Bacterial DMSP metabolism during the senescence of the spring diatom bloom in the Northwest Atlantic

The impact of the decline of the vernal bloom on the bacterial metabolism of dimethyl- sulfoniopropionate (DMSP), the precursor of dimethyl- sulfide (DMS), was investigated during a 7 d Lagran- gian study conducted in the Northwest Atlantic in spring 2003. Daily variations in bacterial leucine incor- poration, dissolved DMSP (DMSPd) uptake and DMS production rates were measured in the surface mixed layer (SML) and in the deep chlorophyll a maximum (DCM) that formed as the bloom collapsed. Seawater samples were amended with 35 S-DMSPd, and the prod- ucts of bacterial DMSPd degradation were measured during 3 h on-board incubations. The gradual decrease in phytoplankton biomass and diatom abundance measured in the SML was accompanied by a sharp doubling of the bacterial abundance and a peak in leucine incorporation rate on Day 2, suggesting that bacteria responded to a transient pulse in dissolved organic matter. Bacterial DMSPd uptake and DMS production were highest on Days 1 and 2 (1.2 and 0.10 nmol l -1 h -1 , respectively), but rapidly decreased by Day 3, suggesting that DMSPd was becoming a less important substrate for the growing bacterial assem- blage as other substrates became available. Bacterial DMSPd uptake and DMS production rates were also low in the DCM despite very high DMS yields (40 to 50%), showing that neither the decline of the diatom spring bloom in the SML nor the accumulation of cells in the DCM resulted in a stimulation of bacterial DMSP metabolism or accumulation of DMS. The pre- sent study provides new field evidence for the po- tential uncoupling between bacterial production and DMS dynamics likely due to variations in the availabil- ity of other S-containing organic compounds released during the decay of phytoplankton blooms.