Jacqueline Stefels

Physiological aspects of the production and conversion of DMSP in marine algae and higher plants

Dimethylsulphoniopropionate (DMSP) is a compound produced in several classes of algae and higher plants that live in the marine environment. Considering its generally high intracellular concentrations, DMSP has a function in the osmotic protection of algal cells. Due to the relatively slow adaptation of its intracellular concentrations upon salinity shifts, DMSP should, however, not be considered as an osmoticum in the strict sense of being responsible for osmotic balance, but rather as a constitutive compatible solute. Besides salinity, other factors also appear to affect cellular DMSP quotas, but the exact regulatory mechanisms are still unclear. In this review, a brief discussion is given of the three pathways of DMSP biosynthesis that are currently distinguished. This is followed by an overview of the factors that affect DMSP biosynthesis (light, salinity, temperature and nitrogen limitation) in relation to its physiological functions. A new hypothesis is presented in which DMSP production is described as an overflow mechanism for excess reduced compounds and for energy excess. Finally, the possible functionality of the enzymatic cleavage of DMSP is discussed in the context of an overflow mechanism. q 2000 Elsevier Science B.V. All rights reserved.

A model system approach to biological climate forcing. The example of Emiliania huxleyi

Abstract Particulate inorganic carbon (calcium carbonate mineral) is produced by pelagic calcifying organisms in the upper layers of the open ocean, it sinks to the deep sea, is partly dissolved and partly stored in the geological archive. This phenomenon, known as the carbonate pump, is an important component of the global carbon cycle and exerts a major influence on climate. The amount of carbonate mineral produced depends on the evolutionary and ecological success of calcifying pelagic organisms. The formulation of adequate predictive carbonate pump modules raises the problem that the behaviour of this highly diverse set of organisms needs to be taken into account. To overcome this difficulty, we propose a “model system” approach, whereby a single representative organism, the coccolithophore Emiliania huxleyi , is investigated in detailed interactive experimental and modelling studies. To construct a comprehensive model of the carbonate pump, subsequent research is envisaged on additional representative organisms, but this work is likely to be facilitated by the experience gained with E. huxleyi . The model system approach permits (1) an emphasis on the non-linear character of the fluxes; (2) a focus on the coupling of the carbonate pump with other climatically important phenomena — the organic carbon pump and DMS production; and (3) exploitation of the experimental accessibility of the E. huxleyi system.

An updated climatology of surface dimethlysulfide concentrations and emission fluxes in the global ocean

[1] The potentially significant role of the biogenic trace gas dimethylsulfide (DMS) in determining the Earth’s radiation budget makes it necessary to accurately reproduce seawater DMS distribution and quantify its global flux across the sea/air interface. Following a threefold increase of data (from 15,000 to over 47,000) in the global surface ocean DMS database over the last decade, new global monthly climatologies of surface ocean DMS concentration and sea‐to‐air emission flux are presented as updates of those constructed 10 years ago. Interpolation/extrapolation techniques were applied to project the discrete concentration data onto a first guess field based on Longhurst’s biogeographic provinces. Further objective analysis allowed us to obtain the final monthly maps. The new climatology projects DMS concentrations typically in the range of 1–7 nM, with higher levels occurring in the high latitudes, and with a general trend toward increasing concentration in summer. The increased size and distribution of the observations in the DMS database have produced in the new climatology substantially lower DMS concentrations in the polar latitudes and generally higher DMS concentrations in regions that were severely undersampled 10 years ago, such as the southern Indian Ocean. Using the new DMS concentration climatology in conjunction with state‐of‐the‐art parameterizations for the sea/air gas transfer velocity and climatological wind fields, we estimate that 28.1 (17.6–34.4) Tg of sulfur are transferred from the oceans into the atmosphere annually in the form of DMS. This represents a global emission increase of 17% with respect to the equivalent calculation using the previous climatology. This new DMS climatology represents a valuable tool for atmospheric chemistry, climate, and Earth System models.

