Chameleon-like Anammox Bacteria for Surface Color Change after Suffering Starvation.

Bacteria are often exposed to long-term starvation during transportation and storage, during which a series of enzymes and metabolic pathways are activated to ensure survival. However, why the surface color of the bacteria changes during starvation is still not well-known. In this study, we found black anammox consortia suffering from long-term starvation contained 0.86 mmol gVSS -1 cytochrome c , which had no significant discrepancy compared with the red anammox consortia ( P > 0.05), indicating cytochrome c was not the key issue for chromaticity change. Conversely, we found that under starvation conditions cysteine degradation is an important metabolic pathway for the blackening of the anammox consortia for H 2 S production. In particular, anammox bacteria contain large amounts of iron-rich nanoparticles, cytochrome c , and other iron-sulfur clusters that are converted to produce free iron. H 2 S combines with free iron in bacteria to form Fe-S compounds, which eventually exist stably as FeS 2 , mainly in the extracellular space. Interestingly, FeS 2 could be oxidized by air aeration, which makes the consortia turn red again. The unique self-protection mechanism makes the whole consortia appear black, avoiding inhibition by high concentrations of H 2 S and achieving Fe storage. This study expands the understanding of the metabolites of anammox bacteria as well as the bacterial survival mechanism during starvation.