Hilton B. Swan

Influence of different water masses and biological activity on dimethylsulphide and dimethylsulphoniopropionate in the subantarctic zone of the Southern Ocean during ACE 1

Measurements of salinity, temperature, phytoplankton biomass and speciation, dissolved nitrate, dimethylsulfide (DMS) in seawater and air, and dimethylsulfoniopropionate (DMSP), were made in the subantarctic zone of the Southern Ocean from 40°-54°S, and 140°-153°E during the southern hemisphere marine First Aerosol Characterization Experiment (ACE 1). DMSP concentrations were highest in subtropical convergence zone (STCZ) waters, intermediate in subantarctic waters, and lowest in polar waters. DMSP appeared to decrease at frontal regions between these major water masses. In subantarctic waters, high levels of DMSP were generally associated with an increase in dinoflagellate biomass and low microzooplankton grazing rates. Lower DMSP concentrations occurred in polar waters when the diatom biomass and grazing rates were high. DMS levels measured on Southern Surveyor ranged from not detectable (nd) to 5.6 nM (mean 1.7 nM), with below average levels in subantarctic waters (mean 1.25 nM), and above average levels (mean = 1.93 nM) in polar waters. Pulses of DMS occurred as Southern Surveyor traveled south into polar waters, with a large pulse (mean = 2.3 nM) highlighted as the vessel traveled back into subantarctic waters (46°-47°S, 148°-151°E) in early December. By using the dissolved DMSP (DMSPd) to DMS ratio as an index of the bacterial conversion of DMSPd to DMS some evidence was found that, in polar waters, increased microzooplankton (MZP) grazing in diatom dominated waters, may lead to above average concentrations of DMS. This does not appear to be the case when the biomass was dominated by dinoflagellates in subantarctic waters.

Air mass characteristics, aerosol particle number concentrations, and number size distributions at Macquarie Island during the First Aerosol Characterization Experiment (ACE 1)

During the First Aerosol Characterization Experiment (ACE 1), continuous measurements were made of the particle number size distribution (between 18 and 540 nm diameter (Dp)) and total particle number concentration (Dp > 3 nm and Dp > 12 nm) on Macquarie Island, Tasmania (54°30′S, 158°57′E, 7 m above sea level). Periodic real-time measurements of dimethyl sulfide were also made. Sampled air masses were separated into clean marine and those influenced by Tasmania or Antarctica. Observations were compared to those from a southern hemisphere midlatitude site (Cape Grim) and to sites on the Antarctic continent. It was found that the average total number concentration observed during clean marine conditions, 675 cm−3, was about 21% higher than values observed at Cape Grim during ACE 1 and was similar to the high end of the historical range of number concentrations reported by Gras [1995] for Cape Grim during the same time of year. During both clean marine and influenced conditions, the Aitken and accumulation modes dominate the number size distribution, with a Young Aitken mode observed less often. The number size distribution between 18 and 540 nm exhibited two and three modes 75% and 25% of the time, respectively, during clean marine conditions, more consistent with previous observations at Cape Grim than with those from coastal Antarctica. The typical bimodal number distribution at Macquarie Island exhibited average modal diameters of 33 and 113 nm during clean marine conditions, corresponding to the smaller Aitken mode and larger accumulation mode, respectively. The 50 to 70 nm diameter range corresponds to the minimum in the bimodal size distribution at Macquarie Island, except for continentally influenced periods when the size distribution exhibits an Aitken mode near 50 nm and an accumulation mode near 128 nm. The Young Aitken mode appeared most often during or immediately after periods of precipitation associated with both warm and cold fronts, when the Aitken and accumulation mode number concentrations were depleted. Evidence for possible cloud processing of aerosol was found during two Antarctic influenced periods. Variability in observed aerosol characteristics was found to coincide with changes in air mass source region as indicated by back trajectories and frontal passages.

