旋链角毛藻和小普林藻对二甲基硫和二甲巯基丙酸的释放研究

通过实验室培养研究了旋链角毛藻(Chaetoceros curvisetus Cleve)和小普林藻(Prymnesium parvum Carter)生长周期内培养液中二甲基硫(DMS)和二甲巯基丙酸(DMSP)的含量。结果表明,2种微藻均能释放DMS,但小普林藻单细胞释放的DMS浓度约是旋链角毛藻的500倍。在藻类生长的不同阶段,它们释放DMS和DMSP的能力存在较大差异,但2种藻类DMS大量释放均出现在衰亡期。同时研究了盐度对2种微藻DMS释放的影响,结果表明高盐度会促进小普林藻DMS和DMSP的释放,而对旋链角毛藻DMSP的释放未有显著影响。     

秋季东海二甲基硫和二甲巯基丙酸内盐浓度分布及降解速率研究

于2013年10~11月现场测定了东海中二甲基硫(DMS)及其前体物质二甲巯基丙酸内盐(DMSP,分为溶解态DMSPd和颗粒态DMSPp)的含量,研究其水平分布特征、DMSPp的粒径分布及DMSPd的降解速率,并对DMS的海-气交换通量进行了探讨。研究结果表明,表层海水中DMS、DMSPd和DMSPp的浓度平均值分别为(4.84±0.40)、(5.84±0.93)和(13.01±0.52)nmol·L-1。海水中DMSPd的降解速率在2.59~16.36nmol·L-1·d-1之间,平均值为(6.78±0.84)nmol·L-1·d-1。调查海域范围内,小型浮游植物(20μm)是DMSPp和叶绿素a(Chl a)重要贡献者。此外,秋季东海表层海水DMS的海-气交换通量为0.66~31.73μmol·m-2·d-1,平均值为(11.63±0.71)μmol·m-2·d-1。     

Biogeochemistry of Dimethylsulfoniopropionate (DMSP) in the Surface Microlayer and Subsurface Seawater of Funka Bay,Japan

Twenty-eight sea surface microlayer samples, along with subsurface bulk water samples were collected in Funka Bay, Japan during October 2000–March 2001 and analyzed for dimethylsulfoniopropionate, dissolved (DMSP_d) and particulate (DMSP_p), and chlorophyll a. The aim of the study was to examine the extent of enrichment of DMSP in the microlayer and its relationship to chlorophyll a, as well as the production rate of dimethylsulfide (DMS) from DMSP and the factors that influence this. The enrichment factor (EF) of DMSP_d in the surface microlayer ranged from 0.81 to 4.6 with a mean of 1.85. In contrast, EF of DMSP_p in the microlayer varied widely from 0.85–10.5 with an average of 3.21. Chlorophyll a also appeared to be enriched in the microlayer relative to the subsurface water. This may be seen as an important cause of the observed enrichment of DMSP in the microlayer. The concentrations of DMSP_p in the surface microlayer showed a strong temporal variation, basically following the change in chlorophyll a levels. Moreover, the microlayer concentrations of DMSP_p were, on average, 3-fold higher than the microlayer concentrations of DMSP_d and there was a significant correlation between them. Additionally, there was a great variability in the ratios of DMSP_p to chlorophyll a over the study period, reflecting seasonal variation in the proportion of DMSP producers in the total phytoplankton assemblage. It is interesting that the production rate of DMS was enhanced in the microlayer and this rate was closely correlated with the microlayer DMSP_d concentration. Microlayer enrichment of chlorophyll a and higher DMS production rate in the microlayer provide favorable evidence supporting the view that the sea surface microlayer has a greater biological activity than the underlying water.     

