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Among planktonic communities haloperoxidase enzymes may play a role in the control of intracellular and extracellular reactive oxygen species, in the generation of halogenated organic compounds and in chemical interactions between microbes. We introduce a sensitive fluorometric assay with a large dynamic range that is based on the dearylation of aminophenyl fluorescein (APF) to fluorescein by highly reactive oxygen species. Bromoperoxidase and chloroperoxidase enzymes catalyze the reaction between hydrogen peroxide and halides to generate highly reactive hypohalite intermediates able to dearylate APF. The fundamentals and standardization of the approach are illustrated using a partially purified, vanadium-dependent bromoperoxidase from the red seaweed Corallina officinalis. Laboratory cultures of two polar diatoms, Porosira glacialis and Fragilariopsis cylindrus, are used to illustrate the sensitivity and potential applications of the approach for in vitro, in vivo and in situ measurements of bromoperoxidase activity. These two diatoms differ in biovolume-specific bromoperoxidase activity by 2-orders of magnitude, from 5.4 to 0.044 fmol fluorescein μm-3 h-1, respectively. The approach is also used to investigate the partition of haloperoxidase activity between different size fractions of summer coastal planktonic communities, illustrating that generally more than 50% of the haloperoxidase activity occurred in a >10 μm size fraction that was dominated by diatoms. The assay has the potential to be of value in many aspects of haloperoxidase research, including developing an improved understanding of the roles of haloperoxidase enzymes in microbial planktonic communities.

The coccolithophore Emiliania huxleyi forms some of the largest phytoplankton blooms in the ocean. The rapid demise of these blooms has been linked to viral infections. E. huxleyi abundance, distribution, and nutritional status make them an important food source for the heterotrophic protists which are classified as microzooplankton in marine food webs. In this study we investigated the fate of E. huxleyi (CCMP 374) infected with virus strain EhV-86 in a simple predator-prey interaction. The ingestion rates of Oxyrrhis marina were significantly lower (between 26.9 and 50.4%) when fed virus-infected E. huxleyi cells compared to non-infected cells. Despite the lower ingestion rates, O. marina showed significantly higher growth rates (between 30 and 91.3%) when fed infected E. huxleyi cells, suggesting higher nutritional value and/or greater assimilation of infected E. huxleyi cells. No significant differences were found in O. marina cell volumes or fatty acids profiles. These results show that virally infected E. huxleyi support higher growth rates of single celled heterotrophs and in addition to the “viral shunt” hypothesis, viral infections may also divert more carbon to mesozooplankton grazers.

An approach combining flow-cytometric fluorescence-activated cell sorting (FACS) of specific phytoplankton groups and subsequent quantification of dimethylsulphoniopropionate (DMSP) content by gas chromatography (GC) is introduced and applied to cultivated microalgae and natural seawater samples. The FACS-GC based method produces statistically robust estimates of the intracellular DMSP content of a variety of phytoplankton groups ranging from prokaryotic Synechococcus spp. to eukaryotic nanophytoplankton. Comparison between FACS and traditional filtration approaches to determine intracellular DMSP content of cultivated microalgae illustrates variable responses between species to the 2 methods. Natural photosynthetic nanoeukaryote populations contained high intracellular DMSP content; comparable to many cultivated prymnesiophyte taxa. A surprisingly high DMSP content was found in natural populations of Synechococcus spp., in contrast to previous values measured in cultivated strains, but possibly confirming the uptake and retention of DMSP by this photosynthetic prokaryote. When combined, the DMSP contributions of each phytoplankton group amounted to between 40 and 57 % of total DMSP in each water sample, with the bulk of the total comprising nanoeukaryote-DMSP. The approach has high potential to provide information that enables better parameterisation of phytoplankton functional types in mechanistic dimethyl sulphide models and to help elucidate the environmental controls that govern the extent and diversity of DMSP production in the oceans.

Microzooplankton are ecologically important grazers of phytoplankton and bacteria in aquatic systems. Inhabiting a three-dimensional realm, the ability of a microzooplankter to detect and respond to infochemical cues from its prey will confer a strong selective advantage. The heterotrophic dinoflagellate Oxyrrhis marina has been repeatedly used to study the effect of infochemicals on foraging success and the results from such investigations have stimulated further research on other microzooplankton. Here, we present a case study that highlights the utility of O. marina for infochemistry research and review current knowledge on its behavioural response to dissolved infochemicals. Through a series of microcapillary assays, we show that 0.1–100 µM source concentrations of the algal secondary metabolite dimethylsulphoniopropionate elicit a motile chemosensory response in O. marina. We outline some of the technical limitations in infochemistry research and provide examples where studies on O. marina resulted in recent progress, before emphasizing the role of the diffusion limited boundary layer or “phycosphere” in chemodetection. The ability of O. marina to detect prey exudates in the phycosphere effectively increases the predator–prey encounter radius and has significant implications to the outcome of encounter rate models. Finally, we introduce a new direction of research that utilizes O. marina as a model microzooplankter to elucidate the function of infochemicals in multi-trophic interactions.

