DMSP Production Research Articles

The effect of temperature and salinity on DMSP production in Gephyrocapsa oceanica (Isochrysidales, Coccolithophyceae)

ABSTRACT A subtropical clone of Gephyrocapsa oceanica was grown over the temperature and salinity range 10–30°C and 20‰–45‰ respectively. Cellular DMSP increased with increasing salinity, compatible with the hypothesis that DMSP is a compatible osmolyte. Cellular DMSP content was highest at colder temperatures and decreased as temperature increased. Net DMSP production rate also depended on cell size and growth rate was greatest about 2°C below the optimum growth temperature of 20°C for this clone. This resulted in a unimodal response of net DMSP production to increasing temperature: net DMSP production increased with increasing temperature when the cells were growing at temperatures below optimum for growth. At and above optimum growth temperature, further warming decreased net DMSP production. For the effect of temperature alone, in the subtropical oceans, where G. oceanica is growing at or above its optimum, further warming due to climate change will result in decreased net DMSP production and so a probable decrease in the flux of DMS to the atmosphere and sulphate aerosol production. Inasmuch as these aerosols modulate cloud albedo and longevity then these too will both decrease, resulting in a positive feedback response for temperature. The reverse effect may occur in higher latitude oceans where growth temperature is below optimum. The exact response in both regions is complicated because warming will also enhance water column stratification and may reduce mixed layer depths, affecting both nutrient and light regimes, as well as possible species succession effects. Further work is required to investigate these other indirect temperature effects.

Bacterial Composition Associated With Giant Colonies of the Harmful Algal Species Phaeocystis globosa.

The cosmopolitan algae Phaeocystis globosa forms harmful algal blooms frequently in a number of tropical and subtropical coastal regions in the past two decades. During the bloom, the giant colony, which is formed by P. globosa, is the dominant morphotype. However, the microenvironment and the microbial composition in the intracolonial fluid are poorly understood. Here, we used high-throughput 16S rRNA amplicon sequencing to examine the bacterial composition and predicted functions in intracolonial fluid. Compared with the bacterial consortia in ambient seawater, intracolonial fluids possessed the lower levels of microbial richness and diversity, implying selectivity of bacteria by the unique intracolonial microenvironment enclosed within the P. globosa polysaccharide envelope. The bacterial consortia in intracolonial fluid were dominated by Balneola (48.6% of total abundance) and Labrezia (28.5%). The bacteria and microbial function enriched in intracolonial fluid were involved in aromatic benzenoid compounds degradation, DMSP and DMS production and consumption, and antibacterial compounds synthesis. We suggest that the P. globosa colonial envelope allows for the formation of a specific microenvironment; thus, the unique microbial consortia inhabiting intracolonial fluid has close interaction with P. globosa cells, which may benefit colony development.

The effect of CO2 concentration on DMSP production in Gephyrocapsa oceanica (Isochrysidales, Coccolithophyceae)

ABSTRACT Atmospheric CO2 is increasing at unprecedented rates, with modelling studies indicating this as a significant cause of continued warming of the planet. It has been suggested that if these conditions in some way lead to enhanced DMSP production by marine algae, then this compound, via the sulphate aerosols that result from its breakdown, might act to mitigate global warming by altering cloud albedo and so act as a natural thermostat. Among the major producers of DMSP in the oceans are the coccolithophores, with Gephyrocapsa oceanica being a common species that prefers warmer conditions than Emiliania huxleyi, and that arose in the Miocene when CO2 levels were higher than present. In this study we grew G. oceanica under CO2 concentrations equivalent to atmospheric values of 200, 390 and 1000 ppm, representative of glacial, near present and potential 2100 CE scenarios. As CO2 concentration increased, so did cell size, while both growth rates and intracellular DMSP concentrations decreased, consistent with the hypothesis that DMSP may act as part of an antioxidant mechanism within the cell. This led to a 50% decrease in net DMSP production under 1000 ppm CO2 relative to the 390 ppm treatment. While there are many variables in the open ocean environment that ultimately determine net DMSP production rates and flux of sulphur to the atmosphere, for the cellular response to increased atmospheric CO2 alone we conclude that elevated CO2 would reduce net DMSP production and, inasmuch as this reduces the production of sulphate aerosol in the atmosphere, reducing cloud albedo, then rather than acting to limit global warming, it will instead amplify it.