Seasonal dimethylsulfoniopropionate (DMSP) variability in Dona Paula bay

Data on temporal variations of total dimethylsulfoniopropionate (DMSP_t) and the environmental factors that influence DMSP_t concentrations are important in understanding the biogeochemical cycling of organic sulfur compounds. Annual and diurnal variations of DMSP_t were investigated in relation to environmental variables at a fixed station in Dona Paula bay (west coast of India). DMSP concentrations were high in the day and low at night and ranged from 3.69 to 84 nM with a maximum at 17.00 h. The high concentrations of DMSP_t during daytime closely followed that of Chl a concentrations. The DMSP utilizers averaged 0.8 ± 0.3 × 10³ cells l^?1 during night and 0.4 ± 0.1 × 10³ cells l^?1 during the day. The diel variation of DMSP_t was influenced more by biological variables than hydrographic parameters. In the year-round study, the concentrations ranged from 0.69 to 15.8 nM. It was fourfold higher during the southwest monsoon season (13.4 ± 2 nM) and threefold higher during the post-monsoon season (9.96 ± 5 nM) compared to the pre-monsoon season (3.1 ± 1 nM). DMSP_t concentrations showed temporal variability, both during diurnal and annual studies. Diatoms were identified as producers of DMSP in Dona Paula bay. Dinoflagellates also contributed during the non-monsoon seasons. Another factor involved in the variability of DMSP_t was DMSP utilizing bacteria, which ranged from 1 to 10% of the total heterotrophic count.     

Seasonal variations of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the sea-surface microlayer and subsurface water of Jiaozhou Bay and its adjacent area

The distributions of DMS and its precursor dimethylsulfoniopropionate, in both dissolved (DMSP_d) and particulate fractions (DMSP_p) were determined in the seasurface microlayer and corresponding subsurface water of the Jiaozhou Bay, China and its adjacent area in May and August 2006. The concentrations of all these components showed a clear seasonal variation, with higher concentrations occurring in summer. This can be mainly attributed to the higher phytoplankton biomass observed in summer. Simultaneously, the enrichment extents of DMSP_d and DMSP_p in the microlayer also exhibited seasonal changes, with higher values in spring and lower ones in summer. Higher water temperature and stronger radiant intensity in summer can enhance their solubility and photochemical reaction in the microlayer water, reducing their enrichment factors (the ratio of concentration in the microlayer to that in the corresponding subsurface water). A statistically significant relationship was found between the microlayer and subsurface water concentrations of DMS, DMSP and chlorophyll a, demonstrating that the biogenic materials in the microlayer come primarily from the underlying water. Moreover, our data show that the concentrations of DMSP_p and DMS were significantly correlated with the levels of chlorophyll a, indicating that phytoplankton biomass might play an important role in controlling the distributions of biogenic sulfurs in the study area. The ratios of DMS/chlorophyll a and DMSP_p/chlorophyll a varied little from spring to summer, suggesting that there was no obvious change in the proportion of DMSP producers in the phytoplankton community. The mean sea-to-air flux of DMS from the study area was estimated to be 5.70 μmol/(m²·d), which highlights the effects of human impacts on DMS emission.     

Seasonal variations of dimethylsul.de (DMS) and dimethylsulfoniopropionate (DMSP) in the sea-surface microlayer and subsurface water of Jiaozhou Bay and its adjacent area

The distributions of DMS and its precursor dimethylsulfoniopropionate, in both dissolved (DMSPd) and particulate fractions (DMSPp) were determined in the sea-surface microlayer and corresponding subsurface water of the Jiaozhou Bay, China and its adjacent area in May and August 2006. The concentrations of all these components showed a clear seasonal variation, with higher concentrations occurring in summer. This can be mainly attributed to the higher phytoplankton biomass observed in summer. Simultaneously, the enrichment extents of DMSPd and DMSPp in the microlayer also exhibited seasonal changes, with higher values in spring and lower ones in summer. Higher water temperature and stronger radiant intensity in summer can enhance their solubility and photochemical reaction in the microlayer water, reducing their enrichment factors (the ratio of concentration in the microlayer to that in the corresponding subsurface water). A statistically signi.cant relationship was found between the microlayer and subsurface water concentrations of DMS, DMSP and chlorophyll a, demonstrating that the biogenic materials in the microlayer come primarily from the underlying water. Moreover, our data show that the concentrations of DMSPp and DMS were signi.cantly correlated with the levels of chlorophyll a, indicating that phytoplankton biomass might play an important role in controlling the distributions of biogenic sulfurs in the study area. The ratios of DMS/chlorophyll a and DMSPp/chlorophyll a varied little from spring to summer, suggesting that there was no obvious change in the proportion of DMSP producers in the phytoplankton community. The mean sea-to-air .ux of DMS from the study area was estimated to be 5.70 μmol/(m2 ·d), which highlights the e.ects of human impacts on DMS emission.     