The response in intracellular dimethyl sulfoniopropionate (DMSP) and dissolved DMSP and dimethyl sulfide (DMS) concentrations when Emiliania huxleyi was exposed to acute (1-h) increases in photon flux densities of photosynthetically active radiation (PAR) and ultraviolet (UV) radiation was examined in cells acclimated to low light (LL, 30 mmol photons m22 s21) and high light (HL, 300 mmol photons m22 s21). LL-acclimated cells displayed greater photoinhibition, assessed as a decrease in maximum photochemical efficiency (Fv:Fm). Photoinhibition was increased by exposure to UV wavelengths. LL-acclimated cells also exhibited more light dissipation through the xanthophyll cycle, evident as changes in de-epoxidation state. Greater photoinhibition in LL-acclimated cells corresponded with increased accumulation of DMSP of 21% 6 4% relative to initial concentrations, contrasting with a slight decrease of 5% 6 6% in HL-acclimated cells. Exposure to UV appeared to decrease the rates of intracellular accumulation of DMSP. Conversely, PAR + UV exposure stimulated the net production of dissolved DMSP and DMS in both HL-acclimated and LL-acclimated cultures, compared with high PAR alone. The results indicate a direct link between acute photo-oxidative stress and DMSP synthesis by E. huxleyi. The physiological basis for increased release of DMSP and DMS from cells due to high PAR + UV exposure is unclear. However, the timescales of changes in intracellular DMSP, dissolved DMSP, and DMS are consistent with variations in light intensity experienced by phytoplankton in a turbulent mixed layer and are similar to rates of change in photosynthetic parameters associated with photoacclimation. Emission of dimethyl sulfide (DMS) from the oceans may contribute to the nucleation and growth of aerosol particles, enhancing formation of cloud condensation nuclei in the marine boundary layer; and thereby increasing the Earth’s albedo (Shaw 1983; Charlson et al. 1987). DMS is generally supersaturated in oceanic waters and its sea-toair flux is dictated to a large extent by surface seawater concentration and the gas transfer velocity. Understanding

The influences of physico-chemical and biological variables on the concentrations of di- methyl sulphide (DMS) and its precursor β-dimethylsulphoniopropionate (DMSP) were investigated through an annual cycle in the temperate shelf seas of the western English Channel. Total DMSP to chlorophyll a ratios (DMSPt/chl a) varied seasonally by 40-fold, and DMS and DMSP concentrations became temporally uncoupled, with elevated relative DMS concentrations during spring and mid- summer. Taxonomic succession of high DMSP-producing phytoplankton, including Phaeocystis pouchetii, Scrippsiella trochoidea and Prorocentrum minimum, is apparent in the seasonal pattern of DMSPt concentrations. Peridinin and DMSPt concentrations showed similar seasonal trends (p < 0.0001), illustrating the substantial contribution by dinoflagellate taxa to DMSP production. Summer- time stratification of the water column coincided with increased mixed layer doses of photosyntheti- cally active radiation (PAR), increased surface ultraviolet-B (UVB) irradiance relative to PAR and a de- crease in nitrate and phosphate availability. PAR dose explained 68% of the variability in DMSP/chl a during the seasonal study; whilst nitrate concentrations were inversely related to DMSP/chl a and explained 64% of the variability in log-transformed DMSP/chl a. PAR dose explained only 25% of the variation in DMS concentration, whilst nitrate concentration was inversely related to DMS and explained 49% of the variation in log-transformed DMS concentration. The highly significant relation- ship between DMSP/chl a and PAR dose was similar to those observed for the chlorophyll-specific accumulation of the photoprotective xanthophyll compounds diadinoxanthin and diatoxanthin and the chlorophyll-specific concentrations of UV-absorbing mycosporine-like amino acids. These results lend further, indirect evidence for a photoprotective role of DMSP, possibly associated with physiological stress caused by high PAR and UV radiation and intensified by nutrient limitation.

This study tested the validity of the modified dilution method approach to partition nat- ural phytoplankton mortality into grazer versus virus-induced fractions. A series of experiments was conducted weekly between August and October 2005 from an offshore site in the western English Channel. The phytoplankton community during this period was dominated numerically by Syne- chococcus spp. and picoeukaryotes. Despite highly significant regression results, no discernible viral lysis of Synechococcus spp. or picoeukaryote populations was apparent during the 3 mo period. Microzooplankton grazing was the major source of mortality for Synechococcus spp. and picoeukary- otes during this time. Statistical analysis indicates that, using our experimental design, the modified dilution approach was not sensitive enough to be able to measure significant viral lysis during the experimental period. Sensitivity analysis indicated that viral mortality occurred at rates of

