Demethylation and cleavage of dimethylsulfoniopropionate in marine intertidal sediments

Abstract Demethylation and cleavage of dimethylsulfoniopropionate (DMSP) was measured in three different types of intertidal marine sediments: a cyanobacterial mat, a diatom-covered tidal flat and a carbonate sediment. Consumption rates of added DMSP were highest in cyanobacterial mat slurries (59 μmol DMSP 1⁻¹) and lower in slurries from a diatom mat and a carbonate tidal sediment (24 and 9 μmol DMSP 1⁻¹ h⁻¹, respectively). Dimethyl sulfide (DMS) and 3-mercaptopropionate (MPA) were produced simultaneously during DMSP consumption, indicating that cleavage and demethylation occurred at the same time. Viable counts of DMSP-utilizing bacteria revealed a population of 2 × 10⁷ cells cm⁻³ sediment (90% of these cleaved DMSP to DMS, 10% demethylated DMSP to MPA) in the cyanobacterial mat, 7 × 10⁵ cells cm⁻³ in the diatom mat (23% cleavers, 77% demethylators), and 9 × 10⁴ cells cm⁻³ (20% cleavers and 80% demethylators) in the carbonate sediment. In slurries of the diatom mat, the rate of MPA production from added 3-methiolpropionate (MMPA) was 50% of the rate of MPA formation from DMSP. The presence of a large population of demethylating bacteria and the production of MPA from DMSP suggest that the demethylation pathway, in addition to cleavage, contributes significantly to DMSP consumption in coastal sediments.     

A hypersaline microbial mat from the Pacific Atoll Kiritimati: insights into composition and carbon fixation using biomarker analyses and a 13C-labeling approach

Modern microbial mats are widely recognized as useful analogs for the study of biogeochemical processes relevant to paleoenvironmental reconstruction in the Precambrian. We combined microscopic observations and investigations of biomarker composition to investigate community structure and function in the upper layers of a thick phototrophic microbial mat system from a hypersaline lake on Kiritimati (Christmas Island) in the Northern Line Islands, Republic of Kiribati. In particular, an exploratory incubation experiment with ¹³C-labeled bicarbonate was conducted to pinpoint biomarkers from organisms actively fixing carbon. A high relative abundance of the cyanobacterial taxa Aphanocapsa and Aphanothece was revealed by microscopic observation, and cyanobacterial fatty acids and hydrocarbons showed ¹³C-uptake in the labeling experiment. Microscopic observations also revealed purple sulfur bacteria (PSB) in the deeper layers. A cyclic C_19:0 fatty acid and farnesol were attributed to this group that was also actively fixing carbon. Background isotopic values indicate Calvin–Benson cycle-based autotrophy for cyc C_19:0 and farnesol-producing PSBs. Biomarkers from sulfate-reducing bacteria (SRB) in the top layer of the mat and their ¹³C-uptake patterns indicated a close coupling between SRBs and cyanobacteria. Archaeol, possibly from methanogens, was detected in all layers and was especially abundant near the surface where it contained substantial amounts of ¹³C-label. Intact glycosidic tetraether lipids detected in the deepest layer indicated other archaea. Large amounts of ornithine and betaine bearing intact polar lipids could be an indicator of a phosphate-limited ecosystem, where organisms that are able to substitute these for phospholipids may have a competitive advantage.     

