Uptake of Organic Substrates by Cyclotella Cryptica (Bacillariophyceae): Effects of Ions, Ionophores and Metabolic and Transport Inhibitors¹,²

The uptake of glucose and amino acids by the euryhaline diatom Cyclotella cryptica Reimann, Lewin & Guillard does not appear to be related to proton gradients. Instead, the transport systems for these organic solutes show a strong requirement for the presence of NaCl. The relationship between uptake and NaCl concentration is hyperbolic, with optimal uptake rates being approached at 100 mM NaCl. High concentrations of KCl cause strong reductions in uptake rates. The (Na + , K + )‐stimulated ATPase inhibitor ouabain has no effect on glucose uptake, whereas the diphenolic glucoside phlorizin and its aglucone phloretin are strongly inhibitory. The proton translocating uncoupler CCCP (carbonylcyanide m‐ chlorophenyl hydrazone) and the ATPase inhibitor DCCD (dicyclohexylcarbodiimide) both almost completely abolish glucose transport, and low concentrations of the ionophares monensin and valenomycin strongly inhibit glucose uptake by the diatom. The requirement of high external NaCl concentrations for glucose transport, and the inhibitory effect an transport of the Na + ‐specific ionophore monensin are consistent with a coupling of Na + and organic substrate transport, but could also be explained by a Na + requirement for glucose binding to a transport carrier, and/or a possible interference with energy producing reactions associated with a monensin‐induced collapse of the normal Na + gradient.