The algal osmolyte DMSP as a microzooplankton grazing deterrent in laboratory and field studies

Using laboratory cultures, microcosm experiments in northern Puget Sound and field experiments in the coastal Gulf of Alaska, the role of dissolved DMSP as a protist grazing deterrent was examined. DMSP (20 mM) added to laboratory cultures of two ciliates (Strombidinopsis acuminatum and Favella sp.) and one dinoflagellate (Noctiluca scintillans) caused a 28–75% decrease in feeding rates; decreases were concentration-dependent with 20 nM as the lower threshold for an effect. Partial but not complete recovery of grazing rates occurred during long-term ( 24 h) exposure to dissolved DMSP, as long as concentrations remained above 12 mM. Additions of 10–30 mM dissolved DMSP to coastal planktonic communities had either no effect or mildly stimulated both phytoplankton growth and microzooplankton grazing rates, probably due to the use of DMSP as a carbon source for the growth of bacteria along with direct and indirect stimulation of phytoplankton growth. However, DMSP did strongly inhibit grazing rates in three experiments. The sensitivity of these communities to the DMSP deterrent signal could have resulted from relatively high ambient dissolved DMSP levels, or from the presence of susceptible protist grazer species. Grazer inhibition as observed in the laboratory and lack of inhibition in natural communities may arise from grazer acclimation to DMSP during 24-h field incubations. Alternatively, inhibitory effects on some grazer species could have been masked by DMSP’s stimulatory effect on other portions of the food web. This study illustrates the dual role of DMSP as both a growth substrate and signaling molecule in coastal planktonic food webs.