Characteristics of maltose transport system in the endosymbiont Chlorella variabilis of Paramecium bursaria

Paramecium bursaria has hundreds of chlorella in the cell. These symbiotic chlorella cells supply maltose, a photosynthetic product, to the host. Only symbiotic chlorella retains the ability to release maltose to the extracellular space, whereas free‐living chlorella does not. To provide the host with maltose, a maltose transporter localized on the symbiotic chlorella plasma membrane is required. However, the molecular identity of the maltose transporter and its properties are unknown. To better understand the characteristics of the maltose transporter, we investigated how maltose release by Chlorella variabilis is affected by the excess external maltose and several agents that inhibit ATP synthesis or disrupt the ion gradient across cell membranes. Our overall findings, including the observation that C. variabilis did not utilize maltose in the growth medium, led to the conclusion that the maltose transporter is likely a unidirectional and active transporter, which utilizes a proton gradient across the plasma membrane and does not use ATP as its energy source.