Cobalamin Scarcity Modifies Carbon Allocation and Impairs DMSP Production Through Methionine Metabolism in the Haptophyte Microalgae Tisochrysis lutea

Cobalamin (vitamin B12) is a cobalt-containing enzymatic cofactor involved in methionine synthesis. Provided only by select bacteria and archaea in marine systems, this vitamin is known to limit primary production in different oceanic areas. Understanding the consequences of cobalamin limitation on phytoplankton physiology is of great interest, notably for cobalamin-dependent haptophytes that significantly contribute to oceanic carbon fixation and sulfur cycle through DMSP synthesis. Here, the effect of cobalamin limitation was compared to nitrogen limitation on the model haptophyte Tisochrysis lutea grown in chemostats, combining comparative proteomics with the analysis of major macromolecules and specific osmolytes. Our results highlight the interconnection of carbon and DMSP metabolisms through the cobalamin-dependent methionine synthesis by showing that cobalamin scarcity impacts the mechanisms of carbon allocation and reduces DMSP quota. Conversely, proline production seemed to anticorrelate with cobalamin availability. In a boarder context, analysis of transcriptomes or genomes of main DMSP producers from different phytoplankton lineages suggests that most of them are cobalamin-dependent, which means that prokaryotic cobalamin synthesis exerts an important control on phytoplankton DMSP production in some regions of the world ocean.

Dynamics of dimethylsulphide and associated compounds in the coastal waters of Goa, west coast of India

Abstract Concentrations of total dimethylsulphoniopropionate (DMSPt), Dimethylsulphide (DMS) and total dimethylsulphoxide (DMSOt) were measured at a coastal time series site, west coast of India, from September 2009 to December 2013. While, DMSPt varied between 0.35 and 252 nM (avg. 24 ± 32.3 nM), DMS varied from 0.5 to 442 nM (avg. 22.5 ± 48.3 nM). The annual phytoplankton regime and succession was observed to play a key role in DMSP and DMS production. DMSPt, DMS and DMSOt showed high temporal variability with maximum concentrations observed during the southwest monsoon (SWM; June to September). While the least DMSP and DMS (

The role of differential DMSP production and community composition in predicting variability of global surface DMSP concentrations

AbstractDimethylsulfoniopropionate (DMSP) is an important labile component of the marine dissolved organic matter pool that is produced by the majority of eukaryotic marine phytoplankton and by many prokaryotes. Despite decades of research, the contribution of different environmental drivers of DMSP production to regional and seasonal variability remains unknown. A synthesis of the current state‐of‐knowledge suggested that approximately half of confirmed DMSP producers are low producers (intracellular DMSP < 50 mM). Low DMSP producers (LoDPs; e.g., diatoms) were shown to strongly regulate intracellular DMSP concentrations (~ 16‐fold change) as a predictable function of nutrient stress. By comparison, high DMSP producers (HiDPs; e.g., coccolithophores) showed very little response (~ 1.5‐fold change). To assess the importance of differential DMSP production by low and high producers, DMSP concentrations were predicted for two time‐series sites (a high‐ and low‐productivity site) and for the global ocean by explicitly incorporating both community composition and mechanistic nutrient stress. Despite large, predictable intracellular DMSP changes, low producers contributed less than 5% to global DMSP. This indicates that, while variations in DMSP production by low producers could be important for predicting microbial interactions and low producer physiology, it is not necessary for predicting global DMSP concentrations. Our analysis suggests that community composition, particularly HiDP biomass, is the dominant driver of variability in in situ DMSP concentrations, even in low‐productivity regions where high producers are typically the subdominant group. Accurate predictions of in situ DMSP concentrations require improved representation of subdominant community dynamics in ecosystem models and remote‐sensing algorithms.

