Glycolysis has been generally accepted to be the source of reducing power used for the synthesis of polyhydroxyalkanoates (PHAs) in the anaerobic metabolism of polyphosphate accumulating organisms (PAOs). However, the tricarboxylic acid cycle (TCA) has also been suggested to contribute to the generation of reducing equivalents, creating some controversy in this research field over the last two decades. Various research approaches have been applied in order to clarify this issue, including the analysis of intracellular compounds, labelled carbon substrates and gene expression. However, the conclusions of these studies seem not to be consolidated. The extent of TCA cycle involvement could be related to the experimental methods employed, the community structure of the cultures used, and also the operational conditions employed. This mini-review analyses the historical findings related to the source of reducing power in PAOs, and highlights the different approaches used in the previous studies. Key factors influencing the generation of reducing power through different metabolic pathways are discussed, and further research directions are also proposed.

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