Feasibility of Fe Autoradiography as Performed on N(2)-Fixing Anabaena spp. Populations and Associated Bacteria.

Fe emits low-energy X rays and Auger electrons by electron capture decay. Auger electrons are useful for autoradiographic examination of Fe incorporation among microbial communities. Attainable resolution, in terms of silver grain deposition, is excellent and comparable to H. Two known Fe-demanding processes, photosynthetic CO(2) fixation and N(2) fixation, were examined by autoradiography of Anabaena populations. During photosynthetically active (illuminated) N(2)-fixing periods, biological incorporation of FeCl(3) by vegetative cells and heterocysts was evident. When N(2) fixation was suppressed by NH(4) additions, heterocysts revealed no incorporation of Fe. Conversely, when N(2)-fixing Anabaena filaments were placed in darkness, Fe incorporation decreased in vegetative cells, whereas heterocysts showed sustained rates of Fe incorporation. Bacteria actively incorporated Fe under both light and dark conditions. The chelated (by Na(2)-ethylenediaminetetraacetate) form of FeCl(3) was more readily incorporated than the nonchelated form. Furthermore, abiotic adsorption of Fe to filters and nonliving particles proved lower when chelated Fe was used in experiments. Fe autoradiography is useful for observing the fate and cellular distribution of various forms of Fe among aquatic microbial communities.