Dimethylated sulfur compounds in coral-reef ecosystems

Environmental context Dimethylated sulfur compounds can exert multiple biological and environmental effects including climate regulation. Climate change and other anthropogenic factors are predicted to affect coral-reef ecosystems where these sulfur compounds are particularly abundant. We review the processes that regulate the production of dimethylated sulfur compounds in coral reefs and the potential consequences of environmental changes on their biogenic cycle in such fragile ecosystems under future climate change scenarios. Abstract Dimethylsulfoniopropionate (DMSP) and its main breakdown products dimethylsulfide (DMS) and dimethylsulfoxide (DMSO) are biogenic species in the marine environment. In coral reefs, these dimethylated sulfur compounds (DSCs) have been reported at greater concentrations than in other marine ecosystems, which is most likely attributable to the extraordinary large biodiversity of coral reef communities (e.g. corals, macroalgae, coralline algae, invertebrates) and to the unique ability of zooxanthellate corals to synthesise DMSP from both the animal host and algal symbionts. Besides the various biological functions that have been attributed to DSCs, including thermoregulation, osmoregulation, chemoattraction and antioxidant response, DMS is suspected to take part in a climate feedback loop that could help counteract global warming. Nowadays, anthropogenic effects such as pollution, overfishing, increased sedimentation and global climate change are imminently threatening the health of coral reef communities around the world, with possible consequences on the natural cycle of DSCs within these ecosystems. This review provides insight into the biogeochemistry of DSCs in coral reefs and discusses the implications of projected changes in DSC production in these increasingly stressed ecosystems under future climate change scenarios. It shows that DSC dynamics will incontestably be affected in the near future, with possible feedback consequences on local climate.

The validation and measurement uncertainty of an automated gas chromatograph for marine studies of atmospheric dimethylsulfide

An automated gas chromatograph (GC) is described that can reliably quantify atmospheric dimethylsulfide (DMS) in near real-time at low nmol m−3 concentrations or pmol mol−1 (ppt) mixing ratios. Features of the automated GC include: removal of atmospheric oxidants and moisture; cryogenic pre-concentration of DMS; methylethylsulfide internal standard calibration; and pulsed flame photometric sulfur specific detection. This automated instrument is suitable for field deployment; it was recently used to obtain a continuous DMS dataset at Heron Island in the southern Great Barrier Reef over 14 days during the austral summer. Detailed analysis of the measurement uncertainty of this automated GC was conducted, according to Eurachem/CITAC guidelines, by quantifying and combining the uncertainties of the components that contributed to the analytical result. This gave a relative standard uncertainty of 6.5% which was expanded, using a coverage factor of two, for an interval containing approximately 95% of the expected distribution of values. When applied to the Heron Island summer dataset, the mean atmospheric DMS concentration and expanded uncertainty was 3.9 ± 0.5 nmol m−3 (n = 651). This value and uncertainty interval falls within the range of mean atmospheric DMS values obtained from other studies at the Great Barrier Reef. Wider adoption of chemometrics to quantify atmospheric DMS measurement uncertainty will enable improved comparison of data and assist climate modelling.

Quantification of dimethylsulfoniopropionate (DMSP) in Acropora spp. of reef-building coral using mass spectrometry with deuterated internal standard

AbstractDimethylsulfoniopropionate (DMSP) in scleractinian coral is usually analysed indirectly as dimethylsulfide (DMS) using gas chromatography (GC) with a sulfur-specific detector. We developed a headspace GC method for mass spectral analysis of DMSP in branching coral where hexa-deuterated DMSP (d6-DMSP) was added to samples and standards to optimise the analytical precision and quantitative accuracy. Using this indirect HS-GC-MS method, we show that common coral sample handling techniques did not alter DMSP concentrations in Acropora aspera and that endogenous DMS was insignificant compared to the store of DMSP in A. aspera. Field application of the indirect HS-GC-MS method in all seasons over a 5-year period at Heron Island in the southern Great Barrier Reef indicated that healthy colonies of A. aspera ordinarily seasonally conserve their branch tip store of DMSP; however, this store increased to a higher concentration under extended thermal stress conditions driven by a strong El Niño Southern Oscillation event. A liquid chromatography mass spectral method (LC-MS) was subsequently developed for direct analysis of DMSP in branching coral, also utilising the d6-DMSP internal standard. The quantitative comparison of DMSP in four species of Acropora coral by indirect HS-GC-MS and direct LC-MS analyses gave equivalent concentrations in A. aspera only; in the other three species, HS-GC-MS gave consistently higher concentrations, indicating that indirect analysis of DMSP may lead to artificially high values for some coral species. Graphical AbstractDimethylsulfoniopropionate (DMSP) was quantified in Acropora spp. of branching coral using deuterated stable isotope dilution mass spectrometry