Experimental studies on dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) production by four marine microalgae

The production of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) by marine microalgae was investigated to elucidate more on the role of marine phytoplankton in ocean-atmosphere interactions in the global biogeochemical sulfur cycle.Axenic laboratory cultures of four marine microalgae–Isochrysis galbana 8701,Pavlova viridis,Platymonas sp.and Chlorella were tested for DMSP production and conversion into DMS.Among these four microalgae,Isochrysis galbana 8701 and Pavlova viridis are two species of Haptophyta,while Chlorella and Platymonas sp.belong to Chlorophyta.The results demonstrate that the four algae can produce various amounts of DMS(P),and their DMS(P) production was species specific.With similar cell size,more DMS was released by Haptophyta than that by Chlorophyta.DMS and dissolved DMSP (DMSPd) concentrations in algal cultures varied significantly during their life cycles.The highest release of DMS appeared in the senescent period for all the four algae.Variations in DMSP concentrations were in strong compliance with variations in algal cell densities during the growing period.A highly significant correlation was observed between the DMS and DMSPd concentrations in algal cultures,and there was a time lag for the variation trend of the DMS concentrations as compared with that of the DMSPd.The consistency of variation patterns of DMS and DMSPd implies that the DMSPd produced by phytoplankton cells has a marked effect on the production of DMS.In the present study,the authors’ results specify the significant contribution of the marine phytoplankton to DMS(P) production and the importance of biological control of DMS concentrations in oceanic water.     

Grazing suppression of dimethylsulphoniopropionate (DMSP) accumulation in iron-fertilised, sub-Antarctic waters

The impact of in situ iron fertilisation on the production of particulate dimethylsulphoniopropionate (DMSPp) and its breakdown product dimethyl sulphide (DMS) was monitored during the SOLAS air-sea gas exchange experiment (SAGE). The experiment was conducted in the high nitrate, low chlorophyll (HNLC) waters of the sub-Antarctic Southern Ocean (46.7°S 172.5°E) to the south-east of New Zealand, during March-April, 2004. In addition to monitoring net changes in the standing stocks of DMSPp and DMS, a series of dilution experiments were used to determine the DMSPp production and consumption rates in relation to increased iron availability. In contrast to previous experiments in the Southern Ocean, DMS concentrations decreased over the course of the 15-d iron-fertilisation experiment, from an integrated volume-specific concentration in the mixed layer on day 0 of 0.78 nM (measured values 0.65-0.91 nM) to 0.46 nM (measured values 0.42-0.47 nM) by day 15, in parallel with the surrounding waters. DMSPp, chlorophyll a and the abundance of photosynthetic picoeukaryotes exhibited indiscernible or only moderate increases in response to the raised iron availability, despite an obvious physiological response by the phytoplankton. High specific growth rates of DMSPp, equivalent to 0.8-1.2 doublings d⁻¹, occurred at the simulated 60% light level of the dilution experiments. Despite the high production rates, DMSPp accumulation was suppressed in part by microzooplankton grazers who consumed between 61% d⁻¹ and 126% d⁻¹ of the DMSPp production. Temporal trends in the rates of production and consumption illustrated a close coupling between the DMSP-producing phytoplankton and their microzooplankton grazers. Similar grazing and production rates were observed for the eukaryotic picophytoplankton that dominated the phytoplankton biomass, partial evidence that picoeukaryotes contributed a substantial proportion of the DMSP synthesis. These rates for DMSPp and picoeukaryotes were considerably higher than for chlorophyll a, indicating higher cycling rates of the DMSP-producing taxa than for the bulk phytoplankton community. When compared to the total phytoplankton community, there was no evidence of selection against the DMSP-containing phytoplankton by the microzooplankton grazers; the opposite appeared to be the case. SAGE demonstrated that increased iron availability in the HNLC waters of the Southern Ocean does not invariably lead to enhanced DMS sea-air flux. The potential suppression of DMSPp accumulation by grazers needs to be taken into account in future attempts to elevate DMS emission through in situ iron fertilisation and in understanding the hypothesised link between levels of Aeolian iron deposition in the Southern Ocean, DMS emission and global albedo.     

Simple determination of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) using solid-phase microextraction and gas chromatography-mass spectrometry

Solid-phase microextraction (SPME) is a simple, sensitive and less destructive method for the determination of dimethylsulfide (DMS) in seawater. Combined with detection by gas chromatography-mass spectrometry (GC-MS), the method had sufficient sensitivity (minimum detectable concentration of DMS was 0.05 nM), and practical levels of reproducibility (relative standard deviation ≤7%) and linearity (r ² > 0.995) over a wide concentration range (0.5 to 910 nM). The protocol developed was applied to a Sagami Bay water sample to determine concentrations of DMS and DMSP, and in situ DMSP-lyase activity.     

