Effect of calcium ion on uptake of amino acids by symbiotic Chlorella F36-ZK isolated from Japanese Paramecium bursaria

Symbiotic Chlorella F36-ZK isolated from Paramecium bursaria F36 lacks nitrate reductase activity and has constitutive amino acid transport systems. Divalent cations generally accelerate amino acid uptake and Ca2+ affects membrane permeability and H+-ATPase activity. However, Ser uptake by F36-ZK was decreased by divalent cations, especially Ca2+, and the cation had no effect on membrane permeability or H+-ATPase activity. Extracellular Ca2+ contributed to inhibit Ser uptake. The cation selectively inhibited uptake of Ser, Ala and Gln, which are transported via the same system, but not Arg. Based on kinetic analysis of Ser transport, inhibition was noncompetitive, i.e., Ca2+ affected only the Vmax of Ser uptake. Calcium ions were found in symbiotic Chlorella cells associated with their host by transmission electron microscopy using an oxalate-pyroantimonate method, suggesting the transfer of Ca2+ during symbiosis. From these results obtained in this paper, we discuss the role of divalent cations on amino acid transport in the Paramecium symbiosis.