The relative abundance of dimethylsulfoniopropionate (DMSP) among other zwitterions in branching coral at Heron Island, southern Great Barrier Reef

AbstractDimethylsulfoniopropionate (DMSP) and eleven other target zwitterions were quantified in the branch tips of six Acropora species and Stylophora pistillata hard coral growing on the reef flat surrounding Heron Island in the southern Great Barrier Reef (GBR), Australia. Hydrophilic interaction liquid chromatography mass spectrometry (HILIC-MS) was used for sample analysis with isotope dilution MS applied to quantify DMSP. The concentration of DMSP was ten times greater in A. aspera than A. valida, with this difference being maintained throughout the spring, summer and winter seasons. In contrast, glycine betaine was present in significantly higher concentrations in these species during the summer than the winter. Exposure of branch tips of A. aspera to air and hypo-saline seawater for up to 1 h did not alter the concentrations of DMSP present in the coral when compared with control samples. DMSP was the most abundant target zwitterion in the six Acropora species examined, ranging from 44-78% of all target zwitterions in A. millepora and A. aspera, respectively. In contrast, DMSP only accounted for 7% in S. pistillata, with glycine betaine and stachydrine collectively accounting for 88% of all target zwitterions in this species. The abundance of DMSP in the six Acropora species examined points to Acropora coral being an important source for the biogeochemical cycling of sulfur throughout the GBR, since this reef-building branching coral dominates the coral cover of the GBR. Graphical AbstractHILIC-MS extracted ion chromatogram showing zwitterionic metabolites from the branching coral Acropora isopora

Sea spray aerosol in the Great Barrier Reef and the presence of nonvolatile organics

Sea spray aerosol (SSA) particles produced from the ocean surface in regions of biological activity can vary greatly in size, number and composition and in their influence on cloud formation. Algal species such as phytoplankton can alter the SSA composition. Numerous studies have investigated nascent SSA properties, but all of these have focused on aerosol particles produced by seawater from non-coral related phytoplankton and in coastal regions. Bubble chamber experiments were performed with seawater samples taken from the reef flat around Heron Island in the Great Barrier Reef during winter 2011. Here we show that the SSA from these samples was comprised of an internal mixture of varying fractions of sea salt, semi-volatile organics as well as non-volatile (below 550°C) organics. A relatively constant volume fraction of semi-volatile organics of 10%-13% was observed while non-volatile organic volume fractions varied from 29%-49% for 60 nm SSA. SSA organic fractions were estimated to reduce the activation ratios of SSA to cloud condensation nuclei by up to 14% when compared with artificial sea salt. Additionally, a sea salt calibration was applied so that a compact Time-of-Flight Aerosol Mass Spectrometer could be used to quantify the contribution of sea salt to sub-micron SSA, which yielded organic volume fractions of 3%-6%. Overall, these results indicate a high fraction of organics associated with wintertime Aitken mode SSA generated from Great Barrier Reef seawater. Further work is required to fully distinguish any differences coral reefs have on SSA composition when compared to open oceans.

Coral reef aerosol emissions in response to irradiance stress in the Great Barrier Reef, Australia

We investigate the correlation between stress-related compounds produced by corals of the Great Barrier Reef (GBR) and local atmospheric properties—an issue that goes to the core of the coral ecosystem’s ability to survive climate change. We relate the variability in a satellite decadal time series of fine-mode aerosol optical depth (AOD) to a coral stress metric, formulated as a function of irradiance, water clarity, and tide, at Heron Island in the southern GBR. We found that AOD was correlated with the coral stress metric, and the correlation increased at low wind speeds, when horizontal advection of air masses was low and the production of non-biogenic aerosols was minimal. We posit that coral reefs may be able to protect themselves from irradiance stress during calm weather by affecting the optical properties of the atmosphere and local incident solar radiation.