EFFECTS OF IRON AND LIGHT STRESS ON THE BIOCHEMICAL COMPOSITION OF ANTARCTIC PHAEOCYSTIS SP. (PRYMNESIOPHYCEAE). I. INTRACELLULAR DMSP CONCENTRATIONS

Iron is essential for phytoplankton growth, as it is involved in many metabolic processes. It controls photosynthesis as well as many enzymatic processes. As such, iron affects the cell’s energy supply and contributes to the assimilation of carbon and nitrogen. To determine whether iron limitation would result in energy stress or induced nitrogen deficiency, an Antarctic Phaeocystis sp. (Prymnesiophyceae) strain was studied for its biochemical composition, with the main emphasis on intracellular production of dimethylsulfoniopropionate (DMSP). DMSP is suggested to replace nitrogen containing solutes under conditions of nitrogen deficiency. Batch cultures of Antarctic Phaeocystis sp. were grown under iron‐rich and iron‐poor conditions and simultaneously subjected to high and low light intensities. Iron depletion induced chlorosis and suppressed growth rates as well as the maximum yield of the cultures; these effects were reinforced by low light intensities. Cell volumes were strongly reduced under iron‐limited conditions. However, this reduction in cell volume was accompanied by a reduced DMSP content only in cultures experiencing low light intensities. Under high light conditions, no reduction of DMSP was observed; hence, intracellular DMSP concentrations increased. These observations are discussed relative to carbon and nitrogen metabolism and the biosynthetic pathway of DMSP. It is argued that under high light, low iron conditions, the cells were bordering on nitrogen deficiency induced by iron limitation, whereas under low light, low iron conditions, the cells were energy limited resulting in overall suppressed metabolic rates. Between treatments, DMSP to chlorophyll‐a ratios varied by a factor of 5, demonstrating the dependence of this parameter on the physiological state of the cell.

EFFECTS OF IRON AND LIGHT STRESS ON THE BIOCHEMICAL COMPOSITION OF ANTARCTIC PHAEOCYSTIS SP. (PRYMNESIOPHYCEAE). II. PIGMENT COMPOSITION

A strain of Phaeocystis sp., isolated in the Southern Ocean, was cultured under iron‐ and light‐limited conditions. The cellular content of chlorophyll a and accessory light‐harvesting (LH) pigments increased under low light intensities. Iron limitation resulted in a decrease of all light‐harvesting pigments. However, this decrease was greatly compensated for by a decrease in cell volume. Cellular concentrations of the LH pigments were similar for both iron‐replete and iron‐deplete cells. Concentrations of chlorophyll a were affected only under low light conditions, wherein concentrations were suppressed by iron limitation. Ratios of the LH pigments to chlorophyll a were highest for iron‐deplete cells under both light conditions. The photoprotective cycle of diato/diadinoxanthin was activated under high light conditions, and enhanced by iron stress. The ratio of diatoxanthin to diadinoxanthin was highest under high light, low iron conditions.  Iron limitation induced synthesis of 19′‐hexanoyloxyfucoxanthin and 19′‐butanoyloxyfucoxanthin at the cost of fucoxanthin. Fucoxanthin formed the main carotenoid in iron‐replete Phaeocystis cells, whereas for iron‐deplete cells 19′‐hexanoyloxyfucoxanthin was found to be the main carotenoid. This shift in carotenoid composition is of importance in view of the marker function of both pigments, especially in areas where Phaeocystis sp. and diatoms occur simultaneously. A hypothesis is presented to explain the transformation of fucoxanthin into 19′‐hexanoyloxyfucoxanthin and 19′‐butanoyloxyfucoxanthin, referring to their roles as a light‐harvesting pigment.