Determinations of dimethylsulphoniopropionate (DMSP) lyase activity using headspace analysis of dimethylsulphide (DMS)

The osmolyte dimethylsulphoniopropionate (DMSP) can be enzymatically cleaved to dimethylsulphide (DMS), acrylate and a proton. The enzyme involved in this reaction is dimethylpropiothetin dethiomethylase (DMSP lyase; enzyme classification number 4.4.1.3.). Although the importance of this reaction for the global sulphur cycle, the influence of DMS on atmospheric acidity and the possible effect on climate regulation have been widely recognised, our knowledge of DMSP lyases is limited to just a few studies. Activity measurements of DMSP lyases offer an important step towards a better understanding of the conditions under which DMS is produced. In the available published data somewhat similar methods have been used before, but a critical examination of the method limitations has not been reported. To encourage further research on this enzyme, we suggest and detail two protocols for measurements of DMSP lyase activity: An in vitro assay for crude cell extracts or purified enzyme and an in vivo method for whole cells, which we recently started to use. After addition of DMSP, samples incubated in a gas tight vial may produce DMS from enzymatic cleavage under suitable conditions, and a DMS production rate can be estimated from time-series measurements of DMS in the headspace of the vial. Headspace analysis of DMS is a useful and rapid technique to estimate and compare DMSP lyase activities from different sources. The relative rates of DMS production in the liquid and of the gas transfer between liquid and headspace, determine the rate of DMS production measured via headspace analysis. If DMS production in the liquid is higher than the rate of transfer, headspace measurements will not reflect the actual amount of DMS produced in the liquid. In this case, extracts have to be diluted to a level that ensures linearity between dilution factor and reduction of enzyme activity. Additionally, incubation volumes and vials should be selected to provide a high surface-to-volume ratio to ensure maximum flux of DMS from the aqueous phase into the headspace. The methods can be adapted to further investigate species- and strain-specific activities, biogeographical distribution, cellular location and biochemical properties of various DMSP lyases.     

Biogeochemistry of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the surface microlayer and subsurface water of the western North Atlantic during spring

Sixteen surface microlayer samples and corresponding subsurface water samples were collected in the western North Atlantic during April–May 2003 to study the distribution and cycling of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) and the factors influencing them. In the surface microlayer, high concentrations of DMS appeared mostly in the samples containing high levels of chlorophyll a, and a significant correlation was found between DMS and chlorophyll a concentrations. In addition, microlayer DMS concentrations were correlated with microlayer DMSPd (dissolved) concentrations. DMSPd was found to be enriched in the microlayer with an average enrichment factor (EF) of 5.19. However, no microlayer enrichment of DMS was found for most samples collected. Interestingly, the DMS production rates in the microlayer were much higher than those in the subsurface water. Enhanced DMS production in the microlayer was likely due to the higher concentrations of DMSPd in the microlayer. A consistent pattern was observed in this study in which the concentrations of DMS, DMSPd, DMSPp (particulate) and chlorophyll a in the microlayer were closely related to their corresponding subsurface water concentrations, suggesting that these constituents in the microlayer were directly dependent on the transport from the bulk liquid below. Enhanced DMS production in the microlayer further reinforces the conclusion that the surface microlayer has greater biological activity relative to the underlying water.     

Evidence for aggregate formation in a nepheloid layer and its possible role in the sedimentation of particulate matter

During cruise 73-A-3 of the R/V “Alaminos”, seawater samples from the nepheloid layer and overlying water were collected and filtered through 0.4 μm Millipore^® Ultra Thin filters for microscopic analysis with a Jeol JSM-U3 scanning electron microscope. The results of the studies of the suspended matter with the scanning electron microscope indicate that there is a significant increase in the percentage of the total number of particles that are associated as aggregates for the nepheloid layer relative to the overlying water. This suggests that once nepheloid layers are formed, they might represent regions of high sedimentation due to aggregate formation.     