The impact of microzooplankton grazing upon phytoplankton production was quantified in surface waters of the Southern Bight of the North Sea, during April 1998. Two sites were studied in order to examine the impact of microzooplankton grazing on phytoplankton communities dominated by either Phaeocystis globosa and large phytoplankton or small phytoplankton taxa. The nearshore site was characterised by a phytoplankton community comprised mainly of P. globosa and chains of diatoms with high productivity (av. 346 F 185 AgC l � 1 d � 1 ) and biomass (280 F 171 AgC l � 1 d � 1 ). In contrast, in the offshore waters relatively small diatoms dominated the phytoplankton where productivity and biomass were more than ca. five times lower than in nearshore waters. Contrary to expectations, the nearshore site supported a high biomass of microzooplankton (av. 22.4 F 10.6 AgC l � 1 d � 1 ) which was dominated by large heterotrophic dinoflagellates, mostly Gyrodinium cf. spirale. Offshore the microzooplankton community contained one-third the biomass of the nearshore community and was dominated by smaller individuals, in particular oligotrich ciliates, Strombidium spp. Dilution experiments were conducted in order to quantify phytoplankton growth and losses due to microzooplankton grazing in the

Microzooplankton herbivory is considered to be a key process by which dimethylsulphoniopropionate (DMSP) in phytoplankton is transformed to climatically active dimethyl sulphide (DMS). However, there is little firm evidence to show that this occurs in natural waters. We used direct measurements of microzooplankton grazing rates and net DMS production in the southern North Sea to examine the impact of herbivory on DMS production. Estimates of the particulate DMSP ingested by microzooplankton in the form of Phaeocystis sp. were found to account for the DMS production rates observed.

During a mesocosm study in Raunefjorden, Norway, a Micromonas spp. population, ini- tially showing exponential net growth, dramatically declined after Day 4 of the experiment. Using a modification of the dilution approach originally developed to quantify grazing by microzooplankton on phytoplankton, it was possible to partition the mortality of Micromonas spp. between grazing and viral lysis on Days 5, 6 and 7 during the population decline. Parallel dilution experiments were car- ried out in which 0.2 µm- and 10 kDa-filtered water was used as the diluents. In this way, gradients of grazing pressure (0.2 µm series) and grazing pressure + viral concentration (10 kDa series) were produced. Model 1 linear regression of the fraction of whole water versus the apparent growth rate of chlorophyll a and Micromonas spp. provided an estimate of mortality in the 0.2 µm and 10 kDa dilution series. On Days 5, 6 and 7, the slopes of the linear regressions of 0.2 µm and 10 kDa dilution series were significantly different at p = 0.083, 0.001 and 0.093 respectively. From the differences in slope between the series, estimates of viral mortality amounted to a turnover rate of the Micromonas spp. standing stocks of 10, 25 and 9% d -1 . This compares to a turnover rate by the microzooplankton of 48, 26 and 23% d -1 . On all 3 d the combined viral lysis and grazing mortality exceeded estimates of the potential production of Micromonas spp., in part accounting for the population decline. This study demonstrates that the dilution approach can be adapted to directly determine virus-induced mortality rates of specific phytoplankton. However, further work is required to determine how the specificity of viral infection and variety of viral infection cycles affect the results of this modified dilution approach when applied to other phytoplankton taxa and communities.

During an experiment in the North Atlantic in June 1998, water samples were collected approximately 400 km south of Iceland inside and outside of a bloom of the coccolithophorid Emilia- nia huxleyi. In vitro dimethylsulphoniopropionate (DMSP) lyase activity (DLA) was quantified using gas chromatography and found to vary from 0.1 to 142.3 nM dimethyl sulphide (DMS) h -1 . Inside the bloom area the majority of DLA (> 74%) occurred in particles >10 µm, indicating that E. huxleyi (5 to 7 µm diameter) made only a minor contribution to total DLA. In surface waters, phototrophic dino- flagellates (>10 µm) made up a high proportion of the total phytoplankton biomass (~27%) towards the end of the coccolithophorid bloom and may have been the source of most of the DLA. This was also indicated by a significant correlation (p < 0.02) between DLA and the concentration of peridinin, a pigment used as a chemotaxonomic marker for dinophytes. The data presented here are the first field measurements of DLA in a coccolithophorid bloom and suggest that even a relatively low con- centration of photosynthetic dinoflagellates larger than 10 µm (during our study 18 to 105 cells ml -1 ) may contribute significantly to DMS production. Although dinoflagellates are recognised as an important source of particulate DMSP, recognition of their significance for DMS production in the field has previously been limited to a few observations in highly concentrated coastal and shelf blooms. Very little information exists on DLA in dinophytes and further investigations are warranted in order to improve our understanding of the biogeochemical and ecophysiological significance of DMSP lyases in this group of phytoplankton.

