Characterization of polyphosphate dynamics in the widespread freshwater diatom Achnanthidium minutissimum under varying phosphorus supplies

Polyphosphates (polyP) are ubiquitous biomolecules that play a multitude of physiological roles in many cells. We have studied the presence and role of polyP in a unicellular alga, the freshwater diatom Achnanthidium minutissimum. This diatom stores up to 2.0 pg·cell⁻¹ of polyP, with chain lengths ranging from 130 to 500 inorganic phosphate units (P_i). We applied energy dispersive X-ray spectroscopy, Raman/fluorescence microscopy, and biochemical assays to localize and characterize the intracellular polyP granules that were present in large apical vacuoles. We investigated the fate of polyP in axenic A. minutissimum cells grown under phosphorus (P), replete (P₍₊₎), or P deplete (P₍₋₎) cultivation conditions and observed that in the absence of exogenous P, A. minutissimum rapidly utilizes their internal polyP reserves, maintaining their intrinsic growth rates for up to 8 days. PolyP-depleted A. minutissimum cells rapidly took up exogenous P a few hours after P_i resupply and generated polyP three times faster than cells that were not initially subjected to P limitation. Accordingly, we propose that A. minutissimum deploys a succession of acclimation strategies regarding polyP dynamics where the production or consumption of polyP plays a central role in the homeostasis of the diatom.