Paleoearthquake evidence in Tenerife (Canary Islands) and possible seismotectonic sources

A series of clastic dikes and tubular vents were identified in southern Tenerife (Canary Islands). These features are the result of seismic liquefaction of a Holocene sand deposit, as the consequence of a high intensity paleoearthquake. The peak ground acceleration (pga) and magnitude of the paleoearthquake generating these liquefaction features were estimated by back calculation analysis. A representative value of 0.30 ± 0.05 g was obtained for the pga. From this, an earthquake intensity of IX was estimated for the liquefaction site. Magnitude bound methods and energy based approaches were used to determine the magnitude of the paleoearthquake, providing a moment magnitude M = 6.8. The zone in which the liquefaction structures are found has undergone tectonic uplift and is affected by two faults. One of these faults was responsible for displacing Holocene materials. Dating of the uplifted sand formation indicates an age of 10,081 ± 933 years, the liquefaction features ranging from this age to 3490 ± 473 years BP. This paleoearthquake was of much greater magnitude than those known historically. Faults with neotectonic activity are significant features that should be borne in mind when assessing the seismic hazards of the Canary Islands, presently considered as low and mainly of volcanic origin.     

胶州湾及青岛近海微表层与次表层中二甲基硫(DMS)与二甲巯基丙酸(DMSP)的浓度分布

以胶州湾及青岛近海为研究区域,利用吹扫-捕集气相色谱法研究了二甲基硫(DMS)和二甲巯基丙酸(DMSP,分为溶解态DMSPd和颗粒态DMSPp)在微表层与次表层中的浓度以及它们在微表层中的富集行为。结果表明,DMS、DMSPd和DMSPp在微表层中的浓度高于次表层,它们在微表层中的富集因子分别为1.17、1.84和1.51。研究发现,DMS及DMSPp浓度与叶绿素a(Chl-a)浓度有很好的相关性,但它们的周日变化与Chl-a并不完全同步。DMS/Chl-a和DMSPp/Chl-a的比值在次表层和微表层分别为4.35、13.47mmol/g和3.99、15.88mmol/g。胶州湾及青岛近海生态环境受人为活动干扰严重,使本海域DMS含量较高,从而贡献出较大的DMS海-气通量。     

Comparison of β-dimethylsulfoniopropionate (DMSP) levels in two mediterranean ecosystems with different trophic levels

β-dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) concentrations were recorded from September 1999 to September 2000 in two geographically close ecosystems, differently affected by eutrophication: the Little Bay of Toulon and the Niel Bay (N.W. Mediterranean Sea, France). Little Bay had higher nutrient levels ([NO₃⁻]_max. = 30.3 μM; [PO₄³⁻]_max. = 0.46 μM) and higher chlorophyll a concentrations ([chl a]_mean = 2.4 μg/L) compared to Niel Bay ([NO₃⁻]_max. = 19.7 μM; [PO₄³⁻]_max. = 0.17 μM; [chl a]_mean = 0.4 μg/L). In the two sites, we measured dissolved (DMSP_d < 0.2 μm) and particulate DMSP (DMSP_p > 0.2 μm) concentrations. The DMSP_p was particularly analysed in the 0.2–5, 5–90 and > 90 μm fractions. In the eutrophicated Little Bay, DMSP_d concentrations showed a clear seasonality with high values from January to March (124–148 nM). The temporal profile of the DMSP_p concentrations was similar, peaking in February–March (38–59 nM). In the less eutrophic Niel Bay, DMSP_p concentrations were much lower (6–9 nM in March–April), whereas DMSP_d concentrations were relatively high (110–92 nM in February–March). DMS concentrations were elevated from the end of the winter to the spring in Little Bay, ranging from 3 nM in October to 134 nM in March. In the less eutrophic Niel Bay, lower DMS levels were observed, generally not exceeding 20 nM. Each particulate fraction (0.2–5; 5–90; > 90 μm) contained less DMSP in Niel Bay than in Little Bay. At both sites, the 5–90 μm fraction made up most of the DMSP_p. This 5–90 μm fraction consisted of microphytoplankton, principally Dinophyceae and Bacillariophyceae. The 5–90 μm biomass calculated from cell biovolumes, was more abundant in Little Bay where the bloom at the end of the winter (165 μg/L in March) occurred at the same time as the DMSP peaks. The estimated DMSP_p to biomass ratio for the 5–90 μm fraction was always higher in Little Bay than in Niel Bay. This suggests that the high DMSP levels recorded in Little Bay were not only due to a large Dinophyceae presence in this ecosystem. Indeed, the peak of DMSP_p to biomass ratio obtained from cell biovolumes (0.23 nmol/μg in March) was consistent with the proliferation of Alexandrium minutum. This Dinophyceae species may account for between 50% (2894 cells/L) and 63% (4914 cells/L) of the total phytoplankton abundance in the Little Bay of Toulon.     