This paper presents an overview of dimethyl sulphide biogeochemistry within a coccolithophore bloom (DISCO), an integrated, multidisciplinary Lagrangian process study of the routes, rates and controls on the biogeochemical cycling of dimethyl sulphide (DMS) within a growing bloom of the coccolithophorid alga, Emiliania huxleyi . The Lagrangian study took place between 16 and 26 June 1999 in the northern North Sea. It was preceded by an 8-d survey of ∼52,000 km 2 of the region to locate an E. huxleyi bloom suitable for study. Although not originally planned, the survey was carried out because heavy cloud cover precluded use of remote sensing to locate a suitable bloom. E. huxleyi blooms, typically common in the region during mid-summer, were unusually sparse in the study area. The bloom chosen for the process study was initially centred ∼58°56′N 02°52′E, and a 40-km 2 patch of water was labelled for study with ∼30 g sulphur hexafluoride (SF 6 ) on 16 June. The original patch was reinfused with further SF 6 on 24 June. During the process study, the SF 6 -labelled patch moved in a south-easterly direction and the study ended when the patch subducted underneath less dense Norwegian coastal water. The process study comprised analyses of the time-varying biological, optical and physical properties of the patch as well as studies of DMS, dimethylsulphonioproprionate (DMSP), dimethylsulphoxide, nutrients, halocarbons, methylamines, carbon monoxide, dissolved organic carbon, and total dissolved nitrogen. The role of viruses, bacteria, phytoplankton, microzooplankton, and mesozooplankton, together with the dynamics of primary, new and bacterial production, plankton respiration, microzooplankton grazing, and sedimentation, were studied in relation to the biogeochemical cycling of DMS. Although the coccolithophore bloom water exhibited high optical backscatter, the algal community present was highly heterogeneous. Flagellates other than E. huxleyi were found to dominate the phytoplankton. A budget of the DMSP pools suggested that E. huxleyi accounted for only 13% of the stocks of particulate DMSP, showing that in this “ E. huxleyi bloom”, taxa other than E. huxleyi were important sources of DMSP. In this young bloom, particulate and dissolved DMSP and DMS concentrations averaged 1360, 155 and 60 μM m −2 , respectively, in the surface mixed layer. Surface-water particulate DMSP concentrations increased during the study at a net rate of 13% d −1 , as did concentrations of phytoplankton including E. huxleyi , confirming that the bloom was developing. Nutrient conditions were low in the mixed layer throughout the study, maintained by a strong pycnocline across which nitrate upflux was estimated to be ∼2 nM dm −3 d −1 . Primary production was fuelled by regenerated nutrients, although nitrification rates in surface waters were found to be significant. Microzooplankton grazing accounted for 91% of the particulate DMSP degradation and was considered to be a major control on the DMSP concentration. Vigorous microzooplankton grazing together with rapid uptake of dissolved DMSP by bacteria suggest that microzooplankton were the main route for the production of dissolved DMSP. The bacterial community was dominated by one taxon, an α proteobacteria related to Roseobacter that satisfied its entire sulphur demand by metabolising dissolved DMSP. Bacteriogenic DMS production amounted to 2 nM d −1 and was considered the main route for DMS production. In vitro DMSPlyase activity was very high, but there was little evidence for high in situ activity. Over the study period, DMS flux to the atmosphere was estimated to be 7 μM m −2 d −1 , equivalent to ∼1% of the DMSP sulphur produced in the surface mixed layer. A budget for DMS cycling in the upper mixed layer is presented based on the analytical and experimental measurements made in the DISCO study.

The structure and diversity of the microplankton, between 10 and 200 μm in size, were quantified during a Lagrangian SF 6 tracer experiment in the North Sea (59°N and 03°E) during June 1999. The experiment was conducted within a developing Emiliania huxleyi -rich phytoplankton bloom. Distinct thermoclines characterised the water column. The surface mixed layer extended to between 15 and 30 m, and the subsurface layer reached a depth of between 45 and 55 m. Despite the relatively high abundance of E. huxleyi ( −1 ), a diverse plankton community was observed throughout the study, with 59 autotrophic and 57 heterotrophic microplankton species being identified. Distinct spatial patterns were observed in the plankton, as smaller cells were common in the surface layer and larger species characterised the subsurface waters. The microplankton communities in the two layers, were found to be significantly different from each other. Prorocentrum minimum and P-Nitzschia ‘delicatissima ’ were the dominant microphytoplankton species in surface waters, while the larger diatom species P-Nitzschia ‘seriata’, Chaetoceros radicans , and Rhizosolenia stolterfothii were common in the subsurface layer. The microzooplankton were dominated by oligotrich ciliates and dinoflagellates, with smaller forms such as Strombidium spp., Diplopsalis sp., and Protoperidinium cf. cerasus characterising surface waters, and larger species, Strombidium ovale, Gyrodinium fusiforme , and Gyrodinium sp. dominating the subsurface layer. Significant temporal changes were observed in the composition of the surface-layer microplankton community, as well as in the structure of the subsurface microzooplankton. Measures of diversity indicated that there were significant differences in the microphytoplankton between surface and subsurface waters and that the temporal changes in the surface layer were also significant. These changes coincided with an increase in microzooplankton grazing pressure and a shift in the species composition of the mesozooplankton. Coupled with observations that regenerated nutrients provided an important nitrogen supply for phytoplankton growth during this study, our data suggest that ‘top-down’ control played an important role in enhancing diversity and determining the structure of the plankton in the stratified northern North Sea, during the summer. The significant changes observed in the microplankton during this study, occurred over a relatively short period of ca. 9 days and emphasise the importance of understanding short-term fluctuations in order to make accurate longer-term predictions.

