Biological Dimethylsulfide Consumption Research Articles

Revising upper-ocean sulfur dynamics near Bermuda: new lessons from 3 years of concentration and rate measurements

Environmental context Microscopic marine organisms have the potential to influence the global climate through the production of a trace gas, dimethylsulfide, which contributes to cloud formation. Using 3 years of observations, we investigated the environmental drivers behind the production and degradation of dimethylsulfide and its precursor dimethylsulfoniopropionate. Our results highlight the important role of the microbial community in rapidly cycling these compounds and provide an important dataset for future modelling studies. Abstract Oceanic biogeochemical cycling of dimethylsulfide (DMS), and its precursor dimethylsulfoniopropionate (DMSP), has gained considerable attention over the past three decades because of the potential role of DMS in climate mediation. Here we report 3 years of monthly vertical profiles of organic sulfur cycle concentrations (DMS, particulate DMSP (DMSPp) and dissolved DMSP (DMSPd)) and rates (DMSPd consumption, biological DMS consumption and DMS photolysis) from the Bermuda Atlantic Time-series Study (BATS) site taken between 2005 and 2008. Concentrations confirm the summer paradox with mixed layer DMS peaking ~90 days after peak DMSPp and ~50 days after peak DMSPp:Chl. A small decline in mixed layer DMS was observed relative to those measured during a previous study at BATS (1992–1994), potentially driven by long-term climate shifts at the site. On average, DMS cycling occurred on longer timescales than DMSPd (0.43±0.35 v. 1.39±0.76 day–1) with DMSPd consumption rates remaining elevated throughout the year despite significant seasonal variability in the bacterial DMSP degrader community. DMSPp was estimated to account for 4–5% of mixed layer primary production and turned over at a significantly slower rate (~0.2 day–1). Photolysis drove DMS loss in the mixed layer during the summer, whereas biological consumption of DMS was the dominant loss process in the winter and at depth. These findings offer new insight into the underlying mechanisms driving DMS(P) cycling in the oligotrophic ocean, provide an extended dataset for future model evaluation and hypothesis testing and highlight the need for a reexamination of past modelling results and conclusions drawn from data collected with old methodologies.

Biological consumption of dimethylsulfide (DMS) and its importance in DMS dynamics in the Ross Sea, Antarctica

We studied the biological consumption of dimethylsulfide (DMS) and its role in controlling DMS concentrations in the Ross Sea, Antarctica, during the spring (Nov) and summer (Jan) of 2005. Surface DMS concentrations, measured with a technique that minimized DMS release from Phaeocystis antarctica, increased rapidly in the spring from 0.3 nmol L−1 to 67.7 nmol L−1, paralleling increases in chlorophyll a and bacterial biomass production. Biological DMS consumption (BDMSC) rates were low (0.02 nmol L−1 d−1) at the start of the bloom, but increased to 8.8 nmol L−1 d−1 at the peak of the bloom. Rate constants for BDMSC (kbc) remained relatively low throughout the spring (0.05‐0.21 d−1) and this slow biological turnover contributed to the buildup of DMS during the early bloom. DMS concentrations in the summer (3.2‐16.8 nmol L−1) were much lower than peak springtime concentrations, partly due to the higher BDMSC rate constants (0.22‐0.98 d−1; i.e., faster biological turnover) in the summer. Kinetic analysis suggested that BDMSC rates were nearly saturated at ambient DMS concentrations in the spring but not in summer. BDMSC was mostly carried out in the size fractions ≪1 µm and ≫8 µm, except in the early spring when the ≪1‐µm fraction (likely free‐living bacteria) dominated BDMSC. BDMSC was the main removal pathway for DMS in the surface mixed layer during both the spring and summer, except during the prebloom, when photolysis dominated. BDMSC exerts a major control on DMS concentrations in the Ross Sea throughout the Phaeocystis antarctica bloom.

