A new source of dimethylsulfide (DMS) for the arctic atmosphere: ice diatoms

We report the first evidence that pennate diatoms growing within the bottom layer of first-year ice in the Arctic produce significant amounts of particulate dimethylsulfoniopropionate (DMSP_p) and dissolved DMSP+DMS. In 1992 in Resolute Passage, a tributary of Barrow Strait, DMSP_p concentrations within the bottom layer of ice reached 1055 mg S m^-3 at the end of the vernal bloom, a value one order of magnitude higher than the maximum value reported in antarctic ice. Bottom-ice concentrations in DMSP_p and DMSP_d+DMS were significantly correlated with the abundance of the dominant pennate diatom Nitzschia frigida. Intracellular concentration in DMSP of ice algae was very low (0.001 pg cell^-1) at the end of April when algae were light-limited and reached 1.17 pg cell^-1 in mid-May following an increase in light and algal growth. We calculate that the rapid release of the dissolved DMSP+DMS from the ice into surface waters following the ice break-up will generate a sea-to-air DMS flux of 0.7 mg S m^-2 d^-1, a pulse ten times higher than the mean arctic summer flux. We estimate that this 1-d pulse represents up to 5% of the annual DMS emission in the Arctic.     

DMSP-lyase activity in five marine phytoplankton species: its potential importance in DMS production

Activity of DMSP-lyase, which cleaves dissolved DMSP (henceforth DMSP_d-lyase), was examined in five axenically cultured phytoplankton species, including both DMSP-producing and non-DMSP-producing species. High DMSP_d-lyase activity was found in two DMSP producers, Heterocapsa triquetra strain NIES-7 and Scrippsiella trochoidea strain NIES-369 (Dinophyceae). The DMS production rates at 100 nM DMSP_d were 0.5 fmol cell⁻¹ min⁻¹ for H. triquetra and 0.3 fmol cell⁻¹ min⁻¹ for S. trochoidea. In a non-DMSP producer, Heterosigma akashiwo strain NIES-6 (Raphidophyceae), the DMSP_d-lyase activity was not found. Two DMSP-producing Prymnesiophyceae species, Isochrysis galbana strain CCMP-1323 and Gephyrocapsa oceanica strain NIES-353, did not show any obvious activity either, in contrary to other authors' findings on Phaeocystis sp., another DMSP-producing Prymnesiophyceae species. The comparison of the DMSP_d-lyase activity of the two Dinophyceae species with bacterial DMSP consumption and DMS production activity in Tokyo Bay showed that the DMSP_d-lyase activity of H. triquetra and S. trochoidea could be an important mechanism for DMS production during their blooms. Received: 9 April 1999 / Accepted: 10 December 1999     

Effects of UV-B irradiation on growth and survival of Antarctic marine diatoms

Growth rate, survival, and stimulation of the production of UV-B (280 to 320 nm) absorbing compounds were investigated in cultures of five commonly occurring Antarctic marine diatoms exposed to a range of UV-B irradiances. Experimental UV-B exposures ranged from 20 to 650% of the measured peak surface irradiance at an Antarctic coastal site (0.533 J m^-2 s^-1). The five diatom species (Nitzschia lecointei, Proboscia alata, P. inermis, Thalassiosira tumida and Stellarima microtrias) appear capable of surviving two to four times this irradiance. In contrast to Phaeocystis cf. pouchetii, another major component of the Antarctic phytoplankton, the concentrations of pigments with discrete UV absorption peaks in diatoms were low and did not change significantly under increasing UV-B irradiance. Absorbance of UV-B by cells from which pigments had been extracted commonly greatly exceeded that of the pigments themselves. Most of this absorbance was due to oxidisable cell contents, with the frustule providing the remainder. Survival of diatoms did not correlate with absorption by either pigments, frustules or oxidisable cell contents, indicating that their survival under elevated UV-B irradiances results from processes other than screening mechanisms.     

