Feeding and grazing impact by the bloom-forming euglenophyte <i>Eutreptiella eupharyngea</i> on marine eubacteria and cyanobacteria

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P>The phototrophic euglenophyte <I>Eutreptiella eupharyngea</I> often causes blooms in the coastal waters of many countries, but its mode of nutrition has not been assessed. This species has previously been considered as exclusively auxotrophic. To explore whether <I>E. eupharyngea</I> is a mixotrophic species, the protoplasm of <I>E. eupharyngea</I> cells were examined using light, epifluorescence, and transmission electron microscopy after eubacteria, the cyanobacterium <I>Synechococcus</I> sp., and diverse algal species were provided as potential prey. Furthermore, the ingestion rates of <I>E. eupharyngea</I> KR on eubacteria or <I>Synechococcus</I> sp. as a function of prey concentration were measured. In addition, grazing by natural populations of euglenophytes on natural populations of eubacteria in Masan Bay was investigated. This study is the first to report that <I>E. eupharyngea</I> is a mixotrophic species. Among the potential prey organisms offered, <I>E. eupharyngea</I> fed only on eubacteria and <I>Synechococcus</I> sp., and the maximum ingestion rates of these two organisms measured in the laboratory were 5.7 and 0.7 cells predator<SUP>−1</SUP> h<SUP>−1</SUP>, respectively. During the field experiments, the maximum ingestion rates and grazing impacts of euglenophytes, including <I>E. eupharyngea</I>, on natural populations of eubacteria were 11.8 cells predator<SUP>−1</SUP> h<SUP>−1</SUP> and 1.228 d<SUP>−1</SUP>, respectively. Therefore, euglenophytes could potentially have a considerable grazing impact on marine bacterial populations.</P> <P><B>Highlights</B></P> <P> <UL> <LI> This study reported for the first time that <I>E. eupharyngea</I> is a mixotrophic species. </LI> <LI> <I>E. eupharyngea</I> fed on only eubacteria and <I>Synechococcus</I> sp. </LI> <LI> Its maximum ingestion rates on eubacteria were 5.7 cells predator<SUP>−1</SUP> h<SUP>−1</SUP>. </LI> <LI> Maximum grazing coefficient by euglenophytes on eubacteria was 1.228 d<SUP>−1</SUP>. </LI> <LI> Euglenophytes may sometimes have a considerable grazing impact on bacteria. </LI> </UL> </P>