena bloom by Murphy et al. (1976). Evidence of Fe (III)-complexing agents has been found both in coastal blue-green algal mats and in sea grass beds by a bioassay tcchnique that uses a siderophore auxotroph, Arthrobacter JG-9 (Estep et al. 1975). Neilands (1973) provided a concise and useful outline of the mechanisms involved in microbial iron transport and