The model high molecular weight DOC compound, dextran, is ingested by the benthic ciliate Uronema marinum but does not supplement ciliate growth

Abstract

Phagotrophic ciliates are capable of growth solely on dissolved compounds in laboratory cultures. Whether ciliates use dissolved compounds in the environment for growth is unclear. We investigated the ability of the marine benthic ciliate Uronema marinum to ingest a model high mole- cular weight dissolved organic carbon (HMW-DOC) compound, dextran, at concentrations typical for coastal salt marsh sediments (3 µM to 3 mM C). Ingestion was measured by incubating ciliates with fluorescein-labeled dextran (2000 kDa) and measuring the fluorescence signal of the labeled com- pound in cells via flow cytometry. Ciliates accumulated dextran (relative to formalin-killed controls) at concentrations as low as 0.1 mg l -1 dextran (3 µM C). Labeled dextran accumulated in food vac- uoles and near the buccal cavity; thus, the ingestion of dextran appears to be a consequence of feed- ing activities rather than transport across the cell membrane via parasomal sac formation. Dextran accumulation did not increase with higher bacterial ingestion rates. Instead, dextran accumulation was greatest at intermediate bacterial concentrations and grazing rates. Ciliate growth rates were measured in treatments amended with model carbon compounds—soluble starch, acetate, and glu- cose (3 mM C, final concentrations). There was no significant increase in ciliate growth rates with these compounds in either bacteria-free or bacteria-enriched treatments. Rather, growth rates were significantly lower in treatments with DOC addition, indicating that processing of these DOC com- pounds may incur some energetic cost to these ciliates.