Hot Water Extractable Phosphorus—an Indicator of Nutritional Status of Peridinium Cinctum (Dinophyceae) From Lake Kinneret (Israel)?1

The intracellular levels of hot water extractable and total phosphorus were determined in the dinoflagellate Peridinium cinctum. f. westii (Lemm.) Lef. for natural samples from the bloom in Lake Kinneret and from laboratory cultures. Amounts of phosphorus (P) in the hot water fraction, relative to total cellular phosphorus, were similar in lake Peridinium and in cells grown in high ambient orthophosphate (Pi) media (3–6 mg P · l−1). The absolute amounts of hot water extractable P in natural cell and those cultured at lower Pi concentrations (0.02–0.05 mg P · 1−1) were similar, although average Pi in lake water were 4 μg · l−1. Under most growth conditions the hot water extract contained approximately equal amounts of molybdate reactive phosphorus (MRP) and non‐MRP. Short chain (6–9 units) polyphosphates (mol wt 630–950) probably constituted the bulk of the non‐MRP pool, which was hydrolysable by alkaline phosphatase and may serve as a precursor for a more permanent P store. Intracellular P levels and distribution were not directly dependent on external Pi concentrations but may be determined by the N:P atomic ratio or overall external ionic milieu. Peridinium grown in low ambient Pi released significant amounts of non‐MRP compounds. In Lake Kinneret, for at least most of the bloom period, Peridinium does not appear to be limited by P supply.