Occurrence and turnover of DMSP and DMS in deep waters of the Ross Sea, Antarctica

High concentrations of the phytoplankton metabolite dimethylsulfoniopropionate (DMSP) and its degradation product dimethylsulfide (DMS) are associated with blooms of Phaeocystis antarctica in the Ross Sea, Antarctica. Episodic and rapid vertical export of Phaeocystis biomass to deep water has been reported for the Ross Sea, therefore we examined the distribution and microbial consumption rates of DMSP and DMS throughout the sub-euphotic water column. Total DMSP (dissolved+particulate; DMSPt) was present at 0.5–22 nM at depths between 70 and 690 m during both the early bloom (November) and the late bloom (January). Sub-euphotic peaks of DMSP were sometimes associated with mid-water temperature maxima, and elevated DMSP below 70 m was found mainly in water masses characterized as Modified Circumpolar Deep Water or Antarctic Shelf Water. Overall, 50–94% of the integrated water-column DMSPt was found below the euphotic zone. At one station during the early bloom, local maxima of DMSPt (14 nM) and DMS (20 nM) were observed between 113 and 240 m and these maxima corresponded with high chlorophyll a concentrations, P. antarctica cell numbers, and Fv/Fm (the quantum yield of photosystem II). During the late bloom, a sub-euphotic maximum of DMSPt (15.8 nM) at 250 m cooccurred with peaks of chlorophyll a concentration, DMSP lyase activity, bacterial production and dissolved DMSP consumption rates. DMSP turnover contributed ∼12% of the bacterial carbon demand between 200 and 400 m. DMS concentrations peaked at 286 m but the maximum concentration (0.42 nM) was far lower than observed during the early bloom, probably because of relatively rapid biological consumption of DMS (1–3 turnovers per day) which, in turn, contributed to elevated dissolved dimethylsulfoxide (DMSO) concentrations. Relatively stable DMSPt distributions at some sites suggest that rapid sinking of Phaeocystis biomass is probably not the major mechanism responsible for mesopelagic DMSP accumulations. Rather, subduction of near-surface water masses, lateral advective transport or trapping of slowly sinking P. antarctica biomass in intermediate water masses are more likely mechanisms. We found that a culture of P. antarctica maintained cellular integrity during 34 days of darkness, therefore the presence of intact cells (and DMSP) at depth can be explained even under a slow sinking/advection scenario. Whatever the mechanism, the large pools of DMSP and DMS below the euphotic zone suggest that export exerts a control on potential DMS emission from the surface waters of the Ross Sea.

Dissolved DMSO production via biological and photochemical oxidation of dissolved DMS in the Ross Sea, Antarctica

Dimethylsulfoxide (DMSO) is an important degradation product of the climate-influencing gas dimethylsulfide (DMS). In the Ross Sea, Antarctica, dissolved DMSO (DMSOd) concentrations exhibited substantial seasonal and vertical variations. Surface water DMSOd concentrations in pre-bloom waters were very low (<1 nM) but increased rapidly up to 41 nM during the spring Phaeocystis antarctica bloom (late November). Increases in DMSOd concentrations lagged by several days increases in DMS concentrations. Although DMSOd concentrations reached relatively high levels during the spring bloom, concentrations were generally higher (36.3–60.6 nM) during summer (January), even though phytoplankton biomass and DMS concentrations had decreased by that time. During both seasons, DMSOd concentrations were substantially higher within the surface mixed layer than below it. DMSOd production from biological DMS consumption (BDMSC) was higher during late November (3.4–5.2 nM d −1) than during the summer (0.7–2.4 nM d −1); therefore, production via BDMSC alone could not explain the higher DMSOd concentrations encountered during the summer. Mixed layer-integrated DMSOd production from BDMSC was 2.5–13.7 times greater than production from dissolved DMS photolysis during the P. antarctica bloom, while photolysis contributed 1.3 times more DMSO than BDMSC before the bloom. The DMSO yield from BDMSC was consistently higher within the upper mixed layer than at depths below. Experimental incubations with water from the mixed layer showed that exposure to full spectrum sunlight for 72 h caused an increase in the DMSO yield whereas exposure to only photosynthetically active radiation did not. This suggests that ultraviolet radiation is a potential factor shifting the fate of biologically consumed DMS toward DMSO. In general, the highest DMSO yields from BDMSC were in samples with slow biological DMS turnover, whereas fast turnover favored sulfate rather than DMSO as a major end-product. This study provides the first detailed information about DMSOd distribution and production in the Ross Sea and points to DMSOd as an important biological and photochemical degradation product of DMS and a major reservoir of methylated sulfur in these polar surface waters.