Calculating carbon biomass ofPhaeocystis sp. from microscopic observations

Conversion factors for calculating carbon biomass ofPhaeocystis sp. colonies and free-living cells were determined from microscopic observations and chemical analysis conducted on cultured and naturalPhaeocystis sp. populations originating from the Southern Bight of the North Sea in 1986 and 1987. They allow calculation, in terms of carbon biomass, of the different forms ofPhaeocystis sp. that succeed each other when the population is growing, on the basis of microscopic observations. The latter include enumerations of free-living cells (flagellated and non-motile) and colonies, as well as colonial biovolume measurement. Specific application to natural populations from Dutch coastal waters during spring 1986 shows that more than 90% ofPhaeocystis sp. carbon biomass is under colonial form, most of it exceeding the grazing characteristics of current zooplankton at this period of the year. Detailed analysis of seasonal changes shows in addition that the size of the colonies greatly increases during the course ofPhaeocystis sp. flowering, reaching sizes as high as 1 mm diameter at the top of the bloom when nutrients are depleted. Physiologically this corresponds to an enhanced synthesis of mucilaginous substances, with the decrease of available nutrients leading to an increasing contribution of the matrix to the total colonial carbon during the course of the bloom. Carbon content ofPhaeocystis sp. colonies therefore greatly varies with their size, ranging from 0.3 to 1430 ngC colony^–1.     

Dimethylsulfide and the algaPhaeocystis pouchetii in antarctic coastal waters

The concentrations of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) were measured in water collected from the Southern Ocean 10 km offshore from Davis Station, Antarctica, during the period May 1987 to January 1988, inclusive. During winter and spring, when the sea-ice was up to 1.9 m thick, DMS, DMSP and DMSO concentrations were low (0.2 to 1.5 nM), as were phytoplankton numbers. The maximum concentration of the sulfur compounds generally occurred in the top 10 m of the water column. DMS levels rose dramatically from early December onwards, reaching a peak of 290 nM at a depth of 15 m in January. This concentration is higher than reported elsewhere in the ocean. These high concentrations occurred at the same time as a bloom of the algaPhaeocystis pouchetii. A significant correlation occurred between DMS concentration and cell numbers of the alga. The ratio of DMS concentration to the number of cells of the alga was considerably higher than reported for blooms of this species elsewhere in the ocean. Up to 10% of the total global flux of DMS to the atmosphere may emanate from antarctic seas.     

Dimethylsulfoniopropionate and dimethylsulfide in the marine flatworm Convoluta roscoffensis and its algal symbiont

 A distinct smell of dimethylsulfide (DMS) was noted at the edge of the intertidal mudflat of Marennes-Oléron Bay, at the French Atlantic coast, where dense populations of the marine flatworm Convoluta roscoffensis Graff (Platyhelminthes: Turbellaria) were present. DMS is the cleavage product of dimethylsulfoniopropionate (DMSP). DMSP was shown to be present in high amounts in sediment containing the flatworm as well as in axenic cultures of the symbiotic alga Tetraselmis sp. that was isolated from the flatworm. In untreated sediment samples containing C. roscoffensis the concentration of DMS was as high as ∼55 μmol l⁻¹ sediment, and in samples that were fixed with glutaraldehyde the concentration of DMS was even three orders of magnitude higher (∼66 mmol l⁻¹ sediment). This rapid cleavage of DMSP to DMS in fixed samples was unexpected. Pure DMSP was stable in glutaraldehyde, and it was therefore concluded that a DMSP-lyase was responsible for cleavage in the field samples. The isolated symbiotic alga, Tetraselmis sp., did not show DMSP-lyase activity, indicating that DMSP-lyase may have been present in the flatworm, although the role of bacteria could not be excluded. The Chl a-specific DMSP content of C. roscoffensis (∼200 mmol g⁻¹) was much higher than that of Tetraselmis sp. (∼30 mmol g⁻¹). Possibly, DMSP was not only present in the symbiotic alga, but was also incorporated in the body tissue of the flatworm. It remains unclear what the function of DMSP is in C. roscoffensis. In Tetraselmis sp., but not in C. roscoffensis, DMSP increased with increasing salinity. It was concluded that salinity probably does not play an important role in the dynamics of DMSP and DMS in sediment containing C. roscoffensis. Received: 21 January 2000 / Accepted: 29 August 2000     