Bacterially mediated precipitation in marine stromatolites

Stromatolites are laminated, lithified (CaCO₃) sedimentary deposits formed by precipitation and/or sediment accretion by cyanobacterial–bacterial mat communities. Stromatolites have been associated with these communities as far back as the Precambrian era some 2+ billion years ago. The means by which microbial communities mediate the precipitation processes have remained unclear, and are the subject of considerable debate and speculation. Two alternative explanations for microbially mediated precipitation include: (i) cyanobacterial photosynthesis increases pH in a system supersaturated in respect of CaCO₃, resulting in CaCO₃ precipitation and then laminated lithification, and (ii) decomposition of cyanobacterial extracellular organic matter (e.g. sheaths, mucilage and organic acids) by microheterotrophs leads to release of organic-bound Ca²⁺ ions and CaCO₃ precipitation. We evaluated these explanations by examining metabolically active, lithifying stromatolitic mat communities from Highborne Cay, Bahamas, using microautoradiography. Microautoradiographic detection of ¹⁴CO₂ fixation and ³H organic matter ( d -glucose and an amino acid mixture) utilization by photosynthetically active cyanobacteria and microheterotrophs, combined with community-level uptake experiments, indicate that bacteria, rather than cyanobacteria are the dominant sites of CaCO₃ deposition. In the oligotrophic waters in which stromatolites exist, microheterotrophs are reliant on the photosynthetic community as a main source of organic matter. Therefore, autotrophic production indirectly controls microbially mediated precipitation and stromatolite formation in these shallow marine environments.     

Procedures for large-scale production and purification of Clostridium botulinum C1 toxin for preparation of toxoid

Abstract Fortified cooked meat medium containing calcium carbonate (CaCO₃-FCM) supported toxin production of a strain of Clostridium botulinum type C to a level of 2 × 10⁶ mouse i.p. LD₅₀/ml. C1 toxin was purified by sequential steps of acid precipitation from 5-fold diluted culture supernatant in the presence of RNA, 2nd acid precipitation by dialysis, removal of RNA by protamine treatment, removal of excess protamine and bufferisation by ultrafiltration through Amicon PM-30 membrane, sulphopropyl-Sephadex chromatography, and Sephadex G-200 gel filtration. By these procedures, 25 mg or more of highly purified C1 toxin was constantly obtained from a lot of 600-ml culture.     

INTRACELLULAR CYANOBACTERIAL SYMBIONTS IN THE MARINE DIATOM CLIMACODIUM FRAUENFELDIANUM (BACILLARIOPHYCEAE)

The diatom Climacodium frauenfeldianum Grunow was collected in the tropical Atlantic and Pacific Oceans. Observations with epifluorescence microscopy revealed that this diatom contained coccoid symbionts (2.5–3.5 μm) with a typical cyanobacterial fluorescence in addition to that of their own chloroplasts. Mean concentration of C. frauenfeldianum for 28 stations in the SW tropical Pacific Ocean was 530 x 10³ (SE = 1372) cells·m ^{− 2}, with highest concentration (mean 17.5 cells·L ^{− 1}) at 40-m depth. The symbiosis was only observed at water temperatures between 26.3 and 28.9° C, with highest concentrations at 27.7° C. Three almost complete 16S rDNA sequences from one sample were determined, and they were identical. The phylogenetic analysis of this 16S rDNA sequence and those from other cyanobacteria and plastids revealed that it was closely related to the 16S rDNA sequence from Cyanothece sp. ATCC 51142. Cyanothece sp. ATCC 51142 is a unicellular nitrogen-fixing cyanobacterium isolated from a coastal marine environment and has ultrastructural features similar to the symbionts of C. frauenfeldianum . The close relationship between Cyanothece sp. and the cyanobacterial symbiont in C. frauenfeldianum suggests the potential for nitrogen fixation in the symbiosis.     

Autotrophy of green non-sulphur bacteria in hot spring microbial mats: biological explanations for isotopically heavy organic carbon in the geological record

