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Here we report on a new nanoscale secondary ion mass spectrometry (nanoSIMS) approach based on enzyme-mediated oxygen isotope exchange, which combines the visualization of general metabolic activity in the cytoplasm with insights into the activity of enzymes related to polyphosphate (polyP) inclusions. The polyP-accumulating strain of the large sulfur bacterium Beggiatoa was used as a model organism. Beggiatoa cultures were grown under oxic and anoxic conditions when exposed to either low- or high-sulfide conditions, which are known to influence polyP metabolism in this strain. Subsequent incubation with 18 O-labeled water led to high 18 O enrichments above the natural background in the cytoplasm and polyP granules derived from enzymatically mediated oxygen isotope exchange. The relative importance of polyP under the different sulfide regimes became evident by an apparent continued metabolic activity at polyP inclusions under stressfully high sulfide concentrations, in contrast to a decreased general metabolic activity in the cytoplasm. This finding confirms the role of polyP as a critical component in bacterial stress response and maintenance of a survival metabolism. IMPORTANCE Microbial organisms exert a large influence on the environment as they directly affect the turnover of essential elements. This is particularly true for polyphosphate-accumulating large sulfur bacteria, which can either accumulate phosphate as polyphosphate or degrade it and release phosphate into the environment, depending on environmental conditions. This study presents a new approach to simultaneously visualize general metabolic activity and enzymatic activity at polyphosphate granules by incubation with 18 O-labeled water as the only stable isotope tracer. For this purpose, the well-studied Beggiatoa sp. strain 35Flor was used as a model organism and was exposed to different stress regimes. General metabolic activity was strongly impaired during high-stress regimes. In contrast, intense intracellular polyP cycling was not restricted to favorable or stressful conditions, highlighting the importance of polyP for general cell physiology, especially during hostile conditions. The nanoSIMS approach adds a new tool to study microorganisms involved in phosphorus cycling in the environment together with the identification of general metabolic activity.

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Simultaneous Visualization of Enzymatic Activity in the Cytoplasm and at Polyphosphate Inclusions in <i>Beggiatoa</i> sp. Strain 35Flor Incubated with <sup>18</sup> O-Labeled Water

Simultaneous Visualization of Enzymatic Activity in the Cytoplasm and at Polyphosphate Inclusions in Beggiatoa sp. Strain 35Flor Incubated with 18 O-Labeled Water

Simultaneous Visualization of Enzymatic Activity in the Cytoplasm and at Polyphosphate Inclusions in Beggiatoa sp. Strain 35Flor Incubated with ¹⁸ O-Labeled Water