Production of DMSP and DMS during a mesocosm study of an Emiliania huxleyi bloom: influence of bacteria and Calanus finmarchicus grazing

We investigated the influence of bacteria and metazooplankton on the production of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) during blooms of Emiliania huxleyi (Lohmann) Hay and Mohler in seawater mesocosms. The phytoplankton succession was marked by the rapid collapse of an initial Skeletonema costatum (Greville) Cleve bloom followed by a small E. huxleyi bloom. The collapse of the diatom bloom was accompanied by an increase in concentrations of dissolved DMSP (DMSP_d) and bacterial abundance and activity (as determined by the thymidine incorporation technique). The increase in bacterial activity was followed by a rapid decrease in DMSP_d concentrations which remained low for the rest of the experiment, even during the subsequent collapse of the E. huxleyi blooms. The absence of DMSP_d and DMS peaks during the declining phase of the E. huxleyi blooms was attributed to the high bacterial activity prevailing at that time. The influence of metazooplankton grazing on DMSP and DMS production was investigated by adding moderate (24 mg dry weight m^-3) and high (520 mg dry weight m^-3) concentrations of Copepodite Stage V and adults of Calanus finmarchicus to two of four filtered (200 m mesh net) enclosures during the E. huxleyi blooms. The addition of C. finmarchicus, even in high concentrations, had no apparent effect on the dynamics of E. huxleyi, suggesting that the copepods were not grazing significantly on nanophytoplankton. The addition of copepods in high concentrations favored an accumulation of chlorophyll a and particulate DMSP. These results suggest that copepods were preying on the herbivorous microzooplankton which, in turn, was controlling the biomass of nanophytoplankton. DMS production was also enhanced in the enclosure with maximum metazooplankton biomass, suggesting that the grazing of C. finmarchicus on microzooplankton containing DMSP may contribute to DMS production. These results provide strong support to the emerging idea that bacteria and metazooplankton grazing play a dominant role in determining the timing and magnitude of DMS pulses following phytoplankton blooms.     

Anti-grazing activity and seasonal variation of dimethylsulfoniopropionate-associated compounds in the invasive alga <Emphasis Type="Italic">Codium fragile</Emphasis> ssp. <Emphasis Type="Italic">tomentosoides</Emphasis>

In this study, we present evidence that the invasive alga Codium fragile ssp. tomentosoides is chemically defended against grazing by a wound-activated chemical defense involving dimethylsulfoniopropionate (DMSP) and the products of its cleavage, dimethylsulfide (DMS), and acrylic acid (AA). DMSP in C. fragile ssp. tomentosoides was present throughout the year, but concentrations varied seasonally and were highest in the winter. Intra-thallus variation was neither large, nor consistent over time. High DMSP concentrations were uncommon among northwestern Atlantic macrophytes. Of 26 other species tested, only two, Ulva lactuca and Polysiphonia harveyi contained concentrations comparable to, or higher than, those of C. fragile ssp. tomentosoides. DMS and AA, both individually and together, deterred grazing by the green sea urchin Strongylocentrotus droebachiensis at “natural” concentrations. These results suggest that DMS and AA contribute to the avoidance of C. fragile ssp. tomentosoides by S. droebachiensis. As a result, the production of DMSP and its subsequent cleavage, upon injury, may reduce herbivory on C. fragile ssp. tomentosoides and contribute to its success.     