Inferences about the evidence of life recorded in organic compounds within the Earth's ancient rocks have depended on ¹³C contents low enough to be characteristic of biological debris produced by the well-known CO₂ fixation pathway, the Calvin cycle. 'Atypically' high values have been attributed to isotopic alteration of sedimentary organic carbon by thermal metamorphism. We examined the possibility that organic carbon characterized by a relatively high ¹³C content could have arisen biologically from recently discovered autotrophic pathways. We focused on the green non-sulphur bacterium Chloroflexus aurantiacus that uses the 3-hydroxypropionate pathway for inorganic carbon fixation and is geologically significant as it forms modern mat communities analogous to stromatolites. Organic matter in mats constructed by Chloroflexus spp. alone had relatively high ¹³C contents (−14.9‰) and lipids diagnostic of Chloroflexus that were also isotopically heavy (−8.9‰ to −18.5‰). Organic matter in mats constructed by Chloroflexus in conjunction with cyanobacteria had a more typical Calvin cycle signature (−23.5‰). However, lipids diagnostic of Chloroflexus were isotopically enriched (−15.1‰ to −24.1‰) relative to lipids typical of cyanobacteria (−33.9‰ to −36.3‰). This suggests that, in mats formed by both cyanobacteria and Chloroflexus , autotrophy must have a greater effect on Chloroflexus carbon metabolism than the photoheterotrophic consumption of cyanobacterial photosynthate. Chloroflexus cell components were also selectively preserved. Hence, Chloroflexus autotrophy and selective preservation of its products constitute one purely biological mechanism by which isotopically heavy organic carbon could have been introduced into important Precambrian geological features.     

Calcium-dependent transferrin receptor recycling in bovine chromaffin cells

The release of regulated secretory granules is known to be calcium dependent. To examine the Ca²⁺-dependence of other exocytic fusion events, transferrin recycling in bovine chromaffin cells was examined. Internalised ¹²⁵I-transferrin was released constitutively from cells with a half-time of about 7 min. Secretagogues that triggered catecholamine secretion doubled the rate of ¹²⁵I-transferrin release, the time courses of the two triggered secretory responses being similar. The triggered ¹²⁵I-transferrin release came from recycling endosomes rather than from sorting endosomes or a triggered secretory vesicle pool. Triggered ¹²⁵I-transferrin release, like catecholamine secretion from the same cells, was calcium dependent but the affinities for calcium were very different. The extracellular calcium concentrations that gave rise to half-maximal evoked secretion were 0.1 m m for ¹²⁵I-transferrin and 1.0 m m for catecholamine, and the intracellular concentrations were 0.1 μ m and 1 μ m , respectively. There was significant ¹²⁵I-transferrin recycling in the virtual absence of intracellular Ca²⁺, but the rate increased when Ca²⁺ was raised above 1 n m , and peaked at 1 μ m when the rate had doubled. Botulinum toxin type D blocked both transferrin recycling and catecholamine secretion. These results indicate that a major component of the vesicular transport required for the constitutive recycling of transferrin in quiescent cells is calcium dependent and thus under physiological control, and also that some of the molecular machinery involved in transferrin recycling/fusion processes is shared with that for triggered neurosecretion.     

FAST AXOPLASMIC TRANSPORT OF A CALCIUM-BINDING PROTEIN IN MAMMALIAN NERVE

Abstract— Calcium is transported at a fast rate of 410 mm/day in cat sciatic nerve on injection of ⁴⁵Ca²⁺ into the L7 dorsal root ganglia. Nerve segments corresponding to the crest and the plateau regions of transported activity were analyzed by column chromatography on Sephadex G-100 and Biogel A 5m columns and the fast transported ⁴⁵Ca²⁺ found to be bound to a protein of 15,000 dalton. Using [³H]leucine as a precursor, a labeled calcium binding protein (CaBP) was found located at the same position in elution volumes from the columns as was the protein-bound ⁴⁵Ca^{2 +}. The level of [³H]-labeled CaBP in the crest and plateau regions were compared using column chromatography and polyacrylamide gel electrophoresis techniques and approx 3×4 times more [³H]-labeled activity was found in the crest as compared to the plateau. These findings indicate that Ca²⁺ is fast transported in association with the CaBP. The relation of CaBP to the transport filament model of axoplasmic transport and its possible role in nerve are discussed.     