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

AbstractThe horizontal/geographical variations in dissolved dimethylsulfide (DMS), its precursor dimethylsulfoniopropionate (DMSPd and DMSPp), and chlorophyll a (Chl a), as well as the oceanographic parameters influencing the concentrations of dimethylated sulfur compounds, were investigated in the Bering Sea from July to August 2012. Similar to Chl a, the surface DMS and DMSPp levels, as well as DMS(P) production and consumption rates, exhibited a declining gradient from the central basin to the continental shelf, with high‐value areas appearing in the central basin, the slope regions, and Anadyr Strait but a low‐value area occurring on the outer‐middle continental shelf. Considerably high values of DMS and DMSP were measured in the saline Bering Sea Basin Deep Water (>2,000 m) located at the southwest of the Bering Basin because of the release of resuspension in 2,000 m depth and the DMSP production from endogenous benthic bacteria and cyanobacteria population. Chl a was positively correlated with DMSPp and DMS in the surface waters and the upper water of the basin, whereas significant negative correlations were found between DMS and nutrients (dissolved inorganic nitrogen [DIN], phosphorus, and silicate) in the inner shelf of the Bering Sea. DMS microbial consumption was approximately 6.26 times faster than the DMS sea‐air exchange, demonstrating that the major loss of DMS in the surface water occurred through biological consumption relative to evasion into the atmosphere. Average sea‐to‐air DMS fluxes were estimated to be 4.66 μmol/(m2·d), and consequently oceanic biogenic DMS emission had a dominant contribution to the sulfur budget over the observational area.

Transcriptomic analysis of the response of Acropora millepora to hypo-osmotic stress provides insights into DMSP biosynthesis by corals

BackgroundDimethylsulfoniopropionate (DMSP) is a small sulphur compound which is produced in prodigious amounts in the oceans and plays a pivotal role in the marine sulfur cycle. Until recently, DMSP was believed to be synthesized exclusively by photosynthetic organisms; however we now know that corals and specific bacteria can also produce this compound. Corals are major sources of DMSP, but the molecular basis for its biosynthesis is unknown in these organisms.ResultsHere we used salinity stress, which is known to trigger DMSP production in other organisms, in conjunction with transcriptomics to identify coral genes likely to be involved in DMSP biosynthesis. We focused specifically on both adults and juveniles of the coral Acropora millepora: after 24 h of exposure to hyposaline conditions, DMSP concentrations increased significantly by 2.6 fold in adult corals and 1.2 fold in juveniles. Concomitantly, candidate genes enabling each of the necessary steps leading to DMSP production were up-regulated.ConclusionsThe data presented strongly suggest that corals use an algal-like pathway to generate DMSP from methionine, and are able to rapidly change expression of the corresponding genes in response to environmental stress. However, our data also indicate that DMSP is unlikely to function primarily as an osmolyte in corals, instead potentially serving as a scavenger of ROS and as a molecular sink for excess methionine produced as a consequence of proteolysis and osmolyte catabolism in corals under hypo-osmotic conditions.

Influence of the Mississippi River plume and non-bioavailable DMSP on dissolved DMSP turnover in the northern Gulf of Mexico

