Uptake of Guanine by the Diatom, Phaeodactylum Tricornutum¹

Nitrate‐cultured cells of Phaeodactylum tricornutum Bohlin lack the ability to take up guanine but can do so after a period of nitrogen deprivation, i.e. photosynthesis in nitrogen‐free medium. Maximum rate of uptake occurred after 24 h of nitrogen deprivation. The development of ability to take up guanine required CO 2 fixation and was prevented by cycloheximide, ammonium or nitrate. The guanine taken up accummulated in the cells almost entirely as a compound which is probably methylated hypoxanthine. Guanine uptake was dependent upon metabolism and exhibited Michaelis‐Menten like kinetics with a half‐saturation value of 0.48 ± 0.05 μM guanine and a maximum uptake rate for guanine of ca. 200 nmol · 10 −8 cells · h −1 . Rate of uptake increased hyperbolically with Na + concentration, with 8.25 mM Na + supporting half‐maximal rate, and it was inhibited by K + ions.