A first appraisal of prognostic ocean DMS models and prospects for their use in climate models

Ocean dimethylsulfide (DMS) produced by marine biota is the largest natural source of atmospheric sulfur, playing a major role in the formation and evolution of aerosols, and consequently affecting climate. Several dynamic process-based DMS models have been developed over the last decade, and work is progressing integrating them into climate models. Here we report on the first international comparison exercise of both 1D and 3D prognostic ocean DMS models. Four global 3D models were compared to global sea surface chlorophyll and DMS concentrations. Three local 1D models were compared to three different oceanic stations (BATS, DYFAMED, OSP) where available time series data offer seasonal coverage of chlorophyll and DMS variability. Two other 1D models were run at one site only. The major point of divergence among models, both within 3D and 1D models, relates to their ability to reproduce the summer peak in surface DMS concentrations usually observed at low to mid- latitudes. This significantly affects estimates of global DMS emissions predicted by the models. The inability of most models to capture this summer DMS maximum appears to be constrained by the basic structure of prognostic DMS models: dynamics of DMS and dimethylsulfoniopropionate (DMSP), the precursor of DMS, are slaved to the parent ecosystem models. Only the models which include environmental effects on DMS fluxes independently of ecological dynamics can reproduce this summer mismatch between chlorophyll and DMS. A major conclusion of this exercise is that prognostic DMS models need to give more weight to the direct impact of environmental forcing (e.g., irradiance) on DMS dynamics to decouple them from ecological processes.

Assessment of a global climatology of oceanic dimethylsulfide (DMS) concentrations based on SeaWiFS imagery (1998-2001)

A method is developed to estimate sea-surface particulate dimethylsulfoniopropionate (DMSPp) and dimethylsulfide (DMS) concentrations from sea-surface concentrations of chlorophyll a (Chl a). When compared with previous studies, the 1° × 1° global climatology of oceanic DMS concentrations computed from 4 years (1998-2001) of Chl a measurements derived from SeaWiFS (satellite-based, sea-viewing wide field of view sensor) exhibits lower seasonal variability in the southern hemisphere than in the northern hemisphere. A first evaluation of the method shows that it reasonably well represents DMSPp and DMS in the North Atlantic subtropical gyre, in large blooms of mixed populations of diatoms and Phaeocystis spp., and in massive blooms of Phaeocystis spp. but fails for large, almost pure blooms of diatoms. DMSPp and DMS concentrations derived from SeaWiFS were also compared with spatially and temporally coincident in situ measurements acquired independently in the Atlantic between 39°N and 45°N and in sub- tropical and subantarctic Indian Ocean surface waters. Moderate spring and summer phytoplankton blooms there exhib- ited similar trends in DMSPp and DMS levels vs. moderate blooms of mixed populations of prymnesiophytes and dinoflagellates investigated by others. Measured DMS largely exceeded simulated DMS concentrations, whereas mea- sured and simulated DMSPp levels were in close agreement. DMS accumulation is tentatively attributed to dinoflagellate DMSP lyase activity.

High-resolution dimethyl sulfide and dimethylsulfoniopropionate time series profiles in decaying summer first-year sea ice at Ice Station Polarstern, western Weddell Sea, Antarctica

High-resolution profiles of ice dimethyl sulfide (DMS) and dimethylsulfoniopropionate (DMSP) concentrations were measured together with a suite of ancillary physical and biological properties during a time series of decaying summer-level first-year sea ice throughout December 2004 during the Ice Station Polarstern drift experiment (western Weddell Sea, Antarctica). Ice DMSP and DMS concentrations were always maximum at the bottom of the ice sheet (636-2627 and 292-1430 nM, respectively) where the highest chlorophyll a levels were also found (15-30 μg L-1). Throughout the observation period, the autotrophic surface community (32-205 μg C L-1) was dominated by Phaeocystis sp. while the bottom community (1622-3830 μg C L -1) mainly consisted of pennate diatoms. This illustrates that, although being known for lower DMSP-to-chlorophyll a ratios than Phaeocystis sp., diatoms dominated the overall DMSP production because of their much larger biomass. Decreasing DMSP concentrations and increasing DMS-to-DMSP ratios in the bottom layers with time suggested active DMSP-to-DMS conversion in a slowly degrading environment. Drastic temporal brine volume and brine salinity changes associated with the decaying sea ice cover are shown to directly impact (1) the migration of DMSP and DMS through the brine network, (2) the DMSP-to-DMS conversion processes within the ice interior, and (3) the physiological response of the ice algae. First-order flux estimates show that decaying summer-level first-year sea ice alone can significantly contribute to the regional sulfur budget of the Weddell Sea with an estimated average loss rate of 5.7 mol DMS(P) m-2 d-1) toward the atmosphere and the ocean. Copyright 2010 by the American Geophysical Union.

