Dimethylsulfoniopropionate Levels Research Articles

Production of dimethylsulfoniopropionate, dimethylsulfide and acrylic acid from marine microalgae

Production of dimethylsulfoniopropionate (DMSP), dimethylsulfide (DMS), and acrylic acid (AA) from 22 marine microalgae representing six phyla were examined at different growth stages. The concentrations of DMSP, DMS, and AA per cell in the media showed significant changes during the whole growth period, and the highest concentrations were found at the stationary growth phase or the senescent phase. Different species had different concentration levels of DMSP, DMS, and AA, among which Pyrrhophyta, Bacillariophyte, and Chrysophyta had relatively large release. The highest concentrations of DMSP, DMS, and AA were 56.70, 0.86, and 44.60 fmol cell−1 in Scrippsiella trochoidea, Prymnesiacee, and Karenia mikimotoi, respectively. The ratios of DMS/DMSP and AA/(DMSP+AA) in the 22 marine microalgae differed dramatically over the growth cycle. The DMS/DMSP ratios were <25%, indicating that only a small fraction of DMSP was converted to DMS by enzymatic cleavage. Moreover, there was a higher ratio of enzymatic degradation of DMSP in the senescent growth stage. The AA/(DMSP+AA) ratio, representing the degradation ratio of DMSP, increased significantly at the beginning of growth period and then decreased. Hence, the changes in these ratios could approximately illustrate the degradation mechanism of DMSP among diverse species at different growth phases.

DMSP Production by Coral-Associated Bacteria

Dimethylsulfoniopropionate (DMSP) is an important molecule in the marine sulfur cycle, produced in large amounts by corals and their dinoflagellate endosymbionts, Symbiodiniaceae. Although corals are known to harbour bacteria that can catabolise DMSP, the recent discovery of bacteria capable of producing DMSP in coastal and deep-sea environments raises the possibility of a bacterial contribution to the DMSP output of corals. Here, 157 bacteria associated with four common coral species were isolated and screened for their ability to produce DMSP by targeting dsyB, a key gene involved in DMSP biosynthesis. Approximately 9% (14 out of 157) of the bacterial isolates harboured dsyB, all being members of the Alphaproteobacteria. The ability of these isolates to produce DMSP was confirmed by liquid chromatography-mass spectrometry (LC-MS) and nuclear magnetic resonance (NMR) measurements. A dsyB-harbouring strain, Shimia aestuarii AMM-P-2, was selected for genome sequencing. This strain harbours the complete genetic machinery to (i) assimilate sulfate and synthesise the DMSP precursors, cysteine and methionine; (ii) demethylate DMSP and generate methanethiol; (iii) cleave DMSP, generating dimethyl sulfide (DMS) and acrylate; and (iv) utilise or detoxify acrylate. The impacts of varied environmental factors (temperature, salinity, light and UV radiation) on S. aestuarii AMM-P-2 DMSP biosynthesis were characterised. DMSP levels in S. aestuarii AMM-P-2 increased almost two-fold under both hypersaline conditions (40 PSU) and high UV exposure. DMSP catabolism through the cleavage pathway also increased under these conditions, producing the antioxidants DMS and acrylate, a potential response to the oxidative stress generated. Overall, our results reveal that coral-associated bacteria can synthesize DMSP and may therefore contribute to DMSP production by the coral holobiont.

Spatiotemporal distribution of bacterial dimethylsulfoniopropionate producing and catabolic genes in the Changjiang Estuary.

The osmolyte dimethylsulfoniopropionate (DMSP) is produced in petagram amounts by marine microorganisms. Estuaries provide natural gradients in salinity and nutrients, factors known to regulate DMSP production; yet there have been no molecular studies of DMSP production and cycling across these gradients. Here, we study the abundance, distribution and transcription of key DMSP synthesis (e.g. dsyB and mmtN) and catabolic (e.g. dddP and dmdA) genes along the salinity gradient of the Changjiang Estuary. DMSP levels did not correlate with Chl a across the salinity gradient. In contrast, DMSP concentration, abundance of bacterial DMSP producers and their dsyB and mmtN transcripts were lowest in the freshwater samples and increased abruptly with salinity in the transitional and seawater samples. Metagenomics analysis suggests bacterial DMSP-producers were more abundant than their algal equivalents and were more prominent in summer than winter samples. Bacterial DMSP catabolic genes and their transcripts followed the same trend of being greatly enhanced in transitional and seawater samples with higher DMSP levels than freshwater samples. DMSP cleavage was likely the dominant catabolic pathway, with DMSP lyase genes being ~4.3-fold more abundant than the demethylase gene dmdA. This is an exemplar study for future research on microbial DMSP cycling in estuary environments.

