Assessment of DMSP turnover reveals a non-bioavailable pool of dissolved DMSP in coastal waters of the Gulf of Mexico

Abstract

Environmental context. DMSP is one of the most important substrates for marine bacteria and its cycling contributes substantially to fluxes of carbon and sulfur in the ocean. Accurate determination of the concentration of DMSP available to bacteria is essential to quantifying DMSP consumption rates, and this work improves those determinations by identifying non-bioavailable pools of DMSP that have previously gone unrecognised. Improved estimates of DMSP consumption rates will lead to better understanding of its role in ocean food web and biogeochemical dynamics.Abstract. Dissolved dimethylsulfoniopropionate (DMSPd) is an important substrate for marine microbes and a precursor of sulfur gases. We compared DMSPd turnover flux rates in coastal seawater measured with a 35 S-DMSPd tracer to those obtained with the DMSP-uptake inhibitor glycine betaine (GBT). The 35 S-DMSP tracer method yielded DMSPd turnover fluxes (35.7-215 nM day À1 ) that were 1.7 to 152 times higher than those obtained in parallel samples with the GBT inhibitor method (0.34-21.6 nM day À1 ). Tests confirmed that GBT functioned as planned by strongly inhibiting DMSPd degradation and that 35 S-DMSPd gave accurate estimates of DMSPd loss rate constants. This left the initial DMSPd concentrations, determined by small volume drip filtration (SVDF) through Whatman GF/F filters (0.7-mm nominal retention) ([DMSPd] SVDF ), as a potential cause of the discrepancy in rate estimates. Indeed, GF/F filtrate incubations showed that the initial [DMSPd] SVDF overestimated the bioavailable DMSPd concentrations for at least two reasons: (1) a significant fraction (10-37 %) of DMSP passing through GF/F filters was in particles .0.2 mm (likely bacteria) and therefore not dissolved, and (2) a significant pool (0.44-1.0 nM) of operationally dissolved, non-particle DMSP ([DMSPd] ,0.2 mm ), comprising 40-99 % of [DMSPd] SVDF , was refractory to degradation on a time scale of days. The nature of this refractory DMSP is currently unknown. Accounting for DMSP-containing particles and the refractory DMSP pool in GF/F filtrates is necessary to obtain the true bioavailable DMSPd concentrations, which we estimate to be very low (0.006-1.0 nM; mean of 0.41 nM) in the coastal waters examined, and to avoid overestimation of DMSPd turnover fluxes when using the 35 S-DMSP tracer technique.

Spatial Distribution and Biogeochemical Cycling of Dimethylated Sulfur Compounds and Methane in the East China Sea During Spring

“…Water samples were incubated at the in situ temperature in 1‐L Bioprocess Containers with 5 μmol/L glycine betaine (GBT). GBT is structurally similar to DMSP and is often used as a competitive inhibitor to study the microbial consumption of DMSP (Kiene & Gerard, ; Kiene et al, ; C. Li et al, ). The containers for DMSP degradation were placed in a dark incubator at the in situ temperature.…”

Spatial and Interannual Variability in Distributions and Cycling of Summer Biogenic Sulfur in the Bering Sea

“…The analytical precision was better than 5%, whereas the absolute detection limit of the gas chromatography system was 1 pmol of DMS per injection. The production and consumption rates of DMS and DMSPd in seawater were determined by the inhibitor addition method (Li et al, ). A complete description of measurements above was presented by Li et al ().…”

Occurrence and Turnover of Biogenic Sulfur in the Bering Sea During Summer

“…For the biological production and consumption of DMS, duplicate unamended bottles were incubated as control, along with a separate set of duplicate bottles that were amended with ∼200 μ M dimethyldisulfide (DMDS, 98% purity; Sigma Chemical Co.), which is an effective inhibitor of bacterial DMS consumption (Wolfe & Kiene, ). In parallel with the DMDS inhibitor experiments, for the DMSPd degradation determination, one set was left untreated, and the other set was treated with glycine betaine (GBT, 99% purity; Sigma Chemical Co.), which is an effective inhibitor for blocking DMSP consumption (Li et al, ), to a final concentration of 5 μ M . The production and consumption rates of DMS and DMSPd were determined by incubating a series of Teflon bottles for 6 h (total incubation time) in the dark with continuous flow from seawater intake to maintain the temperature close to that of the sampling depth.…”

“…Furthermore, a part of the intracellular DMSP is cleaved directly into DMS via bacterial or algal DMSP‐lyases (Stefels, ; Stefels et al, ). Once in the water column, dissolved DMSP (DMSPd, Li et al, ) can be utilized by bacteria as sulfur, carbon, or energy source by the demethylation/demethiolation (de/de) pathways (Moran et al, ) or enzymatically cleaved to DMS (Keller et al, ). The non DMS‐producing de/de pathway, which converts approximately 70% of dissolved DMSP into 3‐methyl‐propionate and subsequently to methanethiol and 3‐mercaptopropionate, regularly dominates DMSP metabolism and is preferred over the cleavage route, accounting for 9%–21% of DMSPd consumption when the S and C demands of the bacterial community are high (Kiene et al, ; Moran et al, ; Simó et al, ).…”