Bacterioplankton-driven turnover of the algal osmolyte, dimethylsulphoniopropionate (DMSP), and its degradation product, dimethylsulphide (DMS) the major natural source of atmospheric sulphur, were studied during a Lagrangian SF6-tracer experiment in the North Sea (60°N, 3°E). The water mass sampled within the euphotic zone was characterised by a surface mixed layer (from 0 m to 13–30 m) and a subsurface layer (from 13–30 m to 45–58 m) separated by a 2°C thermocline spanning 2 m. The fluxes of dissolved DMSP (DMSPd) and DMS were determined using radioactive tracer techniques. Rates of the simultaneous incorporation of 14C-leucine and 3H-thymidine were measured to estimate bacterioplankton production. Flow cytometry was employed to discriminate subpopulations and to determine the numbers and biomass of bacterioplankton by staining for nucleic acids and proteins. Bacterioplankton subpopulations were separated by flow cytometric sorting and their composition determined using 16S ribosomal gene cloning/sequencing and fluorescence in situ hybridisation with designed group-specific oligonucleotide probes. A subpopulation, dominated by bacteria related to Roseobacter-(α-proteobacteria), constituted 26–33% of total bacterioplankton numbers and 45–48% of biomass in both surface and subsurface layers. The other abundant prokaryotes were a group within the SAR86 cluster of γ-proteobacteria and bacteria from the Cytophaga–Flavobacterium—cluster. Bacterial consumption of DMSPd was greater in the subsurface layer (41 nM d−1) than in the surface layer (20 nM d−1). Bacterioplankton tightly controlled the DMSPd pool, particularly in the subsurface layer, with a turnover time of 2 h, whereas the turnover time of DMSPd in the surface layer was 10 h. Consumed DMSP satisfied the majority of sulphur demands of bacterioplankton, even though bacterioplankton assimilated only about 2.5% and 6.0% of consumed DMSPd sulphur in the surface and subsurface layers, respectively. Bacterioplankton turnover of DMS was also faster in the subsurface layer (12 h) compared to the surface layer (24 h). However, absolute DMS consumption rates were higher in the surface layer, due to higher DMS concentrations in this layer. The majority of DMS was metabolised into dissolved non-volatile products, and bacteria could satisfy only 1–3% of their sulphur demands from DMS. Thus, structurally similar bacterioplankton communities exerted strong control over DMSPd and DMS concentrations both in the subsurface layer and surface mixed layer.

One of the key steps towards predicting dimethyl sulphide (DMS) emissions to the atmosphere is to understand the production and fate of its precursor, dimethylsulphoniopropionate (DMSP). This study used the framework of a SF 6 -Lagrangian experiment lasting 6 days, to examine the production and turnover of particulate DMSP (DMSPp) within a developing phytoplankton bloom characterised by abundant Emiliania huxleyi . Detailed information on the composition of the phytoplankton community and literature-derived estimates of cell DMSP content were used to partition DMSPp between 6 taxonomic groups. E. huxleyi was estimated to contribute an average of only 16% and 9% of the DMSPp standing stocks in surface and subsurface layers of the water column, respectively. Other phototrophs, including non-lithed nanoflagellates and dinoflagellates, in particular Prorocentrum minimum, made up a substantial proportion of the DMSPp, especially in the subsurface layer. The Lagrangian approach allowed a direct estimate of the net production of DMSPp, chlorophyll a and the 6 phytoplankton taxonomic groups in the surface layer. The net specific accumulation rate of DMSPp was 0.129 day −1 , equivalent to a net production of DMSPp over the 6 days of 37.3 nM averaged through the surface-layer depth. The net phytoplankton growth rate in terms of chlorophyll a was similar, at 0.108 day −1 . However, not all the phytoplankton showed a net growth during the 6 days, indicating that only certain taxa were responsible for the increases in DMSP and chlorophyll a . A significant relationship between 14 C primary production measurements and the potential production of DMSPp derived from dilution experiments was used to estimate an integrated ‘gross’ production and loss rate of DMSPp for the surface mixed layer. High DMSPp production rates were closely matched by high DMSPp loss rates. Ingestion by microzooplankton appeared to be the major cause of DMSPp loss, accounting for an average of 91% of the DMSPp disappearance. A large proportion of the ingested DMSPp was thought to be released as dissolved DMSP rather than DMS. An estimate of the total DMS flux to the atmosphere was equivalent to only 1.3% of the ‘gross’ DMSPp production in the surface layer during the developing phase of the phytoplankton bloom covered by this experiment. Other processes known to cause DMSP release from phytoplankton, such as viral lysis and senescence, may have become more important if the phytoplankton community had reached higher concentrations and/or encountered more growth-limiting conditions. However, in the present experiment horizontal and vertical mixing appeared to play a major role in determining the fate of the SF 6 -labelled water, and hence the progression of the phytoplankton community.

