Does the red tide-forming dinoflagellatenoctiluca scintillansfeed on bacteria?

Investigations were made to detennine if Noctiluca scintillans is able to remove and ingest bacteria and thus influence bacterial numbers in the water column, before and after a Noctiluca ‘bloom’ (red tide). Laboratory experiments were perfonned with bacteria-sized fluorescent latex microspheres of 0.5., 1.1, and 4.5µm diameter, as well as with live cells of the bacteria Vibrio sp. and Serratiaplymuthica. Results indicate that Noctiluca can remove microspheres and bacteria at a rate of 104 to 106 individual-1 hour-1, under the given laboratory conditions. Data on abundance of Noctiluca, different groups of bacteria, yeasts, nanoflagellates, and microbial exoenzyme activity in the southern North Sea are compared. Although removal of bacteria by Noctiluca in the field is generally not reflected in the occurrence of these organisms, it is possible that Noctiluca ingests bacteria in the winter when other food sources are scarce. During its exponential growth, Noctiluca may remove bacteria from the water column together with other potential and actual food organisms trapped in its mucoid webs.