Uptake of Dimethylsulfoniopropionate (DMSP) by Natural Microbial Communities of the Great Barrier Reef (GBR), Australia.

Dimethylsulfoniopropionate (DMSP) is a key organic sulfur compound that is produced by many phytoplankton and macrophytes and is ubiquitous in marine environments. Following its release into the water column, DMSP is primarily metabolised by heterotrophic bacterioplankton, but recent evidence indicates that non-DMSP producing phytoplankton can also assimilate DMSP from the surrounding environment. In this study, we examined the uptake of DMSP by communities of bacteria and phytoplankton within the waters of the Great Barrier Reef (GBR), Australia. We incubated natural GBR seawater with DMSP and quantified the uptake of DMSP by different fractions of the microbial community (>8 µm, 3–8 µm, <3 µm). We also evaluated how microbial community composition and the abundances of DMSP degrading genes are influenced by elevated dissolved DMSP levels. Our results showed uptake and accumulation of DMSP in all size fractions of the microbial community, with the largest fraction (>8 µm) forming the dominant sink, increasing in particulate DMSP by 44–115% upon DMSP enrichment. Longer-term incubations showed however, that DMSP retention was short lived (<24 h) and microbial responses to DMSP enrichment differed depending on the community carbon and sulfur demand. The response of the microbial communities from inside the reef indicated a preference towards cleaving DMSP into the climatically active aerosol dimethyl sulfide (DMS), whereas communities from the outer reef were sulfur and carbon limited, resulting in more DMSP being utilised by the cells. Our results show that DMSP uptake is shared across members of the microbial community, highlighting larger phytoplankton taxa as potentially relevant DMSP reservoirs and provide new information on sulfur cycling as a function of community metabolism in deeper, oligotrophic GBR waters.

Dimethylsulfoniopropionate concentration in coral reef invertebrates varies according to species assemblages

Dimethylsulfoniopropionate (DMSP) is a key compound in the marine sulfur cycle, and is produced in large quantities in coral reefs. In addition to Symbiodiniaceae, corals and associated bacteria have recently been shown to play a role in DMSP metabolism. Numerous ecological studies have focused on DMSP concentrations in corals, which led to the hypothesis that increases in DMSP levels might be a general response to stress. Here we used multiple species assemblages of three common Indo-Pacific holobionts, the scleractinian corals Pocillopora damicornis and Acropora cytherea, and the giant clam Tridacna maxima and examined the DMSP concentrations associated with each species within different assemblages and thermal conditions. Results showed that the concentration of DMSP in A. cytherea and T. maxima is modulated according to the complexity of species assemblages. To determine the potential importance of symbiotic dinoflagellates in DMSP production, we then explored the relative abundance of Symbiodiniaceae clades in relation to DMSP levels using metabarcoding, and found no significant correlation between these factors. Finally, this study also revealed the existence of homologs involved in DMSP production in giant clams, suggesting for the first time that, like corals, they may also contribute to DMSP production. Taken together, our results demonstrated that corals and giant clams play important roles in the sulfur cycle. Because DMSP production varies in response to specific species-environment interactions, this study offers new perspectives for future global sulfur cycling research.

Effect of Salinity on DMSP Production in Gambierdiscus belizeanus (Dinophyceae).

