Feasibility of 55Fe Autoradiography as Performed on N2-Fixing Anabaena spp. Populations and Associated Bacteria

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RESUMO

55Fe emits low-energy X rays and Auger electrons by electron capture decay. Auger electrons are useful for autoradiographic examination of 55Fe incorporation among microbial communities. Attainable resolution, in terms of silver grain deposition, is excellent and comparable to 3H. Two known Fe-demanding processes, photosynthetic CO2 fixation and N2 fixation, were examined by autoradiography of Anabaena populations. During photosynthetically active (illuminated) N2-fixing periods, biological incorporation of 55FeCl3 by vegetative cells and heterocysts was evident. When N2 fixation was suppressed by NH4+ additions, heterocysts revealed no incorporation of 55Fe. Conversely, when N2-fixing Anabaena filaments were placed in darkness, 55Fe incorporation decreased in vegetative cells, whereas heterocysts showed sustained rates of 55Fe incorporation. Bacteria actively incorporated 55Fe under both light and dark conditions. The chelated (by Na2-ethylenediaminetetraacetate) form of 55FeCl3 was more readily incorporated than the nonchelated form. Furthermore, abiotic adsorption of 55Fe to filters and nonliving particles proved lower when chelated 55Fe was used in experiments. 55Fe autoradiography is useful for observing the fate and cellular distribution of various forms of Fe among aquatic microbial communities.

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