Release of dissolved free and combined amino acids by bacterivorous marine flagellates

We measured the release of dissolved free (DFAA) and combined (DCAA) amino acids and ammonium by a microflagellate (Paraphysomonas imperforata) grazing on bacteria. The release rate of DFAA was determined from changes in DFAA concentrations and uptake rates of DFAA. Exponentially growing flagellates released 4–22% of ingested nitrogen as DFAA. The release of DFAA by flagellates was comparable to or higher than ammonium release when ingestion was high, but lower than ammonium release when ingestion was low. DCAA accumulated during the exponential growth phase of flagellates, suggesting that flagellates released DCAA. We also measured the release rate of 3H‐labeled compounds by flagellates grazing on bacteria labeled with [3H]amino acids. The results suggested that flagellates release significant amounts of dissolved organic matter during the exponential growth phase, consistent with the measurements of amino acid release. Bacterivorous flagellates could be an important source of DFAA and DCAA in aquatic environments.