Increased dimethyl sulphide concentrations in sea water from in situ iron enrichment

Addressing Serine Lability in a Paramagnetic Dimethyl Sulfoxide Reductase Catalytic Intermediate

Two new desoxo molybdenum(V) complexes have been synthesized and characterized as models for the paramagnetic high-g split intermediate observed in the catalytic cycle of dimethyl sulfoxide reductase (DMSOR). Extended X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR) data are used to provide new insight into the geometric and electronic structures of high-g split and other EPR-active type II/III DMSOR family enzyme forms. The results support a 6-coordinate [(PDT)₂Mo(OH)(O_Ser)]^- structure (PDT = pyranopterin dithiolene) for a high-g split with four S donors from two PDT ligands, a coordinated hydroxyl ligand, and a serinate O donor. This geometry orients the redox orbital toward the substrate access channel for the two-electron reduction of substrates.

A high-resolution time-depth view of dimethylsulphide cycling in the surface sea

Emission of the trace gas dimethylsulphide (DMS) from the ocean influences the chemical and optical properties of the atmosphere, and the olfactory landscape for foraging marine birds, turtles and mammals. DMS concentration has been seen to vary across seasons and latitudes with plankton taxonomy and activity, and following the seascape of ocean's physics. However, whether and how does it vary at the time scales of meteorology and day-night cycles is largely unknown. Here we used high-resolution measurements over time and depth within coherent water patches in the open sea to show that DMS concentration responded rapidly but resiliently to mesoscale meteorological perturbation. Further, it varied over diel cycles in conjunction with rhythmic photobiological indicators in phytoplankton. Combining data and modelling, we show that sunlight switches and tunes the balance between net biological production and abiotic losses. This is an outstanding example of how biological diel rhythms affect biogeochemical processes.

The effects of natural iron fertilisation on deep-sea ecology: the Crozet Plateau, Southern Indian Ocean

The addition of iron to high-nutrient low-chlorophyll (HNLC) oceanic waters stimulates phytoplankton, leading to greater primary production. Large-scale artificial ocean iron fertilization (OIF) has been proposed as a means of mitigating anthropogenic atmospheric CO₂, but its impacts on ocean ecosystems below the photic zone are unknown. Natural OIF, through the addition of iron leached from volcanic islands, has been shown to enhance primary productivity and carbon export and so can be used to study the effects of OIF on life in the ocean. We compared two closely-located deep-sea sites (∼400 km apart and both at ∼4200 m water depth) to the East (naturally iron fertilized; +Fe) and South (HNLC) of the Crozet Islands in the southern Indian Ocean. Our results suggest that long-term geo-engineering of surface oceanic waters via artificial OIF would lead to significant changes in deep-sea ecosystems. We found that the +Fe area had greater supplies of organic matter inputs to the seafloor, including polyunsaturated fatty acid and carotenoid nutrients. The +Fe site also had greater densities and biomasses of large deep-sea animals with lower levels of evenness in community structuring. The species composition was also very different, with the +Fe site showing similarities to eutrophic sites in other ocean basins. Moreover, major differences occurred in the taxa at the +Fe and HNLC sites revealing the crucial role that surface oceanic conditions play in changing and structuring deep-sea benthic communities.

Sulfur and phytoplankton: acquisition, metabolism and impact on the environment

Sulfur emission from marine phytoplankton has been recognized as an important factor for global climate and as an entry into the biogeochemical S cycle. Despite this significance, little is known about the cellular S metabolism in algae that forms the basis of this emission. Some biochemical and genetic evidence for regulation of S uptake and assimilation is available for the freshwater model alga Chlamydomonas. However, the marine environment is substantially different from most fresh waters, containing up to 50 times higher free sulfate concentrations and challenging the adaptive mechanisms of primary and secondary S metabolism in marine algae. This review intends to integrate ecological and physiological data to provide a comprehensive view of the role of S in the oceans.

