Uptake, exchange and excretion of orthophosphate in phosphate-starved Scenedesmus quadricauda and Pseudomonas K7

Abstract

Phosphate uptake was investigated in batch cultures of P-starved green algae (Scenedesmus quadricauda) and bacteria of pelagic origin (Pseudomonas K7). Phosphate transport in Scenedesmus was characterized by initial rapid surge uptake enhanced by P starvation, followed by subsequent slower uptake. Two systems, defined as high-affinity (HA) and low-affinity (LA) systems, were found in Scenedesmus. The HA system had high affinity for low phosphate concentrations and bound phosphate in exchangeable form in the cell. Exchangeable phosphate was transformed to nonexchangeable phosphate by the LA system. The phosphate uptake in Pseudomonas followed Michaelis-Menten kinetics and no exchangeable phosphate could be detected. Pseudomonas outcompeted Scenedesmus for low concentrations of phosphate and also for high concentrations if glucose was present. Scenedesmus took up phosphate at about the same rate as Pseudomonas at moderately low concentrations in mixed cultures without glucose. The phosphate transport by the HA system in Scenedesmus was not affected by competition with Pseudomonas at very low phosphate concentrations. It is suggested that the HA system makes the algae competitive with bacteria for low concentrations of phosphate. Both Scenedesmus and Pseudomonas excreted considerable amounts of orthophosphate, which release is suggested to be connected with growth.