Cells of Cylindrotheca fusiformis, a marine littoral pennate diatom, can be induced to take up lactate by merely keeping them in the dark, and to take up succinate, fumarate, and malate as well as lactate by keeping them in the dark in the presence of any of these organic acids. Studies of uptake rates as a function of substrate concentration show a complex relationship indicating the presence of two or more uptake systems for any of these organic acids, each having different affinities and capacities for the substrates. Growth rate also shows a similar complex relationship to substrate concentration.Competition experiments indicate that lactate is taken up by different transport systems from those serving the 4-carbon dicarboxylic acids. Pyruvate is not taken up by C. fusiformis. Correlation of activities of enzymes involved in the immediate assimilation of the organic acids with the presence or absence of the ability to take up these substrates by cells incubated under different conditions show that the necessary assimilatory enzymes are already present and that it is rather the transport systems that are induced. The presence of glucokinase in dark-grown cells indicates that the inability of the cells to take up glucose is due to the lack of induction of a glucose transport system.The metabolic inhibitors dinitrophenol and iodoacetate inhibit the uptake of organic acids. However, since under normal conditions the organic acids are rapidly converted to other metabolites upon entry into the cells, and do not appear to accumulate, the observed inhibition may be at the level of assimilation rather than transport. The uptake of lactate and succinate is temperature dependent with a Q10 of 1.5–1.7, but is almost unaffected by changes in salinity from 10 to 150% of normal seawater. The relation between pH and rates of uptake of lactate and succinate suggests that the completely dissociated form of the organic acids is transported.
Variation of Abundance and Demographic Structure of the Introduced Pumpkinseed, Lepomis Gibbosus (Actinopterygii: Perciformes: Centrarchidae), in Relation to Littoral Habitats and Water Body Use