Vitamine B12 et phytoplancton dans l'Océan Antarctique. Distribution et approche expérimentale

In late summer (Antiprod I cruise, March 1977), antarctic waters of the southern part of the Indian Ocean contain low amounts of dissolved Vitamin B₁₂: 1 ng l^-1 on average. However, higher concentrations were recorded in two areas: in the subantarctic convergence zone and between 50° and 57°S, with values greater than 3,5 ng l^-1. Despite nutrient-rich waters, primary production was low and the phytoplankton sparse. Abundance of diatoms was related, to a certain extent, to the concentration of Vitamin B₁₂. An experimental approach (Antiprod II cruise, February and March 1980) was applied to (1) artificially enriched natural sea water, and (2) cultures of two diatom species isolated from antarctic waters, Nitzschia turgiduloides and Chaetoceros sp. The experiments demonstrated that antarctic diatoms in general do not require any of the three investigated vitamins for growth. However, Vitamin B₁₂ has a slightly stimulating effect on growth. This may explain the observed relation between phytoplankton density and vitamin concentration in the Antarctic Ocean. This experimental approach elucidates the minor role of vitamins in the primary production of antarctic waters and provides indirect evidence of the major importance of turbulence as a limiting factor.     

Dimethylsulfide,algal pigments and light in an Antarctic Phaeocystis sp. bloom

Dimethylsulfide (DMS) concentrations in sea water were found to be high (0.19 to 390 nM) in an Antarctic bloom of Phaeocystis sp. during October and November 1990. DMS concentrations were positively correlated with algal pigments, particularly 19-hexanoyloxyfucoxanthin, a prymnesiophyte pigment. Concentrations of DMS varied diurnally, possibly due to effects of sunlight, although the exact mechanism is unknown. Since oceanic DMS production has been linked to the global albedo through the formation of cloud condensation nuclei, light-mediated changes in DMS concentrations may affect the global climate. The flux of DMS from this bloom into the atmosphere is calculated to be 67±55 mol m^-2 d^-1.     

The effect of egg versus seston quality on hatching success,naupliar metabolism and survival of <Emphasis Type="Italic">Calanus finmarchicus</Emphasis> in mesocosms dominated by <Emphasis Type="Italic">Phaeocystis</Emphasis> and diatoms

We studied the effect of a developing Skeletonema marinoi/Phaeocystis spp. bloom on Calanus finmarchicus hatching success, early naupliar survival and metabolism. Our focus was (1) on the development of reproductive rates during a bloom initiation, peak and decline in relation to the production of potentially toxic algal metabolites and (2) on the proportional importance of female nutrition versus naupliar food environment for the production of viable nauplii. Despite polyunsaturated aldehyde (PUA) production by both S. marinoi and Phaeocystis sp., we did not observe any harmful effects on hatching success or naupliar survival and condition in any stages of the short-term (<1 week) algal bloom. Hatching success appeared to be controlled by egg lipid composition, while the beneficial effect of a high food concentration was reflected in naupliar RNA:DNA ratio, protein content and total production of viable nauplii. The egg lipids reflected seston lipids, indicating that the egg fatty acid composition was not modified by the females. Our results suggest that unselective feeding and/or retention of specific lipids can induce qualitative food limitation, although recruitment during the S. marinoi/Phaeocystis sp. bloom was high.     

Total organic sulfur and dimethylsulfoniopropionate in marine phytoplankton: intracellular variations

Cellular levels of particulate organic sulfur (POS) and dimethylsulfoniopropionate (DMSP) were determined in cultures of five species of marine phytoplankton, Amphidinium carterae, Prorocentrum minimum, Emiliania huxleyii, Dunaliella tertiolecta and Skeletonema costatum. The first three are known producers of DMSP while the latter two produce little or non-detectable amounts of DMSP. In those species which produced significant amounts of DMSP, intracellular levels of POS and DMSP varied for the dinoflagellates (A. carterae and P. minimum) while they remained fairly constant for the prymnesiophyte (E. huxleyii) over the growth cycle (until late stationary phase), and DMSP accounted for the majority of the POS (50 to 100%). In species with low levels of DMSP, intracellular POS and DMSP decreased or remained constant over the growth cycle, and DMSP accounted for a much lower percentage of the POS (0 to 40%). Species with high DMSP concentrations had higher POS levels per unit cell volume as well, and DMSP accounted for substantially higher percentages of the particulate organic carbon (POC). Molar N:S ratios suggest a non-protein origin for much of the sulfur in the species producing DMSP. During late stationary phase, an increasing percentage of the DMSP became extracellular in the dinoflagellate and diatom (S. costatum) cultures, suggesting leakage. In a bacterized algal culture, measured quantities were considerably less than in an axenic counterpart, suggesting consumption.     

