In vivo DMSP‐biosynthesis measurements using stable‐isotope incorporation and proton‐transfer‐reaction mass spectrometry (PTR‐MS)

Abstract

Assessing turnover rates of dimethylsulfoniopropionate (DMSP), precursor of the climate‐active gas dimethylsulfide (DMS), under different environmental conditions is fundamental to accurately modeling the marine sulfur cycle. Current gas chromatographic methods only provide net results of all processes acting on the different sulfur pools and are unable to measure fluxes, leaving room for speculation on the controlling factors and magnitudes of turnover processes. Here we present a new method in which stable isotope additions (D2O or NaH13CO3) are used to follow the in vivo production of DMSP. Incorporation of deuterium or 13C into DMSP was monitored using a proton‐reaction‐transfer mass spectrometer (PTR‐MS). A growth model was developed to calculate specific DMSP synthesis rates from the progress in mass ratio of labeled versus nonlabeled DMSP over time. Instead of measuring the amount of incorporated isotope, our method uses ratios of molecule masses. This approach has the advantage that the effect of the isotope addition on the change in mass ratio is amplified by the number of positions the label can occupy within the molecule. Application of the method was demonstrated in cultures of the ubiquitous DMSP producer Emiliania huxleyi and further developed for use in short‐time incubation studies with water from the Sargasso Sea during July 2004. Very low specific synthesis rates between 0.05 and 0.2 d−1 could be measured. The method is the first to measure in vivo DMSP production rates. It is relatively easy to apply, highly sensitive, and may also be applicable to other biogenic compounds.