THE γ SUBUNITS OF PHYCOERYTHRIN FROM A RED ALGA: POSITION IN PHYCOBILISOMES AND SEQUENCE CHARACTERIZATION

Aglaothamnion neglectum Feldman-Mazoyer has two γ subunits, γ³¹ and γ³³, that are associated with phycoerythrin in the light-harvesting phycobilisomes. We demonstrate that these subunits are spatially separated within the phycobilisome, with the γ³¹ subunit present at the distal end of phycobilisome rods and the γ³³ subunit present on the proximal end. These subunits are thought to link phycoerythrin hexamers together in the rod substructure, serving a role analogous to that of linker polypeptides of cyanobacteria (although unlike the cyanobacterial linker polypeptides they are chromophorylated). The sequencing of tryptic polypeptides of the γ subunits enabled us to prepare oligonucleotides encoding different regions of γ³¹. These oligonucleotides were used as primers to generate a probe for isolating a γ³¹ cDNA clone. Characterization of the cDNA clone predicts a polypeptide of 280 amino acids with a 42 amino acid presequence that is characteristic of a transit peptide, the peptide that targets proteins to chloroplasts of vascular plants. The γ³¹ subunit has 50% similarity to the previously characterized γ³³ subunit but has no identifiable similarity to functionally related polypeptides present in cyanobacterial phycobilisomes or to any other polypeptides in the databases. A repeat of 95 amino acids is present in the red algal γ subunit sequences, suggesting that these proteins were generated by a gene duplication followed by fusion of the duplicate sequences.     

Distribution of Synechococcus spp. determined by immunofluorescent assay

By using two polyclonal antisera against WH 7803 strain (Synechococcus sp.) and WH 5701 strain (Synechococcus bacillaris) it is possible to detect and to enumerate cells of the two cyanobacterial serogroups. The immunofluorescence technique was used to study the distribution of the two serogroups in the estuarine, coastal and upwelling waters of the Mediterranean Sea surrounding Messina. In the estuarine waters of the Alcantara River (Ionian Sea), the WH 7803 serogroup was present at a concentration in the order of 10² cells ml⁻¹ and the WH 5701 serogroup at a concentration of 5·5 × 10² cellsml⁻¹. In the coastal waters of Messina, where urban and industrial wastes are usuallydumped, the concentration of total phycoerythrin- Synechococcus ranged from 1·3 × 10² to 4·1 × 10³ cells ml⁻¹; the WH 7803 serogroup accounted for 50–94% of the totalpopulation in Ionian stations, whereas the WH 5701 serogroup ranged from1·4 × 10¹ to6·7 × 102cells ml⁻¹. In the upwelling area (Straits of Messina) bothserogroups were found. Vertical distribution of two Synechococcus strains had anopposite trend and their concentrations were of the order of 10¹–10²cells ml⁻¹. Theuse of the Scan laser system allows both autofluorescent and labelled organismsto be distinguished in a preparation for optical microscopy. It also allows false-positivecells to be distinguished.     

Otolith resorption induced by anaerobic stress in the goldfish, Carassius auratus

To examine otolith resorption induced by anaerobic stress, ⁴⁵Ca-prelabelled goldfish, Carassius auratus , were kept in oxygen-deficient ambient water (O₂ < 0.5 .1 1⁻¹, 26° C) for 2 days and otoliths (asterisci) were analysed for ⁴⁵Ca retention. In a second experiment, fish were anaerobically stressed for 2 days, and then received a single intraperitoneal injection of ⁴⁵Ca. They were maintained under stress for one more day and killed to examine ⁴⁵Ca deposition in otoliths. Plasma was analysed for total and radioactive calcium. Otoliths (lapilli) were examined for stress-induced check formation by scanning electron microscopy (SEM).
Stress significantly reduced plasma calcium levels and the rate of calcium retention in otoliths, which was calculated from the ⁴⁵Ca specific activity of the plasma. On the other hand, the rate of calcium deposition in otoliths was the same in the stressed as in the control fish. SEM observation revealed that the applied stress resulted in a check formation in otoliths. These results indicate that a 48-h stress of oxygen deficiency induces calcium resorption in otoliths.     

