Ciliate Uronema marinum feeding influences the production of dimethyl sulfur compounds

Dimethyl sulfide (DMS), a climatically important biogenic gas, is produced from the transformation of dimethylsulfoniopropionate (DMSP) and dimethyl sulfoxide (DMSO). While zooplankton grazing is known to promote DMS release, the impacts of ciliated protozoa remain less understood. Here, we studied the effects of feeding behavior of ciliates Uronema marinum on DMS, DMSP, and DMSO generation. The diet, diet concentration, temperature, and salinity experiments showed that the presence of U. marinum increased DMS concentrations by a factor of 2 to 16 while simultaneously decreasing dissolved and particulate DMSP (DMSP_d,p) concentrations. The mean conversion efficiency of grazed DMSP_p to DMS(P_d) was 56%. However, the effect of grazing activity on dissolved and particulate DMSO (DMSO_d,p) concentrations was minor, which may be due to non-activation of DMSO reductase. Furthermore, the diet, temperature, and salinity controlled the ciliate ingestion rates (IRs), and thereby affected DMS production. The increase in DMS concentration was positively correlated with the ciliate IRs. Specifically, the optimal conditions for IRs and growth rates (GRs) of U. marinum were feeding on Isochrysis galbana at 20 °C and salinity 30-32. DMS production was highest (48.48 nmol/L) when fed Prorocentrum donghaiense and lowest (28.4 nmol/L) when fed Phaeodactylum tricornutum. Rising temperatures increased DMS but decreased DMSP_d,p. Rising salinity (peaking at 30-32) also increased DMS, while DMSP_d,p and DMSO_d,p increased with salinity in controls as osmolytes. These findings show that temperature, salinity, and prey characteristics regulate grazing-driven sulfur transformations by modulating algal DMSP/DMSO pools, ciliate physiology, and enzyme activities-key processes determining DMSP and DMSO fate within the microbial food web.