Evidence for grazing-mediated production of dissolved surface-active material by marine protists

Surface-active organic compounds or surfactants play important roles in a variety of upper ocean processes, including surface microlayer physics and gas exchange and the aggregation of colloidal material. Although surfactants are presumed to be produced primarily by phytoplankton, production by protozoan grazers has not been investigated. In general, the processes controlling surfactant abundance in the field are poorly understood. In this study, a two-phase laboratory system containing protists and prey was used to examine the possibility of surfactant production during protozoan grazing. Three protist species were examined, a scuticociliate, Uronema sp. (10–15 μm), and two flagellates, Cafeteria sp. (2–4 μm), and Paraphysomonas imperforata (4–8 μm). For all experimental cultures, protozoan inocula were added to rinsed bacterial suspensions (Halomonas halodurans) in sterile seawater. Surfactants, dissolved organic carbon (DOC) and population dynamics were monitored until protists had reached stationary growth. Surfactant activities increased during protozoan exponential growth. Surfactant production in the ciliate cultures was significantly higher than in either of the flagellate cultures. Bacterial controls maintained low DOC concentrations and surfactant activities. Estimates of protozoan surfactant production rates range from 10−8 to 10−9 mg protist−1 h−1 (Triton X-100 equivalents). Under non-bloom conditions (103 protozoan cells ml−1), we estimated a surfactant production rate of 10−5–10−6 mg h−1 (within 1 ml of seawater), which is comparable to estimates of phytoplankton production of surface-active material during blooms. Thus, protozoan grazers constitute a potentially significant source of surface-active material in areas where protists are abundant, such as the sediment–water interface and microbial loop-dominated oligotrophic regimes.