Inhibition of Na+,K+-ATPase by Ouabain Opens Calcium Channels Coupled to Acetylcholine Release in Guinea Pig Myenteric Plexus

Abstract: Ouabain, an Na⁺,K⁺-ATPase inhibitor, increases the release of acetylcholine (ACh) from various preparations in a Ca²⁺-independent way. However, in other preparations the release of ACh evoked by ouabain is dependent on the presence of extracellular calcium. In the present study, we have labeled the ACh of myenteric plexus longitudinal muscles of guinea pig ileum and compared the effect of calcium channel blockers on ouabain-evoked release of [³H]ACh. Release of [³H]ACh evoked by ouabain is dose dependent and decreased markedly in the absence of calcium or in the presence of cadmium, a nonspecific calcium channel blocker. N-type calcium channel blockage by the ω-conotoxins GVIA (selective N-type calcium channel blocker) and MVIIC (a nonselective calcium channel blocker) inhibited by 45 and 55%, respectively, the release of [³H]ACh. L-type calcium channel suppression by low concentrations of verapamil, nifedipine, and diltiazem had no effect on the release of [³H]ACh. The release of transmitter was also not affected significantly by nickel, a T-type calcium channel blocker. In addition, ω-agatoxin-IVA, at concentrations that block P- and Q-type calcium channels, did not affect significantly the release of [³H]ACh. Thus, extracellular Ca²⁺ is essential for the release of ACh induced by ouabain from guinea pig ileum myenteric plexus. In this preparation, the N-type calcium channel plays a dominant role in transmitter release evoked by inhibition of Na⁺,K⁺-ATPase, but other routes of calcium entry in addition to these channels can also support the release of neurotransmitter induced by ouabain.     

RELEASE OF [3H]GAMMA-AMINOBUTYRIC ACID FROM GLIAL CELLS IN RAT DORSAL ROOT GANGLIA.

Abstract— Desheathed rat dorsal root ganglia were incubated in a medium containing amino-oxyacetic acid and [³H]GABA. Under these conditions, [³H]GABA is taken up exclusively by the satellite glial cells in the ganglia. Efflux of [³H]GABA from the tissue was measured after passing the ganglia through a series of wash solutions. The spontaneous efflux of radioactivity, mostly [³H]GABA, was more rapid in the absence of amino-oxyacetic acid in the incubation and wash media.
Raising the potassium concentration in the wash media caused an increase in the efflux of [³H]GABA. This increase was sigmoidally related to the potassium concentration in the wash media, reaching a maximum at 64 m m -K⁺. The releasing effect of K⁺ was inhibited by removing calcium from the media. Reducing the calcium and raising the magnesium concentration in the wash solutions inhibited the increased efflux of [³H]GABA due to 64 m m -K⁺ by 48 per cent, while 5 mM-La³⁺ and diphenylhydantoin (0·005 and 0·5 m m ) had no effect on this increase.
Only a small increase in the efflux of [¹⁴C]glutamate was produced by 64 m m -K⁺ and it had no effect upon the effluxes of [³H]glycine, [³H]alanine or [³H]leucine. The efflux of lactate dehydrogenase was similarly unaffected by 64 mM-K⁺. The results suggest that glial cells in spinal ganglia can respond to depolarizing concentrations of potassium by releasing GABA in a calcium-dependent process.     

Correlations Between Foliar Stable Carbon Isotope Composition and Environmental Factors in Desert Plant Reaumuria soongorica （Pall.） Maxim.

Leaves of 407 individuals of Reaumuria soongorica （Pall.） Maxim. collected from the major distribution areas were measured to investigate the distribution characteristics of the stable carbon isotope in this desert plant, as well as correlations between δ^13C values and environmental factors. Results showed that δ^13C values in R. soongorica ranged from -22.77‰ to -29.85‰ and that the mean δ^13C value （-26.52‰） was higher than a previously reported δ^13C value for a different desert ecosystem. This indicates that R. soongorica belongs to the C3 photosynthetic pathway and has higher water use efficiency than other species. The correlations between δ^13C values and environmental factors demonstrated that the foliar δ^13C values in R. soongorica increased significantly with decreasing mean annual precipitation and mean relative humidity, and decreased with decreasing duration of sunshine and evaporation. The spatial distribution trend of δ^13C values in R. soongorica was not obvious and there was no significant correlation between the δ^13C values and mean annual temperature. We conclude that different distribution trends in δ^13C values for R. soongorica were likely caused by stomatal limitation rather than by nutrient-related changes in photosynthetic efficiency and that precipitation played an important role in the wide distribution range of R. soongorica. This pattern of δ^13C values for R. soongorica reinforced that it is a super-xerophil in terms of its adaptive strategies to a desert environment.     