Environmental contextDimethylsulfoniopropionate (DMSP) comprises an important fraction of the organic carbon produced by phytoplankton, and is a major source of carbon and sulfur for heterotrophic bacteria. Here, we show that a non-bioavailable fraction of DMSP recently discovered in coastal waters also exists in oligotrophic open-ocean waters. Taking account of the non-bioavailable pool improved estimates of cycling rates of DMSP and its contribution to bacterial nutrition. AbstractMicrobial cycling of dissolved dimethylsulfoniopropionate (DMSPd) and the fate of DMSP-sulfur were measured in the northern Gulf of Mexico off the Louisiana coast in September 2011 using the tracer 35S-DMSPd. Salinity ranged from 31.2ppt in the Mississippi River plume to 36.5ppt offshore. Total DMSP concentrations were significantly higher at the river-influenced stations (12–27nM) than offshore (6–14nM). From 8.7 to 27% of the measured DMSPd, equivalent to 0.1 to 0.23nM, was refractory (i.e. non-bioavailable). We subtracted refractory DMSPd from the measured DMSPd concentrations when calculating DMSPd consumption rates with the tracer 35S-DMSPd. DMSPd consumption and bacterial production were respectively 8 and 7 times higher in the river plume compared with offshore. Incorporation of DMSP-sulfur into biomass was almost three times higher in river-influenced water, whereas the dimethylsulfide (DMS) yield was 3.5 times lower in the river plume compared with offshore. Accordingly, DMSP contributed over 3 times more to the bacterial demand for both carbon and sulfur in the river-influenced water. Despite lower DMS yields in the river plume, the fast DMSP turnover resulted in a 2.7-times higher DMS production compared with offshore waters. Mississippi River inputs and the resulting high productivity led to a rapid turnover of DMSP and high DMS production in the river plume compared with oligotrophic Gulf of Mexico waters. Failure to account for refractory DMSPd when using the 35S-DMSP method will lead to significant overestimation of the DMSPd turnover flux and its contributions to C and S cycling.

The effect of coral bleaching on the cellular concentration of dimethylsulphoniopropionate in reef corals

Measurements of DMSP production from Acropora intermedia collected from Heron Island, in the southern Great Barrier Reef (GBR) from 2001 to 2003, show a distinct seasonal cycle of increased production in summer, and lower production in winter, despite severe coral bleaching in 2002. Increasing seawater temperatures by +2°C in summer and winter increased DMSP production from A. intermedia by approximately 45%. Compared with winter 2001 and summer 2002, marked increases in cellular DMSP occurred in A. intermedia in the winter of 2002 and summer 2003, five to six months after coral bleaching, and seemed to be related to high seawater temperatures and high rainfall. In contrast to these results cellular Chl a concentrations in A. intermedia decreased from 2001 to 2002 and then increased in summer 2003 as the coral slowly recovered. A parallel study conducted on Pocillopera damicornis from a fringing reef off Magnetic Island in the central GBR, highlighted marked variation in cellular concentrations of Chl a, DMSP, and algal symbionts, in colonies that were collected five months after a severe bleaching event. The increases in cellular DMSP at both low and high symbiont concentrations, and the highly significant correlation between cellular DMSP and Chl a, could reflect an adaptive response to enhanced levels of reduced oxygen species produced during the bleaching event, and may have aided the coral's recovery. The increases in cellular DMSP could also be explained by a change in the symbiont community. Comparison with measurements made mainly on Acropora coral from different locations in the GBR over different years, suggests that changes in the cellular or tissue concentration of DMSP are a sensitive indicator of coral stress.

Rapid DMSP production by an Antarctic phytoplankton community exposed to natural surface irradiances in late spring

Natural marine microbial communities sourced from under fast ice at an Antarctic coastal site were incubated in tanks under differently attenuated natural sunlight for 2 wk in late spring (Expt 1) and early summer (Expt 2). In the 18 d period between the 2 sampling episodes, the ice edge retreated from 10 to within 1.5 km of the sampling site, and the fast ice began to break up. Expt 1 rapidly produced significant quantities of total DMSP (DMSPt) with concentrations increasing from 16.6 nmol l �1 to 192.7�204.5 nmol l �1 in 2 d. We believe this is the largest observed increase in DMSPt in a semi-natural community over this time frame. Abundances of Phaeocystis antarctica increased significantly during this initial period, while other phytoplankton species/ groups remained stable. DMSPt concentrations then declined at rates averaging 39.2� 50.0 nmol l �1 d �1 between Days 2 and 4. No major DMSPt production event occurred during Expt 2 despite strong community similarities. Sea ice breakout exposes phytoplankton to significant light-related oxidative stress, and these results suggest the rapid production of DMSPt during Expt 1 was due to the initiation of anti-oxidant mechanisms by a low-light-acclimated community in response to solar radiation stress. DMS concentrations remained comparatively low throughout Expt 1, sug- gesting oxidation of DMSP to products other than DMS. Rapid sea ice breakout in coastal regions of Antarctica may result in similar fast DMSP production events during spring.