Intriguing functionality of the production and conversion of DMSP in Phaeocystis sp

In many areas of the ocean the distribution and conversion of dimethylsulfoniopropionate (DMSP) may to a high degree be influenced by the activity of the Prymnesiophyte alga Phaeocystis sp.: it not only produces DMSP in large amounts, but is also able to convert it enzymatically into dimethylsulfide (DMS) and acrylate. Characteristic properties of DMSP-lyase in Phaeocystis sp. indicate that the enzyme is different from lyase enzymes in other organisms studied. During a spring bloom in Dutch coastal waters, DMSP-lyase activity was strongly correlated with Phaeocystis sp. cell numbers and potentially capable of producing DMS in excess of abiotic loss factors. In Phaeocystis sp. cells, DMSP makes an important contribution to the intracellular osmotic potential, with concentrations of approximately 150 mM. Upon salinity shocks, however, short term regulation of its internal levels was not observed. Although a slow adaptation of the DMSP production in Phaeocystis sp. cells may affect intracellular concentrations on a long term, it is concluded that DMSP-lyase is not involved in the short term osmotic adaptation of the cell. DMSP is a structural component of the cell, being produced continuously in the light as well as in the dark. DMSP-lyase activity facilitates the release of DMSP from the cell, with some intriguing beneficial effects.

Pan-Arctic simulation of coupled nutrient-sulfur cycling due to sea ice biology : Preliminary results

A dynamic model is constructed for interactive silicon, nitrogen, sulfur processing in and below Arctic sea ice, by ecosystems residing in the lower few centimeters of the distributed pack. A biogeochemically active bottom layer supporting sources/sinks for the pennate diatoms is appended to thickness categories of a global sea ice code. Nutrients transfer from the ocean mixed layer to drive algal growth, while sulfur metabolites are reinjected from the ice interface. Freeze, flux, flush and melt processes are linked to multielement geocycling for the entire high-latitude regime. Major element kinetics are optimized initially to reproduce chlorophyll observations, which extend across the seasons. Principal influences on biomass are solute exchange velocity at the solid interface, optical averaging in active ice and cell retention against ablation. The sulfur mechanism encompasses open water features such as accumulation of particulate dimethyl sulfoniopropionate, grazing and other disruptive releases, plus bacterial/enzymatic conversion to volatile dimethyl sulfide. For baseline settings, the mixed layer trace gas distribution matches sparging measurements where they are available. However, concentrations rise to well over 10 nM in remote, unsampled locations. Peak contributions are supported by ice grazing, mortality and fractional melting. The model bottom layer adds substantially to a ring maximum of reduced sulfur chemistry that may be dominant across the marginal Arctic environment. Sensitivity tests on this scenario include variation of cell sulfur composition and remineralization, routings/chemical time scales, and the physical dimension of water layers. An alternate possibility that peripheral additions are small cannot be excluded from the outcomes. It is concluded that seagoing dimethyl sulfide data are far too sparse at the present time to distinguish sulfur-ice production levels. Citation: Elliott, S., C. Deal, G. Humphries, E. Hunke, N. Jeffery, M. Jin, M. Levasseur, and J. Stefels (2012), Pan-Arctic simulation of coupled nutrient-sulfur cycling due to sea ice biology: Preliminary results, J. Geophys. Res., 117, G01016, doi:10.1029/2011JG001649.