The algal osmolyte, dimethylsulphoniopropionate (DMSP), is abundant in the surface oceans and is the major precursor of dimethyl sulphide (DMS), a gas involved in global climate regulation. Here, we report results from an in situ Lagrangian study that suggests a link between the microbially driven fluxes of dissolved DMSP (DMSPd) and specific members of the bacterioplankton community in a North Sea coccolithophore bloom. The bacterial population in the bloom was dominated by a single species related to the genus Roseobacter, which accounted for 24% of the bacterioplankton numbers and up to 50% of the biomass. The abundance of the Roseobacter cells showed significant paired correlation with DMSPd consumption and bacterioplankton production, whereas abundances of other bacteria did not. Consumed DMSPd (28 nM day(-1)) contributed 95% of the sulphur and up to 15% of the carbon demand of the total bacterial populations, suggesting the importance of DMSP as a substrate for the Roseobacter-dominated bacterioplankton. In dominating DMSPd flux, the Roseobacter species may exert a major control on DMS production. DMSPd turnover rate was 10 times that of DMS (2.7 nM day(-1)), indicating that DMSPd was probably the major source of DMS, but that most of the DMSPd was metabolized without DMS production. Our study suggests that single species of bacterioplankton may at times be important in metabolizing DMSP and regulating the generation of DMS in the sea.

Dimethylsulphoniopropionate (DMSP) synthesised by phytoplankton is the principal precursor of the climatically active gas dimethyl sulphide (DMS). The rates of production of particulate DMSP (DMSPp) and turnover by microzooplankton were determined in surface waters of the northern North Sea, using a dilution approach. The phytoplankton communities were characterised by DMSP-rich taxa including Emiliania huxleyi and Prorocentrum minimum and DMSPp:chlorophyll a (chl a) ratios of 64 to 162 nM ?g-1. Microzooplankton biomass varied from 25.5 to 56.7 ?g C l-1 and was dominated by oligotrich ciliates and heterotrophic dinoflagellates. DMSPp production rates ranged from 14.8 to 45.6 nM d-1 and represent a doubling time of the ambient DMSPp pool of between 1.2 and 3.1 d. Consumption rates of DMSPp by microzooplankton varied between 11.4 and 59.9 nM d-1 and were equivalent to turnover rates of the ambient DMSPp pool of between 16 and 43% d-1. In general, production rates of DMSPp were lower than those of chl a and E. huxleyi, with respective mean doubling times of 1.9, 1.5 and 1.3 d. Loss rates due to grazing were similar for DMSPp and E. huxleyi but generally significantly lower than those of the bulk phytoplankton, with mean turnover rates of 31, 30 and 40% d-1 of the standing stock of DMSPp, E. huxleyi and chl a, respectively. E. huxleyi contributed an estimated 2 to 25% of the total DMSPp production and 6 to 23% of the DMSPp ingested by microzooplankton, indicating the importance of other phytoplankton to DMSPp dynamics in ŒE. huxleyi blooms¹. At the depths sampled, DMSPp production was closely coupled to primary production and was equivalent to approximately 11% of the carbon fixation. DMSPp may be an important component of the diets of microzooplankton. Ingested DMSPp could have provided 2 to 3% of the microzooplankton carbon demand and 26 to 44% of their sulphur demand. DMSPp production and turnover rates were closely matched and suggest that in these waters microzooplankton grazing may be the principal determinant of the fate of DMSPp. The quantity of ŒDMSP¹ excreted by microzooplankton, calculated from ingestion rates, biomass and assumed growth rates and growth efficiencies, ranged from 8.0 to 41.9 nM d-1. This is equivalent to 13 to 35% d-1 of the DMSPp standing stocks and sufficient to support a daily turnover of the DMS plus dissolved DMSP (DMS+DMSPd) pool.

A dilution approach for quantifying the grazing-mediated production of dissolved di- methylsulphoniopropionate (DMSPd) and dimethyl sulphide (DMS) was tested using laboratory cul- tures and in natural marine waters. We attempted to make simultaneous estimates of microzoo- plankton grazing rate and grazing-mediated production of DMS and DMSPd. In the laboratory, the dinoflagellate Oxyrrhis marina grazed on the prymnesiophyte Isochrysis galbana at a rate equivalent to a turnover of 57% d -1 of the standing stock. Three experiments using natural waters are presented; 1 in the southern North Sea on 13 April 1998 and 2 in the Iceland Basin on 27 June and 4 July 1998. In all cases there was significant microzooplankton grazing with a turnover of 20% d -1 of the chloro- phyll a in the southern North Sea on 13 April and 19 and 15% d -1 of the nanophytoplankton on 27 June and 4 July in the Iceland Basin, respectively. Production rates of DMS, DMSPd and DMS+DMSPd due to grazing were calculated from the slope of the regression between algal-specific production and the level of dilution, a proxy of the grazing pressure. DMS+DMSPd production rates due to microzooplankton grazing of 31 ± 6, 28 ± 2, 14 ± 3 and 10 ± 1 nM d -1 (± SE), were measured in the laboratory and on 13 April, 27 June and 4 July. These values represent a conversion of 19, 14, 15 and 9% d -1 of the standing stock of DMSPp to DMS+DMSPd. The potential impact of bacterial metabolism of DMS and DMSPd on estimates of grazing mediated production together with other competing production/loss processes are discussed. We suggest that in marine surface waters micro- zooplankton grazing plays an important role in the generation of the dissolved pool of DMS+DMSPd.