Spring bloom sedimentation in a subarctic ecosystem

A 5-yr study (1985 to 1989) of spring bloom sedimentation in Auke Bay, Alaska, indicates that the sinking response of diatoms to ambient nutrients influences both species succession during the spring bloom and the subsequent sedimentation of new production. Diatoms from the genera Thalassiosira, Chaetoceros and Skeletonema formed the bulk of the spring bloom each year. Growth of Thalassiosira spp. consistently initiated the primary bloom, while Skeletonema costatum tended to grow later in, or after, the primary bloom. We postulate that this successional pattern is driven by interspecific nutrient competition. Overall, sedimentation flux of the dominant species of bloom diatoms was correlated with surface concentrations of cells integrated over the bloom period. In fact, different linear relationships existed when Thalassiosira and Chaetoceros spp. were considered separately, but not for Skeletonema sp., indicating that marked differences exist between the sedimentation tendencies of these genera. The observed inter-generic differences are explicable by the different overall sinking rates, as well as different nutrient-sensitivities of the sinking rates of each genus. Thalassiosira spp., the fastestsinking and most nutrient-sensitive species, contributed up to 10 x more carbon to the benthos in all years of the study, reaching a maximum of 11.1 gCm^-2 over a single spring bloom event in 1988. This study indicates that the tendency to sink to the benthos during and/or after a bloom is highly dependent on species-specific cell physiology, and supports the idea that it is the fast-sinking, nutrient-sensitive diatoms, such as Thalassiosira species, that constitute the major source of vertical carbon flux in this embayment and other such coastal ecosystems during the spring bloom.     

Growth and grazing loss rates in single-celledPhaeocystis sp. (Prymnesiophyceae)

Growth and grazing loss rates of naturalPhaeocystis sp. single cells were measured using a seawater dilution technique. Measurements were performed during an intensePhaeocystis sp. bloom in the North Sea between 19 April and 5 May 1988. Experimental results yielded rapid carbon turnover rates. Population growth rates varied from 0.033 to 0.098 h^–1, grazing loss rates from 0.037 to 0.174 h^–1. From measured growth rates, average doubling rages of 1.3 doublings d^–1 were calculated. The growth rates would have resulted in maximum carbon production rates of 146 mg C m^–3 d^–1. Grazing rates increased in the course of the bloom and exceeded growth rates at the end. Grazing loss was caused primarily by microzooplankton feeding. Ciliates and heterotrophic dinoflagellates were identified as the major potential consumers of single cells ofPhaeocystis sp. at the beginning of the bloom. The grazing impact of larger microzooplankton species appeared to increase during the progressing bloom.     

Phaeocystis blooms and eutrophication of the continental coastal zones of the North Sea

It is suggested that novel nuisance algal blooms can result from major shifts in N/P or NH₄ ⁺/NO₃ ^- ratios. Inland hydraulic engineering caused a shift from P-limitation (before 1977) towards N-limitation (after 1977) in the Marsdiep area (Dutch coastal waters). Following this shift the colonial flagellate Phaeocystis sp. became more abundant and started to bloom during the nutrient-controlled period (later spring to autumn). Competition experiments showed that the N/P ratio can influence the species composition of marine phytoplankton. In addition, the natural distribution of some species like Rhodomonas sp. and Emiliania huxleyi may be affected by the frequency of nutrient pulses in the system. Phaeocystis was a poor competitor under P-limitation and a good competitor under N-limitation. Colony formation was absent under P- and NH₄ ⁺-limitation. Colonies were formed under NO₃ ^--limitation. These preliminary results suggest that colony-forming Phaeocystic blooms may be restricted (besides light-controlled environments) to those N-controlled environments where nitrate is consumed by Phaeocystis. The distribution of Phaeocystis along the European continental coast is evaluated on the basis of its ability to compete for nutrients and to form colonies when nitrate is the major N-source.     