Coral animals combat stress with sulphur

Photosynthetic algal symbionts of corals produce sulphur substances that are involved in the regulation of ocean temperatures. In a twist to the tale, it emerges that coral animals produce the same compounds. See Letter p.677 Dimethylsulphoniopropionate (DMSP) is a widely distributed metabolite that is converted by marine bacteria to the volatile gas dimethylsulphide (DMS), a major contributor of sulphur to the atmosphere that contributes to cloud formation and hence influences climate. Here Jean-Baptiste Raina et al. report DMSP formation by two common reef-building coral species, Acropora millepora and Acropora tenuis. This comes as a surprise — previously it was thought that DMSP was produced solely by algae (including species symbiotic in coral) and some plants. DMSP biosynthesis may help the coral animals to survive conditions of thermal stress. This finding could have implications for how DMSP production responds to the effects of global change on corals and their symbionts.

Proteomic Analysis of a Sea-Ice Diatom: Salinity Acclimation Provides New Insight into the Dimethylsulfoniopropionate Production Pathway

Dimethylsulfoniopropionate (DMSP) plays important roles in oceanic carbon and sulfur cycling and may significantly impact climate. It is a biomolecule synthesized from the methionine (Met) pathway and proposed to serve various physiological functions to aid in environmental stress adaptation through its compatible solute, cryoprotectant, and antioxidant properties. Yet, the enzymes and mechanisms regulating DMSP production are poorly understood. This study utilized a proteomics approach to investigate protein changes associated with salinity-induced DMSP increases in the model sea-ice diatom Fragilariopsis cylindrus (CCMP 1102). We hypothesized proteins associated with the Met-DMSP biosynthesis pathway would increase in relative abundance when challenged with elevated salinity. To test this hypothesis axenic log-phase cultures initially grown at a salinity of 35 were gradually shifted to a final salinity of 70 over a 24-h period. Intracellular DMSP was measured and two-dimensional gel electrophoresis was used to identify protein changes at 48 h after the shift. Intracellular DMSP increased by approximately 85% in the hypersaline cultures. One-third of the proteins increased under high salinity were associated with amino acid pathways. Three protein isoforms of S-adenosylhomo-cysteine hydrolase, which synthesizes a Met precursor, increased 1.8- to 2.1-fold, two isoforms of S-adenosyl Met synthetase increased 1.9- to 2.5-fold, and S-adenosyl Met methyltransferase increased by 2.8-fold, suggesting active methyl cycle proteins are recruited in the synthesis of DMSP. Proteins from the four enzyme classes of the proposed algal Met transaminase DMSP pathway were among the elevated proteins, supporting our hypothesis and providing candidate genes for future characterization studies.

Changes in DMS production and flux in relation to decadal shifts in ocean circulation

A fundamental question is are the biological processes regulating dimethylsulphide (DMS) production by the marine ecosystem interconnected and responding to atmospheric or ocean signals at decadal timescales? Related to this is a need to quantify how climate change affects these interconnections and understand the expected levels of natural variability on decadal timescales. To explore this we have used indicators of climate variability [the Gulf Stream North Wall (GSNW) and the North Atlantic Oscillation (NAO) indices] as probes to demonstrate that a marine ecosystem model, incorporating DMS production, can extract and amplify a climatic signal, which is spread across a variety of meteorological variables. The GSNW signal is imparted through the wind and cloud forcing, despite the fact there was not significant relationship observed between the GSNW index and the meteorological forcing data. The model simulations appear to reproduce observed decadal variability in phytoplankton community structure in the eastern North Atlantic and imply that DMS(P) biogeochemistry may vary on decadal timescales as a consequence of changes in community structure. The GSNW index is a potential indicator of such changes and there may have been a regime shift in DMSP production in the eastern North Atlantic coincident with that observed for plankton. Sensitivity analysis indicates that the impact of climate variability on DMS biogeochemistry may potentially be damped by the ability of microbial communities to adapt physiologically to the effects of changes in light and nutrients.