Factors determining the vertical profile of dimethylsulfide in the Sargasso Sea during summer

Abstract The major source of reduced sulfur in the remote marine atmosphere is the biogenic compound dimethylsulfide (DMS), which is ubiquitous in the world's oceans and released through food web interactions. Relevant fluxes and concentrations of DMS, its phytoplankton-produced precursor, dimethylsulfoniopropionate (DMSP) and related parameters were measured during an intensive Lagrangian field study in two mesoscale eddies in the Sargasso Sea during July–August 2004, a period characterized by high mixed-layer DMS and low chlorophyll—the so-called ‘DMS summer paradox’. We used a 1-D vertically variable DMS production model forced with output from a 1-D vertical mixing model to evaluate the extent to which the simulated vertical structure in DMS and DMSP was consistent with changes expected from field-determined rate measurements of individual processes, such as photolysis, microbial DMS and dissolved DMSP turnover, and air–sea gas exchange. Model numerical experiments and related parametric sensitivity analyses suggested that the vertical structure of the DMS profile in the upper 60 m was determined mainly by the interplay of the two depth-variable processes—vertical mixing and photolysis—and less by biological consumption of DMS. A key finding from the model calibration was the need to increase the DMS(P) algal exudation rate constant, which includes the effects of cell rupture due to grazing and cell lysis, to significantly higher values than previously used in other regions. This was consistent with the small algal cell size and therefore high surface area-to-volume ratio of the dominant DMSP-producing group—the picoeukaryotes.

DMSP and DMS dynamics during a mesoscale iron fertilization experiment in the Northeast Pacific–Part II: Biological cycling

Dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) biological cycling rates were determined during SERIES, a mesoscale iron-fertilization experiment conducted in the high-nutrient low-chlorophyll (HNLC) waters of the northeast subarctic Pacific. The iron fertilization resulted in the rapid development of a nanoplankton assemblage that persisted for 11 days before abruptly crashing. The nanoplankton bloom was followed by a diatom bloom, accompanied by an important increase in bacterial abundance and production. These iron-induced alterations of the plankton assemblage coincided with changes in the size and biological cycling of the DMSP and DMS pools. The initial nanoplankton bloom resulted in increases in particulate DMSP (DMSPp; 77–180 nmol L−1), dissolved DMSP (DMSPd; 1–24 nmol L−1), and biological gross (0.11–0.78 nmol L−1 h−1) and net (0.04–0.74 nmol L−1 h−1) DMS production rates. During the nanoplankton bloom, DMSPd consumption by bacteria exceeded their sulfur demand and the excess sulfur was probably released as DMS, consistent with the high gross DMS production rates observed during that period. The crash of the nanoplankton bloom was marked by the rapid decline of DMSPp, DMSPd, and gross DMS production to their initial values. Following the crash of the nanoplankton bloom, bacterial production and estimated sulfur demand reached transient maxima of 9.3 μg C L−1 d−1 and 14.2 nmol S L−1 d−1, respectively. During this period of high bacterial production, bacterial DMSPd consumption was also very high (6 nmol L−1 h−1), but none of the consumed DMSPd was converted into DMS and a net biological DMS consumption was measured. This transient period initiated a rapid decrease in DMS concentrations inside the iron-enriched patch, which persisted during the following diatom bloom due to low biological gross and net DMS production that prevented the replenishment of DMS. Our results show that the impact of Fe fertilization on DMS production in HNLC waters result from a complex interplay between the dynamics of the algal blooms and their influence on bacterial DMSP and DMS metabolism.