Effects of high temperature and CO2 on intracellular DMSP in the cold-water coral Lophelia pertusa

Significant warming and acidification of the oceans is projected to occur by the end of the century. CO₂ vents, areas of upwelling and downwelling, and potential leaks from carbon capture and storage facilities may also cause localised environmental changes, enhancing or depressing the effect of global climate change. Cold-water coral ecosystems are threatened by future changes in carbonate chemistry, yet our knowledge of the response of these corals to high temperature and high CO₂ conditions is limited. Dimethylsulphoniopropionate (DMSP), and its breakdown product dimethylsulphide (DMS), are putative antioxidants that may be accumulated by invertebrates via their food or symbionts, although recent research suggests that some invertebrates may also be able to synthesise DMSP. This study provides the first information on the impact of high temperature (12 °C) and high CO₂ (817 ppm) on intracellular DMSP in the cold-water coral Lophelia pertusa from the Mingulay Reef Complex, Scotland (56°49′N, 07°23′W), where in situ environmental conditions are meditated by tidally induced downwellings. An increase in intracellular DMSP under high CO₂ conditions was observed, whilst water column particulate DMS + DMSP was reduced. In both high temperature treatments, intracellular DMSP was similar to the control treatment, whilst dissolved DMSP + DMS was not significantly different between any of the treatments. These results suggest that L. pertusa accumulates DMSP from the surrounding water column; uptake may be up-regulated under high CO₂ conditions, but mediated by high temperature. These results provide new insight into the biotic control of deep-sea biogeochemistry and may impact our understanding of the global sulphur cycle, and the survival of cold-water corals under projected global change.     

Influence of light and temperature on the growth rate of estuarine benthic diatoms in culture

Four species of estuarine benthic diatoms: Amphiprora c. f. paludosa W. Smith, Nitzschia c. f. dissipata (Kützing) Grunow, Navicula arenaria Donkin, and Nitzschia sigma (Kützing) W. Smith were grown in unialgal cultures. The growth rates of the diatoms were determined as the rate of increase of the chlorophyll a content of the cultures. The diatoms were cultured at different combinations of temperture, daylength, and quantum irradiance. The highest growth rates of Navicula arenaria occurred at 16° to 20°C; the other 3 species had their optimum at 25°C or higher. The small-celled species had higher growth rates at their optimum temperature, but at lower temperatures the growth rates of all 4 species became very similar. The minimum daily quantum irradiance that could effect light-saturated growth at 12° and 20°C ranged from 2.5 to 5.0 E.m^-2.day^-1. At 12°C, two species had their highest growth rates under an 8 h daily photoperiod. At 20°C, the three species tested all had highest growth rates under 16 h daily photoperiod. The growth response of the benthic diatoms is comparable to that of several cultures of planktonic diatoms, as described in the literature. The influence of temperature and quantum irradiance on the diatoms in the present investigation was comparable to the influence of temperature and light intensity on the ¹⁴C-fixation of marine benthic diatoms (Colijn and van Buurt, 1975).     

