Revisiting the dilution technique to quantify the role of microzooplankton in DMS(P) cycling: laboratory and field tests

Abstract

With the aim of re-examining the suitability of the dilution technique in studies of dimethylated sulfur cycling in the sea, we conducted two types of experiments: one with a simplified food chain of cultured organisms, and secondly with field-sampled natural communities. In the case of Isocrhysis galbana being grazed by Oxyrrhis marina, algal growth (μ) and grazing mortality (m) rates calculated using either chlorophyll a (Chla) or particulate dimethylsulfoniopropionate (DMSPp) were very similar. By means of a parallel incubation of prey with no predator, the regression-derived phytoplankton μ was validated. Net 24-h changes in DMS concentration showed a linear trend against DMSP grazing rates, with a slope that allowed estimation of the rate of DMS production due to grazing. When the method was applied to natural communities from coastal NW Mediterranean waters, μ values of the bulk Chla-containing phytoplankton assemblage were higher than that of the DMSP-containing algae, but m were not significantly different. The grazing-mediated DMS production rate was successfully estimated, and parallel incubations with and without a specific inhibitor of bacterial DMS metabolism allowed us to estimate the gross DMS production by the whole community and the bacterial DMS consumption rate. A further experiment showed that bacterial consumption was not affected by dilution.