Numerical simulation of tidally induced eddies in the Bungo Channel: A possible role for sporadic Kuroshio-water intrusion (kyucho)

It is well known that the sudden intrusion of Kuroshio warm water into the Bungo Channel (kyucho) is regulated by spring–neap tidal forcing. In order to clarify the physical background behind this regulation, numerical experiments are carried out using a high-resolution non-hydrostatic three-dimensional model. We first reproduce the strong mixing region off the east coast of the Bungo Channel resulting from tidal flow interaction with complicated land configurations during spring tides; behind islands and headlands, small-scale eddies satisfying an approximate cyclostrophic balance are generated. As a result, averaged over the whole model domain, the tidal-mean energy dissipation rate reaches ≈1.6?×?10^?6?W?kg^?1. The model predicted energy dissipation rates at the location and times of direct microstructure measurements in the Bungo Channel are comparable to the observed values. We next examine whether or not strong tidal mixing thus reproduced can inhibit the northward intrusion of Kuroshio warm water in the Bungo Channel. It is shown that the Kuroshio warm water can (or cannot) pass through the tidal mixing regions off the east coast of the Bungo Channel during periods of weakened (or enhanced) tidal mixing at neap (or spring) tides. This indicates that taking into account the realistic spring–neap modulation of tidal mixing intensity is indispensable to further increase the ability of the existing forecast system for kyucho in the Bungo Channel.     

New and important roles for DMSP in marine microbial communities

The algal osmolyte dimethylsulfoniopropionate (DMSP) is recognised as the major precursor of marine dimethylsulfide (DMS), a volatile sulfur compound that affects atmospheric chemistry and global climate. Recent studies, using ³⁵S-DMSP tracer techniques, suggest that DMSP may play additional very important roles in the microbial ecology and biogeochemistry of the surface ocean. DMSP may serve as an intracellular osmolyte in bacteria that take up phytoplankton-derived DMSP from seawater. In addition, DMSP appears to support from 1 to 13% of the bacterial carbon demand in surface waters, making it one of the most significant single substrates for bacterioplankton so far identified. Furthermore, the sulfur from DMSP is efficiently incorporated into bacterial proteins (mostly into methionine) and DMSP appears to be a major source of sulfur for marine bacterioplankton. Assimilatory metabolism of DMSP is via methanethiol (MeSH) that is produced by a demethylation/demethiolation pathway which dominates DMSP degradation in situ. Based on the linkage between assimilatory metabolism of DMSP and bacterial growth, we offer a hypothesis whereby DMSP availability to bacteria controls the production of DMS by the competing DMSP lyase pathway. Also linked with the assimilatory metabolism of DMSP is the production of excess MeSH which, if not assimilated into protein, reacts to form dissolved non-volatile compounds. These include sulfate and DOM–metal–MeSH complexes, both of which represent major short-term end-products of DMSP degradation. Because production rates of MeSH in seawater are high (3–90 nM d⁻¹), reaction of MeSH with trace metals could affect metal availability and chemistry in seawater. Overall, results of recent studies provide evidence that DMSP plays important roles in the carbon, sulfur and perhaps metal and DOM cycles in marine microbial communities. These findings, coupled with the fact that the small fraction of DMSP converted to DMS may influence atmospheric chemistry and climate dynamics, draws attention to DMSP as a molecule of central importance to marine biogeochemical and ecological processes.     

Spartina maritima (cordgrass) rhizosediment extracellular enzymatic activity and its role in organic matter decomposition processes and metal speciation

Seasonal monitoring was carried out to investigate the influence of extracellular enzymatic activity (EEA) on metal speciation and organic matter cycling in the rhizosediment of Spartina maritima . Heavy metal speciation was achieved by the Tessier scheme, and showed a similar pattern of variation of the organic-bound fraction, indicating a decomposition process in progress. Both humic acid and organic matter showed the same seasonal pattern. The basal respiration of the rhizosediments also presented a similar seasonal pattern, indicating a microbial degradation of organic matter. The high organic-bound fraction found in the summer gradually decreased towards the winter. This decrease was found to be related to the increase of activity of peroxidase, β- N -acetylglucosaminidase and protease. Also the activity of sulphatase was found to be related to the depletion on the exchangeable fraction, probably due to sulphide formation and consequent mobilization. The results show an interaction between several microbial activities, affecting metal speciation.