What biological condition must exist for PAOs to accumulate phosphorus effectively?

For phosphorus-accumulating organisms (PAOs) to effectively accumulate phosphorus, an anaerobic environment is essential. PAOs thrive in conditions where oxygen is absent. In these anaerobic conditions, they utilize stored energy sources to take up organic carbon, which enables them to release phosphorus in the process.

This is in contrast to what happens in aerobic environments, where the presence of oxygen favors different microbial processes that do not allow for the same accumulation of phosphorus. High light intensity is not relevant to PAOs, as their function is not dependent on light but rather on the anaerobic process. Similarly, high nitrate concentrations do not directly contribute to the accumulation of phosphorus by PAOs; instead, they may affect other types of microorganisms that operate differently from PAOs.

Thus, an anaerobic environment is crucial for PAOs to effectively accumulate phosphorus, as it is during this phase that they breakdown organic matter and start the process of phosphorus uptake.