Dimethylsulfoniopropionate (DMSP) is produced by many species of marine phytoplankton and has been reported to provide a variety of beneficial functions including osmoregulation. Dinoflagellates are recognized as major DMSP producers; however, accumulation has been shown to be highly variable in this group. We explored the effect of hyposaline transfer in Gambierdiscus belizeanus between ecologically relevant salinities (36 and 31) on DMSP accumulation, Chl a, cell growth, and cell volume, over 12d. Our results showed that G. belizeanus maintained an intracellular DMSP content of 16.3pmol cell-1 and concentration of 139mM in both salinities. Although this intracellular concentration was near the median reported for other dinoflagellates, the cellular content achieved by G.belizeanus was the highest reported of any dinoflagellate thus far, owing mainly to its large size. DMSP levels were not significantly affected by salinity treatment but did change over time during the experiment. Salinity, however, did have a significant effect on the ratio of DMSP:Chl a, suggesting that salinity transfer of G.belizeanus induced a physiological response other than DMSP adjustment. A survey of DMSP content in a variety of Gambierdiscus species and strains revealed relatively high DMSP concentrations (1.0-16.4pmol cell-1 ) as well as high intrageneric and intraspecific variation. We conclude that, although DMSP may not be involved in long-term (3-12d) osmoregulation in this species, G.belizeanus and other Gambierdiscus species may be important contributors to DMSP production in tropical benthic microalgal communities due to their large size and high cellular content.

Interactions and consequences of silicon, nitrogen, and Fusarium palustre on herbivory and DMSP levels of Spartina alterniflora

Sudden Vegetation Dieback (SVD) has been associated with multiple factors affecting the health of Spartina alterniflora. These include altered nutrition (N, Si and various metals), herbivory from the purple marsh crab, and the association with a fungal pathogen (Fusarium palustre). A metabolite produced by Spartina alterniflora that has been associated with plant health is dimethylsulfoniopropionate (DMSP), but little information exist on how these biotic stressors and nutrition interact to affect DMSP levels. Understanding how these factors might be interrelated might provide insight into the etiology of SVD. Surveys of a marsh affected by SVD confirmed lower levels of DMSP and higher concentrations of Si and other metals were present in Sp. alterniflora when compared to plants from marsh that exhibited no signs of SVD. In repeated greenhouse experiments, the application of Si to Sp. alterniflora had no effect on DMSP concentrations. However, when plants were inoculated with the pathogenic fungus, Fusarium palustre, and then treated with Si, DMSP levels were elevated 27%. Inoculation alone had no effect on DMSP levels. Si application neither favor growth nor suppress the stunting effect of disease by F. palustre. Furthermore, grazing by Sesarma reticulatum, a herbivorous crab, was not affected by Si nutrition. Grazing was increased by nitrogen fertilization and inoculation with F. palustre. Deciphering the role of Si nutrition in Sp. alterniflora and dieback remains unresolved, but no evidence suggests enhancing Si nutrition would directly favor marsh health.

Role of dimethylsulfoniopropionate as an osmoprotectant following gradual salinity shifts in the sea-ice diatom Fragilariopsis cylindrus

Environmental context Dimethylsulfoniopropionate (DMSP), a small sulfur compound biosynthesised by algae, plays an important role in global climate, particularly in polar regions. We investigated salinity effects on DMSP levels, and provide the first experimental measurements of DMSP and associated physiological changes in a polar diatom across to a range of gradual salinity shifts representative of sea-ice conditions. Quantitative estimates of DMSP in polar diatoms following salinity changes will facilitate new mathematical models to predict seasonal responses and reactions to climate change. Abstract Although extreme environmental gradients within sea-ice have been proposed to stimulate dimethylsulfoniopropionate (DMSP) accumulation in diatoms, a taxa whose temperate counterparts show relatively low concentrations, this has yet to be experimentally validated across a range of salinities representative of sea-ice conditions. The present study examined changes in DMSP concentrations in the widespread polar diatom Fragilariopsis cylindrus in response to gradual salinity shifts representative of those encountered during sea-ice formation and melt. DMSP concentrations were elevated up to 127% in 70-salinity cultures. Low-salinity shifts decreased intracellular DMSP concentrations in a gradient-dependent manner that suggests DMSP recycling rather than release under milder hyposalinity shifts. Permeable membranes were detected in ~45% of 10-salinity cells; therefore, loss of membrane integrity may only partially explain DMSP release in the lowest-salinity group. Growth rates, photosynthetic efficiency of photosystem II and reactive oxygen species detection indicated only partial impairment by salinity stress in this organism. Thus, experimental evidence supports the role of DMSP as a compatible solute in the acclimation of a sea-ice diatom across large salinity gradients and measurements of associated physiological changes will improve interpretation of environmental measurements.

