Coming to Terms: The Mechanisms of Overplus and Luxury Phosphorus Uptake for Polyphosphate Storage in Microalgae and Yeast.

Polyphosphate (polyP) synthesis is ubiquitous in organisms, including microorganisms such as microalgae and yeasts, playing a crucial role in phosphorus (P) metabolism (e.g., storage) and in other vital functions. Two mechanisms are broadly recognized for intracellular polyP accumulation: (i) The first involves a response to experiencing P repletion following a period of P depletion, often described as an overplus response because cells accumulate P at levels above what is considered physiologically "normal" if they had never experienced P depletion. PolyP accumulation likely occurs to maintain intracellular P homeostasis, with stored P supporting cell growth if extracellular P becomes depleted again. (ii) Contrarily, the second mechanism of polyP accumulation is not directly related to a change in P supply but is triggered by other conditions, such as the depletion of another nutrient. Because its purpose is less clear, this mechanism is often called "luxury P uptake." Here, we briefly review concepts in polyP synthesis in microalgae and yeast as reasoning for differentiating the mechanisms described above based on environmental P availability, polyP chemistry, genetics, or cellular function. Based on this knowledge, we suggest a common terminology to be used across the different fields to allow comparability and avoid misunderstandings.