Depth-dependent fate of biologically-consumed dimethylsulfide in the Sargasso Sea

Biological consumption is a major sink for dimethylsulfide (DMS) in the surface ocean, but the fate of DMS is poorly known. We determined the fate of sulfur from biologically consumed DMS in samples from the upper 60 m of the Sargasso Sea during July 2004. Using tracer levels of 35S-DMS in dark incubations we found that DMS was transformed into three identifiable non-volatile, sulfur-containing product pools: dimethylsulfoxide (DMSO), sulfate, and particle-associated macromolecules. Together, DMSO and sulfate accounted for most (81–93%) of the non-volatile sulfur products. Only a small fraction (∼ 2%) of the consumed DMS-sulfur was recovered in cellular macromolecules, leaving 5–17% of the metabolic products of DMS consumption unidentified. The relative importance of the two major products varied with depth. DMSO was the main sulfur product (∼ 72%) from DMS metabolism in the surface mixed layer, whereas sulfate was the most important product (∼ 74%) below the mixed layer. Changes in temperature and photosynthetically-active radiation (PAR) did not cause shifts in DMS fate in short term incubations (7–12 h), however these or other factors (e.g., exposure to ultraviolet radiation), operating over longer time scales, could potentially influence the observed pattern of DMS fate with depth. Biological DMSO production rates ranged from 0.07 to 0.33 nM day − 1 , with the highest rate found at 30 m, just below the surface mixed layer. With DMSO concentrations ranging from 4.0 to 8.6 nM, turnover times for DMSO were long (15–61 days) when only the biological production from DMS was considered. Identification of the main sulfur containing products from DMS metabolism improves understanding of this important process in the marine sulfur cycling. Detection and quantification of DMSO production from biological DMS consumption also provides a more complete picture of DMSO biogeochemistry in the ocean.

Effects of solar radiation on dimethylsulfide cycling in the western Atlantic Ocean

The influence of solar radiation on springtime rates of photochemical and biological consumption of dimethylsulfide (DMS) in surface waters from the western Atlantic Ocean was examined by exposing 0.2 μm filtered and unfiltered surface seawater to natural sunlight at five depths in the upper 30 m. Parallel deck incubations of 0.2 μm filtered seawater under various long-pass optical filters were also carried out to aid in assessing the wavelength dependence of DMS photolysis. DMS photolysis rate constants for mid-day exposure (∼10:30–17:30 local time) to surface irradiance ranged from 0.026 to 0.086 h −1 and were highest in coastal and shelf waters. Photolysis rate constants decreased with increasing irradiation depth, in accordance with the attenuation of ultraviolet radiation (UVR, 280–400 nm). Total DMS consumption rates (photochemical+biological) in unfiltered surface samples also decreased with increasing incubation depth and were larger than photolysis rates at nearly all depths and all stations. The decrease in photolysis rate constants with exposure depth was mirrored by biological DMS consumption rate constants that were severely inhibited at surface irradiances, and approached or exceeded dark rate constants at deeper exposure depths. Photolysis rates were 2–19 times greater than estimated biological consumption rates in the surface light exposed samples, while biological consumption rates were significantly larger than photolysis rates at incubation depths below the 1% light level for UV–B radiation (280–320 nm). Total DMS loss rates increased up to nine-fold with UVR exposure, but changes in DMS concentrations were not strongly correlated to light dose, presumably due to parallel, light-mediated DMS production processes. The primary loss process for DMS depended mainly on the depth interval considered and the attenuation of UVR; in general, photochemical removal dominated shallow layers characterized by high UV–B intensities, whereas biological removal dominated in deeper layers where UV–B was absent, but UV–A (320–400 nm) and visible (400–700 nm) light fluxes were still relatively high. These results demonstrate that UVR exposure significantly influences the spatial and temporal pattern of DMS production and loss processes, and ultimately the DMS flux to the atmosphere.