Leaf δ13C reflects ecosystem patterns and responses of alpine plants to the environments on the Tibetan Plateau

Leaf δ¹³C is an indicator of water-use efficiency and provides useful information on the carbon and water balance of plants over longer periods. Variation in leaf δ¹³C between or within species is determined by plant physiological characteristics and environmental factors. We hypothesized that variation in leaf δ¹³C values among dominant species reflected ecosystem patterns controlled by large-scale environmental gradients, and that within-species variation indicates plant adaptability to environmental conditions. To test these hypotheses, we collected leaves of dominant species from six ecosystems across a horizontal vegetation transect on the Tibetan Plateau, as well as leaves of Kobresia pygmaea (herbaceous) throughout its distribution and leaves of two coniferous tree species ( Picea crassifolia, Abies fabri ) along an elevation gradient throughout their distribution in the Qilian Mountains and Gongga Mountains, respectively. Leaf δ¹³C of dominant species in the six ecosystems differed significantly, with values for evergreen coniferous13C values of the dominant species and of K. pygmaea were negatively correlated with annual precipitation along a water gradient, but leaf δ¹³C of A. fabri was not significantly correlated with precipitation in habitats without water-stress. This confirms that variation of δ¹³C between or within species reflects plant responses to environmental conditions. Leaf δ¹³C of the dominant species also reflected water patterns on the Tibetan Plateau, providing evidence that precipitation plays a primary role in controlling ecosystem changes from southeast to northwest on the Tibetan Plateau.     

45Ca2+ Uptake into Rat Whole Brain Synaptosomes Unaltered by Dihydropyridine Calcium Antagonists

Abstract: Voltage-dependent ⁴⁵Ca²⁺ uptake into rat whole brain synaptosomes was measured after 3-s KCl-induced depolarization to investigate possible inhibitory effects of calcium antagonists, nitrendipine, nimodipine, and nisoldipine. At a Ca²⁺ concentration of 1.2 m M , nitrendipine, in concentrations ranging from 0.1 n M to 10 μ M , had no effect on ⁴⁵Ca²⁺ uptake. When the Ca²⁺ concentration was lowered to 0.06 and 0.12 m M , nitrendipine, 10 μ M , inhibited ⁴⁵Ca²⁺ uptake in response to 109 m M KCl depolarization. However, in a separate concentration response study, nitrendipine, nimodipine, and nisoldipine, 0.1 n M to 10 μ M , failed to alter the uptake of ⁴⁵Ca²⁺ (0.06 m M Ca²⁺) into 30 m M KCl-depolarized synaptosomes. The high concentrations of these agents required to depress ⁴⁵Ca²⁺ uptake indicate that the dihydropyridine calcium antagonists are considerably less potent in brain tissue than in peripheral tissue.     

Methanogenesis in the hypersaline Solar Lake (Sinai)

Abstract Enrichment studies on microbial mat sediments (potential stromatolites) from the hypersaline Solar Lake (Sinai) indicated high numbers of methanogenic bacteria (up to 10⁵ ml⁻¹ sediment) in spite of the high sulfate reduction rate, sulfate concentration and salinity. Among H₂/CO₂, acetate and monomethylamine, the methylated amine was the preferred substrate. The predominant species enriched was a Methanosarcina sp. The findings indicate that methanogenic bacteria play an important role in hypersaline sulfate-enriched anoxic sediments and stromatolithic microbial mats.     