Phytoplankton in the Weddell Sea,Antarctica: composition,abundance and distribution in water-column assemblages of the marginal ice-edge zone during austral autumn

At the compacted, north-south line of the ice edge, phytoplankton were sampled during early austral autumn of 1986 in the northwestern Weddell Sea. Cells from discrete water bottle samples from 12 stations on two east-west transects were counted to gain quantitative information on the composition, abundance, distribution, and condition of the phytoplankton in water-column assemblages. Over 70 species were found. The highest numbers of total cells (integrated through the top 150 m) were found in open water, well-separated from and to the east of the ice edge on the southern transect, with 6.01×10¹⁰ cells m^-2. The relative abundance of diatoms was low at ice-convered stations (< 35% of the total phytoplankton in preserved samples) and high at open-water stations (> 80%); however, the relative abundance of the prymnesiophyte Phaeocystis sp. was high at ice-covered stations (> 60%) and low at open-water stations (< 16%), with lower absolute abundances than during a previous austral-spring phytoplankton increase. In the open ocean, the dominants were the pennate diatoms Fragilariopsis cylindrus, Pseudonitzschia prolongatoides, F. curta, and a small form of the centric diatom Chaetoceros dichaeta in chains. Although the three pennate diatoms were frequently dominant in number, they represented less biomass than C. dichaeta in open waters. Mean phytoplankton abundance was low (0.2×10⁶ cells l^-1) but, overall, the diatom cell density (0.14×10⁶ cells l^-1) was similar to that found previously during a northward transect from ice-covered to ice-free water at the Weddell-Scotia Sea ice edge (spring 1983). The phytoplankton spatial patterns in the two autumn transects differed, with the more southerly transect exhibiting a higher abundance of diatoms and dinoflagellates. The ratio of full to empty diatoms was higher on the southern transect, indicating a healthy population, while lower ratios of full/empty frustules on the northern transect suggested a generally declining population. However, Phaeocystis sp. was more abundant on the northern transect.     

Growth pattern andβ-dimethylsulphoniopropionate (DMSP) content of green macroalgae at different irradiances

Growth rates and intracellular-dimethylsulphoniopropionate (DMSP) concentrations of five green algal species collected from different geographic regions in 1986 and 1989 were determined under four photon flux rates. InUlothrix implexa, U. subflaccida andAcrosiphonia arcta from Antarctica, growth was light-saturated at lower irradiances than in temperateUlva rigida from Southern Chile andBlidingia minima from Germany. The DMSP content ofUlothrix implexa, A. arcta andUlva rigida was directly correlated with the light factor: with increasing irradiance, algal DMSP level increased. In contrast, inUlothrix subflaccida andB. minima DMSP concentrations gradually decreased up to a photon flux rate of 30µmol m^–2 s^–1, then increased markedly under the highest photon flux rate tested. In non-growing, dark-incubatedA. arcta DMSP content was reduced by 35%, while the DMSP pool of all other species remained unchanged, at the level of pre-culture conditions. Under full darkness all plants exhibited a significantly higher DMSP concentration compared with algae grown at low photon flux rates of 2 to 30µmol m^–2 s^–1. These data show a correlation between growth pattern and DMSP biosynthesis, and may point to a species-specific minimum amount of light energy necessary for DMSP accumulation.Contribution no. 302 of the Alfred Wegener Institute of Polar and Marine Research     

Dimethylsulphide and ocean–atmosphere interactions

Dimethylsulphide (DMS) is a trace sulphur gas found in most atmospheric and surface water samples, which is derived from dimethylsulphonioproprionate (DMSP). Although it has been extensively studied over the last 50 years, its natural production, consumption and cycling are still not completely understood. Until recently, DMS was believed to originate mainly from marine waters, but later studies have shown that estuaries and lakes are also an important source of DMS. DMS also originates from terrestrial plants such as maize, wheat and lichen, but it is not fully understood why. DMS is believed to have an important impact on the global environment by influencing factors such as the acidity of the atmosphere, cloud condensation nuclei (CNN) and solar insolation. The impact that humans have on the cycling of DMS and on its environmental impact is not well understood either. DMS is affected by temporal and geographical factors, as well as physical factors such as salinity and wind speed, yet when studied under El Niño conditions which modify these physical factors in vivo, there was found to be no fluctuation in the concentration of DMS in the water column. This review outlines our current state of knowledge on DMS.