Eggs and larvae of Acropora palmata and larvae of Porites astreoides contain high amounts of dimethylsulfoniopropionate

Coral holobionts, including their symbionts, are known to produce large amounts of dimethylsulfoniopropionate (DMSP), a compound which some corals use to mitigate oxidative stress; however, very little work has been done on the presence and use of DMSP in early life history stages of coral, such as in eggs and larvae. This study shows that eggs and larvae, from Acropora palmata, and brooded larvae, from Porites astreoides, also contain high amounts of DMSP. Eggs and larvae of wild A. palmata were collected and contained extremely high levels of DMSP: 1.2μmol per larva and 359μmol per 100μL eggs. Larvae of P. astreoides were collected in flow-through laboratory tanks, from wild-collected parent colonies, over the course of the larval release cycle. In brooded larvae of P. astreoides, the amount of DMSP in larvae ranged from 11.6 to 1510nmol per larva; larval DMSP peaked by the second night of release and then decreased over the following release nights. These high levels in aposymbiotic eggs and larvae of A. palmata and the peaking trend in symbiotic larvae of P. astreoides are suggestive of parental provisioning. Given the large amounts of DMSP found in eggs and larvae, this study provides further evidence that even early life stages of the coral holobiont may benefit from the presence of DMSP.

Substrate Kinetics of DMSP-lyases in Various Cultured Symbiodinium Strains

In marine algae, the conversion of the multifunctional metabolite dimethylsulfoniopropionate (DMSP) to dimethylsulfide (DMS) occurs via DMSP-lyase enzymes. Apparent substrate kinetic properties of DMSP-lyases in algae have revealed high levels of variability. Some algal dinoflagellates, including symbiotic zooxanthellae (genus Symbiodinium) produce high intracellular levels of DMSP and DMSP-lyase capabilities vary among specific Symbiodinium strains. To examine the enzyme kinetic properties of DMSP-lyases in zooxanthellae, we used five cultured Symbiodinium microadriaticum (fruedenthal) strains (ccMP 421, 828, 829, 830, 1633) to optimize in vivo DMSP-lyase assay parameters. Optimal activity occurred at ?30 °c with higher enzyme activity at pH 8 compared to pH 7. Further studies were carried out to examine the enzyme kinetic parameters Vmax and Km in two zooxanthellae strains, ccMP 829 and 1633, that differ in their DMSP-lyase basal activity (high and low activity, respectively). Both zooxanthellae strains showed lower half saturation constants for DMSP (range of Km = 0.008-0.124 mM) compared to most other marine phytoplankton tested to date. Vmax values (?mol DMS Lcell volume-1 min-1) increased slightly or by 2.7-fold in strains 1633 and 829, respectively, in conjunction with decreasing growth rates in batch cultures and parallel increases in intracellular DMSP concentrations in strain 829.

Spatial and temporal variation in DMSP content in the invasive seaweed Codium fragile ssp. fragile: effects of temperature, light and grazing

The methionine-derived secondary metabolite dimethylsulfoniopropionate (DMSP) is believed to function in cryoprotection, defense against herbivory, and reduction of oxidative stress in some seaweeds. The ability to produce DMSP, and modify its concentration according to environ- mental conditions, may confer adaptive advantages to these algae. We studied the spatial and tem- poral distribution of DMSP content in Codium fragile off the Atlantic coast of Nova Scotia. Levels of DMSP in C. fragile were highest in boreal spring (~2% of total dry wt) and lowest in fall (~1%), and negatively related to seawater temperature. DMSP content differed among sites and tended to decrease or remain constant with depth. In tide pools, partially bleached algae contained less DMSP than undamaged individuals, consistent with the putative function of DMSP in response to oxidative stress. Results of field- and laboratory-based experiments, designed to examine plasticity in DMSP production by C. fragile, indicate that it increases with light intensity, exposure to sea urchin grazing, and decreasing temperature.