On the variation of alkalinity during phytoplankton photosynthesis

The alkalinity of the organic constituents of marine phytoplankton and their participation in the total alkalinity (TA) change of seawater during photosynthesis are carefully assessed. Quantification of the contribution of phytoplankton chlorophyll, proteins and phosphorus compounds to the hydrogen ion balance of seawater in terms of total inorganic nitrogen (∆[NT] = ∆NH4 +] + ∆[N2] + ∆[NO2–] + ∆[NO3–]) and total inorganic phosphorus (∆[PT]) changes during photosynthesis yielded that the organic components of marine phytoplankton are alkaline by –0.06 × ∆[NT] – 0.49 × ∆[PT], and that the potential total alkalinity (TAP) during photosynthesis is TAP = TA – [NH4– + 0.93 × [NO2–] + [NO3–] + 0.08 × [NT] + 0.23 × [PT] for unfiltered seawater samples and TAP = TA – [NH4–] + 0.93 × [NO2–] + [NO3–] + 0.02 × [NT] + 0.26 × [PT] for filtered seawater samples. These equations correct the traditionally used expression TAP = TA + [NO3–]. The TAP anomalies are produced, in order of increasing importance, by N2 fixation, DMSP production and CaCO3 fixation.

Nitrogen-related versus species-specific variability of DMSP production in marine phytoplankton

Nitrogen-related versus species-specific variability of DMSP production in marine phytoplankton

Controls of dimethyl sulphide in the Bay of Bengal during BOBMEX-Pilot cruise 1998

The air-sea exchange is one of the main mechanisms maintaining the abundances of trace gases in the atmosphere. Some of these, such as carbon dioxide and dimethyl sulphide (DMS), will have a bearing on the atmospheric heat budget. While the former facilitates the trapping of radiation (greenhouse effect) the latter works in the opposite direction through reflectance of radiation back into space by sulphate aerosols that form from oxidation of DMS in atmosphere. Here we report on the first measurements made on DMS in the Bay of Bengal and the factors regulating its abundance in seawater. Phytoplankton alone does not seem to control the extent of DMS concentrations. We find that changes in salinity could effectively regulate the extent of DMSP production by marine phytoplankton. In addition, we provide the first ever evidence to the occurrence of DMS precursor, DMSP, in marine aerosols collected in the boundary layer. This suggests that the marine aerosol transport of DMSP will supplement DMS gaseous evasion in maintaining the atmospheric non-sea salt sulphur budget.

A new route for synthesis of dimethylsulphoniopropionate in marine algae.

The 3-dimethylsulphoniopropionate (DMSP) produced by marine algae is the main biogenic precursor of atmospheric dimethylsulphide (DMS). This biogenic DMS, formed by bacterial and algal degradation of DMSP, contributes about 1.5 x 10(13) g of sulphur to the atmosphere annually, and plays a major part in the global sulphur cycle, in cloud formation and potentially in climate regulation. Although DMSP biosynthesis has been partially elucidated in a higher plant, nothing is known about how algae make DMSP except that the whole molecule is derived from methionine. Here we use in vivo isotope labelling to demonstrate that DMSP synthesis in the green macroalga Enteromorpha intestinalis proceeds by a route entirely distinct from that in higher plants. From methionine, the steps are transamination, reduction and S-methylation to give the novel sulphonium compound 4-dimethylsulphonio-2-hydroxybutyrate (DMSHB), which is oxidatively decarboxylated to DMSP. The key intermediate DMSHB was also identified in three diverse phytoplankton species, indicating that the same pathway operates in other algal classes that are important sources of DMS. The fact that a transamination initiates this pathway could help explain how algal DMSP (and thereby DMS) production is enhanced by nitrogen deficiency.

