| Abstract: | Laboratory batch cultures of the unicellular alga Selenastrum capricornutum Printz (Chlorophyceae) were used as a model system to test the hypothesis that the regulatory molecule cAMP may alter algal photo synthetic carbon assimilation (PCA) rates via an electrogenic mechanism. In one series of experiments, cells were resuspended in cAMP-free media subsequently augmented with 0.5 nmol·L?1to 0.1 μmol·L?1of added cAMP, concentrations paralleling dissolved cAMP levels in lake water and culture filtrates but insufficient to result in short-term increases in intracellular cAMP content. cAMP-treated cells were moderately and transiently hypopolarized concomitant with a reduction in PCA capacity. Media and intracellular pH and quinacrine fluorescence quenching rates were also altered, with similar effects on PCA, but specific effects appeared to be associated with diel cycles. Higher concentrations of added cAMP (10 μmol·L?1to 10 mmol·L?1) further hypopolarized the transmembrane potential, greatly enhanced PCA rates, acidified the cell interior, and enhanced quinacrine quenching rates. With the exception of cGMP, no nucleotide derivative examined elicited the same effects. Inhibitors of plasmalemma ATPase and NADH oxidase activity also hypopolarized the membrane potential but inhibited PCA rates. Collectively, the data support the cAMP-electrogenic thesis, but the mechanism, although resembling features of H+/ ion-cotrans-port, may in fact be more complex. |