The rates of plankton community production and respiration were determined from in vitro changes in dissolved inorganic carbon and dissolved oxygen and the incorporation of (NaHCO3)-C-14 at a coastal site in East Antarctica between 16 December 1993 and 12 February 1994. The breakout of seasonal fast ice was associated with a succession of dominant phytoplankton from Cryptomonas to Phaeocystis to a diatom assemblage. Gross production reached 33 mmol C m(-3) d(-1) and C-14 incorporation peaked at 24 mmol C m(-3) d(-1) on 23 January 1994, at the time of the chlorophyll a maximum (22 mg chi a m(-3)). Dark community respiration reached its maximum (13 mmol C m(-3) d(-1)) 4 d later. Photosynthetic rates calculated from C-14 incorporation were significantly lower (17 to 59%) than rates of gross production. The derivation of plankton processes from changes in both dissolved oxygen and dissolved inorganic carbon allowed the direct measurement of photosynthetic and respiratory quotients. A linear regression of all data gave a photosynthetic quotient of 1.33 +/- 0.23 and a respiratory quotient of 0.88 +/- 0.14. Concurrent determinations of bacterial, heterotrophic dinoflagellate, nanoflagellate and ciliate respiration could account for 15 to 58% of measured dark-community respiration. This study has improved the sparse data set of plankton respiration measurements, confirmed that heterotrophic respiration is a significant process in the carbon flux of coastal Antarctic waters and achieved a first apportionment of community respiration to the major microbial groups in this region

Heterotrophic dinoflagellates and their herbivory were quantified at a coastal site in East Antarctica in the vicinity of the Australian Antarctic station of Davis (68° 35' S, 77° 58' E). The study period, 14 January to 11 February 1994, coincided with the growth and decline of a diatom-dominated phytoplankton bloom. Nine taxa of heterotrophic dinoflagellates, including 2 naked and 7 armoured forms, were identified and selected for the determination of standing stock and grazing rates. All 9 taxa selected for grazing rate measurements showed an increase in abundance and biomass during the phytoplankton bloom. Total abundance and biomass increased exponentially from 14 January to reach a maximum abundance, when the 9 taxa were combined, of 46400 cells l-1 on the 31 January, equivalent to a standing stock of 114.5 µg C l-1. Taxon-specific grazing rates were determined at in situ predator and prey concentrations by tracing 14C through a 3 compartment (water, phytoplankton, heterotrophic dinoflagellate) model. Mean taxon-specific clearance rates varied more than 10-fold from 0.028 µl cell-1 h-1 in Diplopeltopsis spp. to 0.318 µl cell-1 h-1 in a Protoperidinium sp. In contrast, mean taxon-specific rates of ingestion varied only 3-fold from 0.72 pg chl a cell-1 h-1 in Diplopeltopsis spp. to 2.38 pg chl a cell-1 h-1 in the same Protoperidinium sp. The total ingestion rate of the 9 taxa was 29.7 ng chl a l-1 h-1 on 31 January, of which 92% was consumed by the 3 most abundant taxa, Gyrodinium sp.1, Gyrodinium sp.2 and Diplopeltopsis spp. This activity represented 6.7% of the water column cleared, 4.8% of autotrophic biomass and 25% of daily primary production ingested per day. The estimated rates of specific ingestion and growth of heterotrophic dinoflagellates were lower in the coastal waters of East Antarctica than in laboratory studies carried out at higher temperatures. However, when the environmental parameters, predator size and prey type and concentration are taken into account, values measured in the present study are tenable, comparing well with other in situ measurements. Furthermore, the estimates of grazing impact on phytoplankton biomass and production illustrate that heterotrophic dinoflagellates play an important part in the biotic control of phytoplankton production and therefore of carbon flux through the food web of coastal waters of East Antarctica.

The coastal sea ice in the vicinity of Davis Station, Antarctica (68* 35' S, 77* 58' E), supported a diverse microbial community which varied in composition and biomass in response to increasing insolation and temperature during the austral summer. To understand more fully the fate of photosynthetically fixed carbon in sea ice, we examined the dynamics of community composition, biomass and production in autotrophs, heterotrophic protozoa and bacteria. The microbial community inhabiting the bottom few centimeters of land fast ice differed markedly from the interior communities in taxonomic composition and biomass and in the timing and fate of production. Total microbial biomass integrated throughout the ice depth declined during the season from a mean of 1150 mg C m-2 on 17 November to 628 mg C m-2 by 22 December. This largely reflected a decrease in the biomass of the bottom ice community which was dominated by the diatom Entomoneis spp. In contrast, the biomass of the interior ice community increased during summer and was dominated by autotrophic forms

