Microoxic-Anoxic Niche of Beggiatoa spp.: Microelectrode Survey of Marine and Freshwater Strains

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Abstract
Beggiatoa spp. grow optimally in media containing opposed gradients of oxygen and soluble sulfide, although some strains also require an organic substrate. By using microelectrodes, we characterized oxygen and sulfide gradients during their initial development in uninoculated media and in cultures of marine and freshwater strains. In gradient media, Beggiatoa strains always grew some distance below the air/agar interface as a dense “plate” of constantly gliding filaments with sharply demarcated upper and lower boundaries. Within established plates, the maximum oxygen partial pressure was 0.6 to 6.0% of air saturation and not significantly lower if filaments were fixing nitrogen. Oxygen penetrated only 100 to 300 μm into the plate, and the anoxic fraction increased from less than 10% to approximately 90% during later stages of growth. For lithoautotrophically grown marine strains, the linearity of the oxygen profile above the plate plus its drop to zero therein indicated that oxygen uptake for the entire tube occurred only within the Beggiatoa plate. Consequently, oxygen consumption could be predicted solely from the distance between the air/agar interface and the top of a plate, given the diffusion coefficient for oxygen. By contrast, for freshwater strains grown heterotrophically (with sulfide also in the medium), oxygen profiles were frequently nonlinear because of nonbiological reaction with sulfide which had diffused past the aggregated filaments. For all strains tested, microoxic aggregation also occurred in the absence of sulfide, apparently reflecting a step-up phobic response to oxygen.