stable sulfur isotope
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Abstract Aims Sediment composition is highly crucial for seagrass growth and survival. Eutrophication has been suggested a major cause of seagrass decline globally. We investigated the effects of beach sand supplementation to natural sediments under eutrophic condition on the growth and survival of tropical dominant seagrass Thalassia hemprichii. Methods We cultured seagrass T. hemprichii under the controlled laboratory conditions in three sediment types by combining different ratio of in-situ eutrophic sediment and coarse beach sand. We examined the effect of beach sand mixing to natural eutrophic sediments on the growth of seagrass using photobiology, metabolomics and isotope labeling approaches. Results Seagrass grown in eutrophic sediments mixed with sand exhibited significantly higher photosynthetic activity with high relative maximum electron transport rate and minimum saturating irradiance. Simultaneously, considerably greater belowground amino acid and flavonoid concentrations were observed to counteract anoxic stress in eutrophic sediment without mixing sand. This led to more positive belowground stable sulfur isotope in the eutrophic sediment with lower Eh. Conclusions These results indicated coarse beach sand indirectly enhanced photosynthesis and growth for T. hemprichii by reducing sulfide intrusion with lower concentrations of amino acid and flavonoid. This could possibly explain why T. hemprichii often grow better in the coarse sand substrate. Therefore, it is imperative to consider adding sand soil in the sediments to improve the growth condition for seagrass and restoring the seagrass shoots during transplantation in eutrophicated ecosystem.

<p>Stable sulfur isotope analysis is potentially applicable in various fields in forensics and environmental analytics to investigate the sources and degradation of organic compounds, many of them being priority pollutants in groundwater and the atmosphere. A broader use of sulfur isotopes of organic compounds in environmental studies is still hampered by the availability of precise and easy-to-use techniques. Here we present a method for the determination of stable sulfur isotope ratios using gas chromatography coupled with multiple-collector inductively coupled plasma mass spectrometry (GC-MC-ICPMS) which can be used for both δ<sup>34</sup>S and δ<sup>33</sup>S analysis. The method was evaluated using the reference materials IAEA-S-1, IAEA-S-2 and IAEA-S-3 which were converted offline to SF<sub>6</sub> prior to analysis. Standardization was carried out by using a two-point calibration approach. The δ<sup>34</sup>S values obtained by our method are in good agreement (within analytical uncertainty) with the results obtained by the conventional dual inlet method. Additionally, the impact of the used mass resolution (low and medium), the influence of auto-protonation of sulfur isotopes and the effect of isobaric interferences of O<sub>2</sub><sup>+</sup> on the obtained isotopic ratios was investigated. The analytical precision (1σ) for δ<sup>34</sup>S and δ<sup>33</sup>S values was usually better than ±0.1 ‰ for analytes containing >0.1 nmol S. Thus, the presented compound-specific online method should be sufficiently precise to address a wide variety of research questions involving mass independent isotope effects of sulfur-containing organic compounds to discriminate sources or biological and chemical reactions in the environment.</p>

Phytoplankton-bacteria interactions drive the surface ocean sulfur cycle and local climatic processes through the production and exchange of a key compound: dimethylsulfoniopropionate (DMSP). Despite their large-scale implications, these interactions remain unquantified at the cellular-scale. Here we use secondary-ion mass spectrometry to provide the first visualization of DMSP at sub-cellular levels, tracking the fate of a stable sulfur isotope (34S) from its incorporation by microalgae as inorganic sulfate to its biosynthesis and exudation as DMSP, and finally its uptake and degradation by bacteria. Our results identify for the first time the storage locations of DMSP in microalgae, with high enrichments present in vacuoles, cytoplasm and chloroplasts. In addition, we quantify DMSP incorporation at the single-cell level, with DMSP-degrading bacteria containing seven times more 34S than the control strain. This study provides an unprecedented methodology to label, retain, and image small diffusible molecules, which can be transposable to other symbiotic systems.