Variability in dimethylsulfoniopropionate (DMSP) concentrations in Spartina alterniflora and the effect on Littoraria irrorata

We measured concentrations of dimethylsulfoniopropionate (DMSP) in Spartina alterni- flora (Loisel.) in response to the plant hormones abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA) to determine whether DMSP concentration is linked to any of their signaling pathways. DMSP concentrations were also measured in plants at a salt marsh dieback site in Georgetown County, South Carolina (USA), to determine whether dieback conditions affect foliar DMSP concen- trations. We found elevated levels of DMSP in plants receiving SA treatment compared to a control treatment, but we found no treatment effect for either of the other 2 hormone treatments. This sug- gests that DMSP production or transformation is linked to the SA signaling pathway. Diminished lev- els of DMSP were observed in plants nearest the dieback edge compared to those farthest from the dieback in an apparently healthy marsh, which suggests that DMSP concentration responds to envi- ronmental conditions connected with salt marsh dieback. Using a ring assay, we found that Littoraria irrorata snails, potential herbivores of S. alterniflora, were attracted to DMSP. However, we found no correlation between DMSP concentration and L. irrorata density at the acute dieback site. These results suggest that factors such as plant biomass may play a more important role than DMSP in determining L. irrorata distribution in dieback areas.

Alterations in dimethylsulfoniopropionate (DMSP) levels in the coral Montastraea franksi in response to copper exposure

Symbiotic corals routinely experience hyperoxic conditions within their tissues due to the photosynthesis of the endosymbiotic dinoflagellate microalgae ( Symbiodinium spp.). Symbiodinium spp. produce high intracellular levels of the osmolyte dimethylsulfoniopropionate (DMSP). It has recently been discovered in marine algae that DMSP and its enzymatic breakdown products also play a significant role in the scavenging of cellular reactive oxygen species (ROS). To examine this potential for DMSP in corals, we exposed the hard coral Montastraea franksi to 1, 10 and 50 μg L −1 (ppb) concentrations of the oxidative stressor, copper. Levels of total (DMSP t, all coral tissue) were higher than particulate DMSP p (algal component only), demonstrating partitioning of DMSP between algal symbionts and coral host. Significant changes in levels of DMSP t and DMSP p occurred in M. franksi after 48 h, demonstrating a response to copper and indicating a potential antioxidant role for DMSP. DMSP t and DMSP p levels decreased with copper dose, although at the highest copper dose DMSP p levels increased, whereas DMSP t levels did not. This observed differential response to copper between DMSP t and DMSP p demonstrates that physiological changes may be overlooked if conclusions are based upon DMSP t levels alone, which is a common measure used in coral studies. Decreases in chlorophyll a and algal cell numbers in response to elevated copper were also observed. These indices are important physiological indicators and are often used as indices to normalize DMSP levels. Our data suggest that the use of these common indices for normalizing DMSP may not always be appropriate.

Rapid Communication: Experimental Evidence that Juvenile Pelagic Jacks (Carangidae) Respond Behaviorally to DMSP

Dimethylsulfoniopropionate (DMSP) is produced by marine algae and released during foraging activity by zooplankton and fish. Pelagic fishes depend on patchily distributed foraging opportunities, and DMSP may be an important signaling molecule for these events. We have previously shown that the abundance of carangid jacks is positively associated with elevated DMSP levels over coral reefs in the Gulf of Mexico, suggesting that these fishes may use spatial and temporal variation in DMSP to locate foraging opportunities. Here, we extend this work by demonstrating that juveniles of two species of pelagic jack, crevalle jack, Caranx hippos, and bluefin trevally, C. melampygus, detect and respond to DMSP in a flow-through tank in the laboratory. Juveniles of these species showed elevated swimming activity in response to ecologically relevant concentrations of DMSP (10−9 M). These results provide further evidence that this chemical may serve as a chemosensory cue for carangid species.

