Iron-induced alterations of bacterial DMSP metabolism in the western subarctic Pacific during SEEDS-II

The effect of added iron on bacterial cycling of the climate-active gas dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) was tested during the second Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS II) from 19 July to 21 August 2004 aboard the R/V Hakuho-Maru. The study area in the northwest Pacific Ocean (48°N 165°E) was enriched with Fe and the conservative tracer, SF₆, allowing the fertilized patch to be tracked. Microbial DMSP cycling rates were determined in the surface mixed layer (5 m) during incubations using the ³⁵S-DMSP technique. The addition of iron resulted in a 4-fold increase in concentrations of chlorophyll a (chl a) within the surface mixed layer (5 m depth), and the length of the sampling period allowed the observation of both bloom and post-bloom conditions. Inside the fertilized patch, the alleviation of resource limitation gave rise to the concurrent increase in bacterial abundance and production. Changes in the phytoplankton community within the Fe-enriched patch translated into a sustained decrease in chl a-normalized particulate DMSP (DMSP_p) concentrations, suggesting a preferential stimulation of the growth of DMSP_p-poor phytoplankton species. Despite short-lived peaks of DMSP_p within the Fe-enriched area, concentrations of DMSP_p generally remained stable during the entire sampling period inside and outside the fertilized patch. During the Fe-induced bloom, microbial DMSP-sulfur (DMSP-S) assimilation efficiency increased 2.6-fold inside the Fe-enriched area, which indicated that as bacterial production increased, a greater proportion of DMSP-S was assimilated and possibly diverted away from the bacterial cleavage pathway (i.e. production of DMS). Our results suggest that iron-induced stimulation of weak DMSP_p-producers and DMSP-assimilating bacteria may diminish the potential production of DMS and thus limit its flux towards the atmosphere over the subarctic Pacific Ocean.