Mechanistic Studies of Rhodobacter sphaeroides Me2SO Reductase

Studies of the molybdenum-containing dimethyl sulfoxide reductase from Rhodobacter sphaeroides have yielded new insight into its catalytic mechanism. A series of reductive titrations, performed over the pH range 6-10, reveal that the absorption spectrum of reduced enzyme is highly sensitive to pH. The reaction of reduced enzyme with dimethyl sulfoxide is found to be clearly biphasic throughout the pH range 6-8 with a fast, initial substrate-binding phase and substrate-concentration independent catalytic phase. The intermediate formed at the completion of the fast phase has the characteristic absorption spectrum of the established dimethyl sulfoxide-bound species. Quantitative reductive and oxidative titrations of the enzyme demonstrate that the molybdenum center takes up only two reducing equivalents, implying that the two pyranopterin equivalents of the molybdenum center are not formally redox active. Finally, the visible spectrum associated with the catalytically relevant "high-g split" Mo(V) species has been determined. Spectral deconvolution and EPR quantitation of enzyme-monitored turnover experiments with trimethylamine N-oxide as substrate reveal that no substrate-bound intermediate accumulates and that Mo(V) content remains near unity for the duration of the reaction. Similar experiments with dimethyl sulfoxide show that significant quantities of both the Mo(V) species and the dimethyl sulfoxide-bound complex accumulate during the course of reaction. Accumulation of the substrate-bound complex in the steady-state with dimethyl sulfoxide arises from partial reversal of the physiological reaction in which the accumulating product, dimethyl sulfide, reacts with oxidized enzyme to yield the substrate-bound intermediate, a process that significantly slows turnover.

Changing concentrations of CO, CH(4), C(5)H(8), CH(3)Br, CH(3)I, and dimethyl sulfide during the Southern Ocean Iron Enrichment Experiments

Oceanic iron (Fe) fertilization experiments have advanced the understanding of how Fe regulates biological productivity and air-sea carbon dioxide (CO(2)) exchange. However, little is known about the production and consumption of halocarbons and other gases as a result of Fe addition. Besides metabolizing inorganic carbon, marine microorganisms produce and consume many other trace gases. Several of these gases, which individually impact global climate, stratospheric ozone concentration, or local photochemistry, have not been previously quantified during an Fe-enrichment experiment. We describe results for selected dissolved trace gases including methane (CH(4)), isoprene (C(5)H(8)), methyl bromide (CH(3)Br), dimethyl sulfide, and oxygen (O(2)), which increased subsequent to Fe fertilization, and the associated decreases in concentrations of carbon monoxide (CO), methyl iodide (CH(3)I), and CO(2) observed during the Southern Ocean Iron Enrichment Experiments.

Determination of Trace Levels of Iron in a Seawater Sample Using Isotope Dilution/Inductively Coupled Plasma Mass Spectrometry

An analytical method for trace levels of iron in a seawater sample using isotope dilution ICP-MS was developed. Preconcentration of iron and the removal of major elements in seawater such as alkali and alkaline-earth elements can be carried out quickly using a chelating resin disk by adjusting the sample pH to 3. The collision cell option of the ICP-MS instrument method was used to improve the performance of the instrument for iron measurements since ArO and ArN interferences could be reduced using this analytical method. About 4 ml min(-1) helium, as the collision gas, were introduced into the cell. 40Ar14N and 40Ar16O which interfere with 54Fe and 56Fe in water had their amounts decreased by 5 orders of magnitude. Then, the isotope dilution method was used for iron determination below ng g(-1) level of trace iron in four environmental reference materials (river water standard sample JAC-0031 (Japan Soc. for Analytical Chemistry), estuarine standard sample SLEW-2 (NRC Canada) and seawater standard samples CASS-3 and NASS-5 (NRC Canada)) were measured. Good agreement between analytical results and certified values of reference materials was obtained, which confirmed the effectiveness of this method.