Dimethylsulfoniopropionate (DMSP) lyase activity in different strains of the symbiotic alga Symbiodinium microadriaticum

Dimethylsulfoniopropionate (DMSP) lyases are responsible for the enzymatic conver- sion of the algal metabolite DMSP into dimethylsulfide (DMS) and other products. DMSP lyase poten- tial activity (DLA) was assayed in 5 cultured strains of Symbiodinium microadriaticum to investigate whether DLA was present and whether variations in DLA occurred among strains. S. microadri- aticum are important primary producers symbiotic with corals and other cnidarians. Four of the 5 tested S. microadriaticum strains were capable of performing enzymatic lysis of DMSP, and the lev- els of DLA varied significantly among the strains. Average DLA normalized to chlorophyll a (chl a) was significantly higher in strain CCMP 829 (5.3 ± 0.9 nmol DMS · min -1 · µg -1 ) compared with all other strains investigated. DLA was not detected in strain CCMP 830, suggesting that DMSP lyase may not be a universal enzyme in S. microadriaticum. DMSP levels also varied between strains, but there was no correlation between cellular DMSP and DLA. Additionally, our data indicate that in non-axenic S. microadriaticum strains, the bacterial contribution to DLA was not substantial in any of the algal cultures investigated. These results are supportive of the idea that S. microadriaticum contain DLA, that DMSP producers are not necessarily capable of DLA, and that, even within a spe- cies, DMSP and DLA levels vary significantly. These findings suggest important differences in the regulation of DMSP and DLA in S. microadriaticum, but the reasons for these differences remain to be elucidated.

Dimethylsulfoniopropionate (DMSP) accumulation in abalone Haliotis midae (Mollusca: Prosobranchia) after consumption of various diets, and consequences for aquaculture

Macroalgae, some of which produce dimethylsulfoniopropionate (DMSP), are important feeds for aquacultured abalone Haliotis midae Linnaeus. DMSP levels were high in Ulva lactuca Linnaeus (up to 6977 ± 1161 μg g − 1 , mean ± 1 SD) but low in Gracilaria gracilis (Stackhouse) Steentoft et Farnham and the kelp Ecklonia maxima (Osbeck) Papenfuss (0.8 ± 0.3 to 26.8 ± 20.6 μg g − 1 ), and increased in abalone tissue after feeding on these algae. A volatile breakdown product of DMSP, dimethylsulfide (DMS), was formed in abalone during canning, causing repellent tastes and odours in some batches of canned meat. Abalone adductor muscle displayed high DMSP concentrations compared to other tissues. Further, the feeding regime determined the level to which DMSP accumulated: abalone previously fed Ulva accumulated DMSP to 16162 ± 3408 μg g − 1 . This was higher than in wild-collected abalone (471 ±339 μg g − 1 ) or those fed with manufactured feeds (up to 46.2 ± 9.6 μg g − 1 ) or Ecklonia (198 ± 73 μg g − 1 ). A depuration phase prior to processing allowed for the reduction of tissue DMSP levels to those seen in wild abalone. Knowing the DMSP levels in feeds and its behaviour in abalone tissue will lead to the development of new strategies for controlling abalone taste characteristics.

Capillary electrophoretic analysis of dimethylsulfoniopropionate in sugarcane and marine algal extracts

A simple and high-resolution analytical method for the determination of dimethylsulfoniopropionate (DMSP) in sugarcane and marine algae is described. Effective extraction of DMSP from plant samples was also investigated using organic solvents, 5% perchloric acid or deionised water. To increase the sensitivity, DMSP in the extracts was first converted to a phenacyl ester, and the reaction mixture was applied directly to capillary electrophoresis without any pretreatment. Water extraction followed by esterification in a pH 4 reaction buffer was found most suitable for the measurement of alkaline-labile DMSP. This method was applied to the determination of DMSP levels in marine algae samples collected from the seashore of Nagasaki, Japan. An increase in DMSP content in Ulva pertusa in the winter period was observed.