[3H]Imipramine Is Accumulated but Not Released from Slices of the Rabbit Caudate and Hypothalamus

Abstract: Slices of rabbit caudate and hypothalamus take up and accumulate [³H]imipramine. In superfused slices of both structures electrical stimulation or exposure to tyramine failed to release recently taken up [³H]imipramine. De-polarization by exposure to 30–60 mm-potassium caused only a small release of [³H]imipramine that was not concentration-dependent. The release of [³H]imipramine by high potassium was independent of the presence of calcium ions in the superfusion medium. These results contrasted with those obtained for the release of [³H]dopamine from the caudate and [³H]noradrenaline from the hypothalamus, where tyramine, electrical stimulation, and high potassium caused a significant release of the labeled neurotransmitters. The release of [³H]dopamine from the caudate and [³H]noradrenaline from the hypothalamus elicited by electrical stimulation or high potassium was entirely calcium-dependent. It is concluded that [³H]imipramine is taken up into the two brain regions and is accumulated in a nonvesicular site from which it is not released by calcium-dependent depolarizing stimuli.     

Isotopic analysis of the sources of organic carbon for zooplankton in shallow subarctic and arctic waters

Shallow high-latitude lakes and ponds are usually characterized by an oligotrophic water column overlying a biomass-rich, highly productive benthos. Their pelagic food webs often contain abundant zooplankton but the importance of benthic organic carbon versus seston as their food sources has been little explored. Our objectives were to measure the δ¹³C and δ¹⁵N isotopic signatures of pelagic and benthic particulate organic matter (POM) in shallow water bodies in northern Canada and to determine the relative transfer of this material to zooplankton and other aquatic invertebrates. Fluorescence analysis of colored dissolved organic matter (CDOM) indicated a relatively strong terrestrial carbon influence in five subarctic waterbodies whereas the CDOM in five arctic water columns contained mostly organic carbon of autochthonous origin. The isotopic signatures of planktonic POM and cohesive benthic microbial mats were distinctly different at all study sites, while non-cohesive microbial mats often overlapped in their δ¹³C signals with the planktonic POM. Zooplankton isotopic signatures indicated a potential trophic link with different fractions of planktonic POM and the non-cohesive mats whereas the cohesive mats did not appear to be used as a major carbon source. The zooplankton signals differed among species, indicating selective use of resources and niche partitioning. Most zooplankton had δ¹³C values that were intermediate between the values of putative food sources and that likely reflected selective feeding on components of the pelagic or benthic POM. The results emphasize the likely importance of benthic-pelagic coupling in tundra ecosystems, including for species that are traditionally considered pelagic and previously thought to be dependent only on phytoplankton as their food source.     

Microbial consumption of dimethyl sulfide and methanethiol in coastal marine sediments

Abstract: Samples were taken from oxic and anoxic zones of three ecosystems: a cyanobacterial mat, a diatom film and a carbonate sediment. Dimethylsulfide (DMS) concentrations were determined by headspace analysis of sediment slurries; maximal amounts were in the upper 5–10 mm of the sediments of 20 μM (cyanobacterial mat), 8 μM (diatom film) and < 1 μM in the carbonate sediment. Dissolved DMS in the cyanobacterial mat, determined by centrifugation and cryogenic trapping, was about two orders of magnitude lower than from slurry estimations but its variation with depth was similar. CH₃SH concentrations in slurried samples, determined after treatment with tributylphosphine, ranged from 2 to 7 μM in the diatom mat and was below the limit of detection (< 0.1 μM) in the carbonate sediment. MPN counts of bacteria that grew on DMS under oxic and anoxic (nitrate added) conditions were determined at all three sites. Aerobic DMS utilizers peaked in the surface and decreased with depth, while the population of anaerobic DMS utilizers was relatively constant in the top 20 mm. Populations of DMS utilizers were highest in the cyanobacterial mat and lowest in the carbonate sediment. MPN's of thiosulfate utilizers, aerobic and anaerobic (nitrate added) were determined in the cyanobacterial mat. Populations of aerobic and anaerobic S₂O₃²⁻ utilizers were similar throughout the top 20 mm and comparable to those of DMS utilizers in the top 5 mm, but higher by about 100-fold below that zone. DMS and CH₃SH consumption rates were measured in slurries of sediments and aerobic rates were similar or only slightly higher than anaerobic rates; the latter were stimulated by nitrate.