Increasing atmospheric CO2 is decreasing ocean pH most rapidly in colder regions such as the Arctic. As a component of the EPOCA (European Project on Ocean Acidification) pelagic mesocosm experiment off Spitzbergen in 2010, we examined the consequences of decreased pH and increased pCO2 on the concentrations of dimethylsulphide (DMS). DMS is an important reactant and contributor to aerosol formation and growth in the Arctic troposphere. In the nine mesocosms with initial pHT 8.3 to 7.5, equivalent to pCO2 of 180 to 1420 μatm, highly significant but inverse responses to acidity (hydrogen ion concentration [H&plus;]) occurred following nutrient addition. Compared to ambient [H&plus;], average concentrations of DMS during the mid-phase of the 30 d experiment, when the influence of altered acidity was unambiguous, were reduced by approximately 60% at the highest [H&plus;] and by 35% at [H&plus;] equivalent to 750 μatm pCO2, as projected for 2100. In contrast, concentrations of dimethylsulphoniopropionate (DMSP), the precursor of DMS, were elevated by approximately 50% at the highest [H&plus;] and by 30% at [H&plus;] corresponding to 750 μatm pCO2. Measurements of the specific rate of synthesis of DMSP by phytoplankton indicate increased production at high [H&plus;], in parallel to rates of inorganic carbon fixation. The elevated DMSP production at high [H&plus;] was largely a consequence of increased dinoflagellate biomass and in particular, the increased abundance of the species Heterocapsa rotundata. We discuss both phytoplankton and bacterial processes that may explain the reduced ratios of DMS:DMSPt (total dimethylsulphoniopropionate) at higher [H&plus;]. The experimental design of eight treatment levels provides comparatively robust empirical relationships of DMS and DMSP concentration, DMSP production and dinoflagellate biomass versus [H&plus;] in Arctic waters.

Muscle capillary supply, mitochondrial distribution, and the surface density of mitochondrial cristae have been determined in the pectoralfin adductorprofundis muscles of fish that use a labriform mode of locomotion. Species studied were the hemoglobinless Antarctic icefishes Chaenocephalus aceratus and Champsocephalus gunnari, the red-blooded Antarctic fishes Notothenia gibberifrons and Psilodraco breviceps, and the Northern Hemisphere temperate-water fish Callionymus lyra. The volume density of mitochondria in slow-twitch fibers decreased in the order C. gunnari (0.51) > C. aceratus (0.49) > P. breviceps (0.46) > N. gibberifrons (0.25). Surface densities of mitochondrial cristae were lower in C. gunnari (25.2 μm² μm⁻ ³) and C. aceratus (28.2 μm² μm⁻ ³) than in the red-blooded species (32.2-37.0 μm² μm⁻ ³). This is consistent with the observation that O₂ consumption per unit volume of mitochondria is lower in the hemoglobinless species. The density of mitochondria adjacent to capillaries is not significa...

Size related changes in muscle twitch kinetics, morphometrics and innervation have been examined in cod, Gadus morhua. Fish size ranged from 9 cm to 45 cm in total length (L). Twitch contraction time (time to 90% relaxation), scaled in proportion to L0.29. Scaling of morphometric parameters was essentially geometric. Mean cross‐sectional area and weight of the myotomal muscle scaled in proportion to L2.05 and L3.08 respectively. These results are discussed in the light of alterations in length specific swimming performance and kinematics as fish grow. During growth, the number of endplates terminating on each fast fibre increased, from around 10 on fibres 2 mm in length (∼10 cm fish) to 20 on 10 mm fibres (—40 cm fish). However, mean spacing between endplates increased from around 0.25 mm to 0.50 mm. The functional significance of polyneuronal innervation in teleost fast muscle fibres is discussed.

A histochemical study was carried out on muscle fibre types in the myotomes of post-larval and adult stages of seven species of notothenioid fish. There was little interspecific variation in the distribution of muscle fibre types in post-larvae. Slow fibres (diameter range 15–60 μm) which stained darkly for succinic dehydrogenase activity (SDHase) formed a superficial layer 1–2 fibres thick around the entire lateral surface of the trunk. In all species a narrow band of very small diameter fibres (diameter range 5–62 μm), with only weak staining activity, occurred between the skin and slow fibre layer. These have the characteristics of tonic fibres found in other teleosts. The remainder of the myotome was composed of fast muscle fibres (diameter range 9–75 μm), which stain weakly for SDHase, α-glycerophosphate dehydrogenase, glycogen and lipid. Slow muscle fibres were only a minor component of the trunk muscles of adult stages of the pelagic species Champsocephalus gunnari and Pseudochaenichthys georgianus, consistent with a reliance on pectoral fin swimming during sustained activity. Of the other species examined only Psilodraco breviceps and Notothenia gibberifrons had more than a few percent of slow muscle in the trunk (20%–30% in posterior myotomes), suggesting a greater involvement of sub-carangiform swimming at cruising speeds. The ultrastructure of slow fibres from the pectoral fin adductor and myotomal muscles of a haemoglobinless (P. georgianus) and red-blooded species (P. breviceps), both active swimmers, were compared. Fibres contained loosely packed, and regularly shaped myofibrils numerous mitochondria, glycogen granules and occasional lipid droplets. Mitochondria occupied >50% of fibre volume in the haemoglobinless species P. georgianus, each myofibril was surrounded by one or more mitochondria with densely packed cristae. No significant differences, however, were found in mean diameter between fibres from red-blooded and haemoglobinless species. The activities of key enzymes of energy metabolism were determined in the slow (pectoral) and fast (myotomal) muscles of N. gibberifrons. In contrast to other demersal Antarctic fish examined, much higher glycolytic activities were found in fast muscle fibres, probably reflecting greater endurance during burst swimming.