PARTICULATE DIMETHYLSULFOXIDE IN ARCTIC SEA‐ICE ALGAL COMMUNITIES: THE CRYOPROTECTANT HYPOTHESIS REVISITED

The particulate‐phase concentrations of dimethyl sulfoxide (DMSOp) and dimethylsulfoniopropionate (DMSPp) in sea‐ice algal communities from the North Water, northern Baffin Bay, were examined from April to June 1998. The concentrations of these compounds were measured in the bottom 2 cm of the ice at 36 locations throughout this region and are compared with results from water‐column samples collected for a complementary study. In general, levels of DMSPp (8.66–987 nmol·L−1, average 126 nmol·L−1) in sea‐ice algal communities were slightly less than those found in bottom sea‐ice algal communities from other polar locations but greater than those found in phytoplankton in other polar environments or at more temperate latitudes. Furthermore, DMSPp :chl a ratios (0.02–14.8 nmol·μg−1, average 1.91 nmol·μg−1) in the sea‐ice algal community were slightly less than those found in other polar environments. DMSOp was measured for the first time in sea‐ice algal communities. DMSOp concentrations varied from 1.35 to 102 nmol·L−1 (average 13.7 nmol·L−1). DMSOp:chl a ratios varied from 0.01 to 4.5 nmol·μg−1 (average 0.22 nmol·μg−1) and were significantly lower than the DMSPp:chl a ratios observed in this study. It has been hypothesized that DMSO can act as a cryoprotector in phytoplankton cells. However, the low concentrations of DMSO observed in the ice algae during this study indicate that intracellular concentrations of DMSO are unlikely to have a significant influence on the freezing point depression of intracellular fluids.

Dimethyl sulfide in the Southern Ocean: Seasonality and flux

The first flux estimate of dimethyl sulfide (DMS) from the Australian sector of the Southern Ocean (63°E to 162°E) has been calculated from seven voyages, which span spring and summer seasons from 1991 to 1995. Increases in seawater DMS and its precursor, dimethyl sulfoniopropionate (DMSP) generally occurred in Southern Ocean surface waters during the transition from spring to summer. DMS flux from the Subantarctic Zone (SAZ), Antarctic Zone (AZ), and Seasonal Ice Zone (SIZ) ranged from 1.7 to 49 μmol/m2/d with a mean value of 9.4 μmol/m2d. These flux calculations are believed to be underestimates, and do not include potential contributions from sea ice. Very high levels of DMSP in sea ice suggest that the SIZ may be a source of DMS to the atmosphere. The different types of vertical DMSP profiles found in sea ice possibly reflect the type of algal assemblage present and the age of the sea ice. Without considering contributions of DMS from sea ice, the overall Southern Ocean DMS emission estimate from this work was 139 Gmol S/yr. The emission estimate for the Antarctic region alone (AZ and SIZ) was 85 Gmol S/yr. This represents 17% of the global emission estimate, from 6% of the ocean surface area. This emission estimate is almost double that of an earlier estimate by Berresheim [1987] of 48 Gmol S/yr, and is likely to be higher when the amount released from the sea ice surrounding Antarctica is more accurately characterized.

Production of glycine betaine and dimethylsulfoniopropionate in marine phytoplankton. II. N-limited chemostat cultures

The nitrogenous organic osmolyte glycine betaine (GBT) and its sulfur analog dimethylsulfoniopropionate (DMSP) are quantitatively significant solutes in many marine algae. Although an inverse relationship has been suggested between these two compounds in marine phytoplankton that may be regulated by nitrogen availability, our results in Part I of this work (same issue) showed no such relationship in batch cultures of six species. In experiments conducted in August 1994, we reexamined this relationship in three axenic strains of phytoplankton [Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, Emiliania huxleyi Hay et Mohler, and Amphidinium carterae Hulbert] in chemostat cultures at three N-limited growth rates. Levels of DMSP, GBT, and homarine, another nitrogenous osmolyte, were inversely related in T. pseudonana, with GBT and homarine preferentially produced at the highest growth rate when cells were N-replete. DMSP concentrations did not change in E. huxleyi, although GBT and homarine levels did increase at the highest growth rate. In A. carterae, neither DMSP nor GBT varied in any systematic way. In all three algae, additions of nitrogen to N-limited cultures resulted in short-term increases in GBT production. Levels of cellular DMSP remained fairly constant in E. huxleyi and A. carterae and were much greater than levels of comparable nitrogenous osmolytes like GBT and homarine. The dominance of DMSP makes a reciprocal relationship with the nitrogenous osmolytes unlikely in most species. Phytoplankton appear to be capable of directly assimilating extracellular GBT, although it is not known if they eliminate equivalent amounts of DMSP in the process.