Évidence en faveur de la présence du 3-diméthylsulfoniopropionate chez une large gamme d'Angiospermes

3-Dimethylsulfoniopropionate (DMSP) is an osmoprotectant compound that serves as the biogenic precursor of dimethyl sulfide (DMS), an important atmospheric sulfur gas. DMSP is known to be accumulated by many marine algae but has been little studied in higher plants; it has previously been identified in only four angiosperm genera (one genus from the Asteraceae and three from the Poaceae), at levels of about 5 to 30 μmol g−1 fresh weight. Leaves of 177 species of angiosperms representing 90 families from 55 orders were screened for DMSP. An indirect assay was used in which DMSP treated with a cold NaOH solution released acrylic acid and DMS, the latter being analyzed by gas chromatography. The detection limit was 0.01 μmol g−1 fresh weight. Twenty-nine species (from 22 families and 22 orders) had detectable levels of DMSP, all fairly low (≤ 1 μmol g−1 fresh weight). In vivo radiotracer labeling results indicated that species from the Asteraceae, Poaceae, and Rosaceae containing DMSP synthesize it from methionine via S-methylmethionine, and that this pathway may be present at a low level in species of Asteraceae that do not accumulate detectable amounts of DMSP. Taken together, these data imply that the capacity for DMSP production is widespread among Angiosperms. Key words: 3-dimethylsulfonioproprionate, S-methylmethionine, dimethyl sulfide.

Light-dependence of carbon and sulfur production by polar clones of the genus Phaeocystis

Blooms of the marine prymnesiophyte genus Phaeocystis link the oceanic and atmospheric compartments of the carbon and sulfur cycles. Modeling the fluxes of dimethylsulfide from the ocean to the atmosphere has been limited due to a lack of information on functional responses to environmental variables. In this study, the light-dependence of extracellular carbon production and dimethyl sulfide (DMS) production by non-axenic polar clones of Phaeocystis spp. was examined at different growth stages. Comparative experiments were run with non-axenic arctic clones of the diatoms Thalassiossira nordenskioeldii and Skeletonema costatum. A large portion of carbon incorporated by the colonial stage of Phaeocystis spp. is released extracellularly, in particular in stationary colonies. This extracellular production can be modeled as a function of irradiance, as for carbon incorporation. In Phaeocystis spp., cellular and extracellular carbon incorporation represent different uptake rates, indicating the formation of two distinct carbon pools. The release of extracellular carbon by polar Phaeocystis spp. was not a constant fraction of total production over the irradiance range used. We observed little extracellular carbon production by cells at high irradiance, and maximal rates were observed at intermediate irradiance. Newly incorporated carbon that accumulates in the mucilage of the colonial stage of antarctic Phaeocystis sp. during photosynthesis was not reutilized for cellular growth during the dark period, as observed for temperate clones. In contrast, only a minor fraction of the radiocarbon incorporated by the diatoms was released extracellularly for all growth stages. The production of DMS was an order of magnitude higher for Phaeocystis spp. than for diatoms. The chlorophyll-specific production of DMS and DMSP (dimethylsulphoniopropionate, the precursor to DMS) by Phaeocystis spp. showed a hyperbolic response to irradiance, while arctic diatoms (weak or non-producers of DMS), on the other hand, did not show any light-dependency of DMS production. An inverse relationship between DMS and DMSP production in stationary clones of arctic P. pouchetii was observed, but not for the exponentially growing antarctic clone. Stationary colonies also had higher DMS and dissolved DMSP production rates than exponentially growing ones. These relationships can be extrapolated to the field in areas where Phaeocystis spp. dominates.