Letter pubs.acs.org/ac
Improved Method for Minimizing Sulfur Loss in Analysis of Particulate Organic Sulfur Ki-Tae Park,† Kitack Lee,*,† Kyoungsoon Shin,‡ Hae Jin Jeong,§ and Kwang Young Kim∥ †
School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea Korea Institute of Ocean Science and Technology/South Sea Institute, Jangmok, 656-830, Korea § School of Earth and Environmental Sciences, Seoul National University, Seoul, 151-747, Korea ∥ Department of Oceanography, College of Natural Sciences, Chonnam National University, Gwangju 500-757, Korea ‡
S Supporting Information *
ABSTRACT: The global sulfur cycle depends primarily on the metabolism of marine microorganisms, which release sulfur gas into the atmosphere and thus affect the redistribution of sulfur globally as well as the earth’s climate system. To better quantify sulfur release from the ocean, analysis of the production and distribution of organic sulfur in the ocean is necessary. This report describes a wet-based method for accurate analysis of particulate organic sulfur (POS) in the marine environment. The proposed method overcomes the considerable loss of sulfur (up to 80%) that occurs during analysis using conventional methods involving drying. Use of the wet-based POS extraction procedure in conjunction with a sensitive sulfur analyzer enabled accurate measurements of cellular POS. Data obtained using this method will enable accurate assessment of how rapidly sulfur can transfer among pools. Such information will improve understanding of the role of POS in the oceanic sulfur cycle.
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sulfur compounds,11 and are less sensitive than is needed for studying the marine sulfur cycle.12−14 An analytical procedure is needed that minimizes sulfur loss during analysis and is accurate at submicromolar levels of sulfur because the POS concentration in the ocean is generally lower than 1 μmol L−1. In this study, we developed a novel analytical method that enables accurate measurement of organic sulfur by minimizing sulfur loss during analysis. We compared the new method with the conventional drying procedure by assessing sulfur loss from 18 phytoplankton species (including diatoms, prasinophytes, cryptophytes, dinoflagellates, and prymnesiophytes) and natural seawater samples.
he main sulfur reservoir on Earth is the ocean, in which sulfur mostly occurs in the form of dissolved sulfate. Sulfur is an essential element for the growth of all organisms and is present in a wide variety of metabolites, each having distinctive biological functions.1,2 In the marine ecosystem, sulfur flow involving the conversion of sulfate to organic sulfur compounds depends primarily on sulfate uptake and reduction pathways in microorganisms.2,3 The metabolism of organic sulfur compounds in the ocean is a key to the global sulfur cycle.1−3 Particulate organic sulfur (POS) in the marine environment has a planktonic origin.4 The release of organic sulfur from marine phytoplankton has been recognized as an important factor for the global climate and is a key part of the biogeochemical sulfur cycle.1,2,4 In particular, the biological production of the volatile sulfur compound dimethylsulfide (DMS) in the ocean is the major natural source of tropospheric sulfur, which can influence the earth’s radiation balance.5−8 DMS is produced by the enzymatic breakdown of dimethylsulfoniopropionate (DMSP) derived from marine phytoplankton. In phytoplankton cells that produce DMSP, most of the cellular organic sulfur is in this form; the DMSP is subsequently transformed into gaseous DMS through complex food web processes.9,10 To gain an understanding of the sulfur cycle in the ocean−atmospheric system, it is critical to know the concentrations of various organic sulfur pools among the diverse marine phytoplankton species and their dynamics under differing oceanic conditions. This critical information is not currently available, largely because the conventional analysis methods are inherently subject to loss of potentially volatile © 2014 American Chemical Society
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EXPERIMENTAL SECTION Particulate Organic Sulfur. The most common published method for analysis of POS involves the collection of particulate matter on a precombusted filter, rinsing of the filtered sample with isotonic ammonium formate, and drying at >50 °C overnight (in general for approximately 12 h). When the filter is dry it is rolled inside a tin disk, and the filter/tin is compressed using a stainless steel rod, then placed into an autosampler.11−17 The drying step in these published methods can result in the loss of potentially volatile sulfur compounds including DMSP, methionine, and cysteine, which account for up to 90% of the total intracellular organic sulfur in marine Received: November 11, 2013 Accepted: January 15, 2014 Published: January 15, 2014 1352
dx.doi.org/10.1021/ac403649m | Anal. Chem. 2014, 86, 1352−1356
Analytical Chemistry
Letter
Sample water (10−15 mL) was filtered through a 25-mm GF/F filter. The filter was placed into a cryo-capsule and stored at −80 °C. The DMSP lyase was extracted by sonication in 10 mL of 0.3 M sterile BTP buffer (1,3-bis[tris(hydroxymethyl)methylamino] propane) amended with 0.5 M NaCl (pH 8.2). A volume of 10−100 μL of extract was assayed, and the linear production of DMS at 20 °C was quantified for 15−60 min following the addition of 10 μL of 1 M DMSP-Cl stock (the final DMSP concentration was 2 mM). Culturing of Phytoplankton. The phytoplankton species investigated include three diatoms (Skeletonema costatum, Melosira nummuloides, and Thalassiosira weissf logii), two prasinophytes (Pyramimonas grossii and Tetraselmis sp.), one cryptophytes (Teleaulax sp.), seven dinoflagellates (Prorocentrum minimum, two strains of Amphidinium carterae, Heterocapsa triquetra, Scrippsiella trochoidea, Gymnodinium aureolum, and Alexandrium minutum), and five prymnesiophytes (four strains of Emiliania huxleyi and Isochrysis sp.) (Table S1 in the Supporting Information). The methods for phytoplankton culture are described in the Supporting Information.
phytoplankton.11 This makes the methods unreliable for measurements of POS concentrations. In addition, the elemental analyzers associated with the use of these methods require the filtration of large volumes (many liters) of seawater to ensure the quantity of analyte exceeds the analyzer sensitivity.14−17 The requirement to filter large volumes of seawater creates difficulties in field studies12,14 and is probably one reason for the relative lack of comprehensive POS measurements reported for the marine environment. However, critical modifications to sample preparation procedures in conjunction with a highly sensitive sulfur analyzer could make accurate measurements of POS possible. To this end we developed a liquid phase extraction procedure that made the drying procedure, noted above, unnecessary, with the aim of reducing the loss of volatile organic sulfur compounds. Depending on the cell density, 10−50 mL of each cultured phytoplankton sample was filtered onto a precombusted 25 mm GF/F filter under low vacuum (
