Article pubs.acs.org/est
Chemical Characterization of Dissolved Organic Compounds from Coastal Sea Surface Microlayers (Baltic Sea, Germany) Manuela van Pinxteren,† Conny Müller,† Yoshiteru Iinuma,† Christian Stolle,‡ and Hartmut Herrmann†,* †
Leibniz-Institut für Troposphärenforschung (IfT), Permoserstrasse 15, D-04318 Leipzig, Germany Leibniz-Institute for Baltic Sea Research Warnemuende (IOW), Seestrasse 15, 18119 Rostock, Germany
‡
S Supporting Information *
ABSTRACT: The physicochemical properties of the sea surface microlayer (SML), i.e. the boundary layer between the air and the sea, and its impact on airsea exchange processes have been investigated for decades. However, a detailed description about these processes remains incomplete. In order to obtain a better chemical characterization of the SML, in a case study three pairs of SML and corresponding bulk water samples were taken in the southern Baltic Sea. The samples were analyzed for dissolved organic carbon and dissolved total nitrogen, as well as for several organic nitrogen containing compounds and carbohydrates, namely aliphatic amines, dissolved free amino acids, dissolved free monosaccharides, sugar alcohols, and monosaccharide anhydrates. Therefore, reasonable analytical procedures with respect to desalting and enrichment were established. All aliphatic amines and the majority of the investigated amino acids (11 out of 18) were found in the samples with average concentrations between 53 ng L−1 and 1574 ng L−1. The concentrations of carbohydrates were slightly higher, averaging 2900 ng L−1. Calculation of the enrichment factor (EF) between the sea surface microlayer and the bulk water showed that dissolved total nitrogen was more enriched (EF: 1.1 and 1.2) in the SML than dissolved organic carbon (EF: 1.0 and 1.1). The nitrogen containing organic compounds were generally found to be enriched in the SML (EF: 1.9−9.2), whereas dissolved carbohydrates were not enriched or even depleted (EF: 0.7−1.2). Although the investigated compounds contributed on average only 0.3% to the dissolved organic carbon and 0.4% to the total dissolved nitrogen fraction, these results underline the importance of single compound analysis to determine SML structure, function, and its potential for a transfer of compounds into the atmosphere.
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dry and wet surfactants4 or as a gelatinous-like matrix.5 Independent of the model, the accumulation of surface active material, such as fatty acids, lipids, proteins, monosaccharides, and amino acids is evident. Kuznetsova et al. reported an enrichment of amino acids (free dissolved and combined) in the SML by a factor up to 50.6 Further investigations showed enrichment of anthropogenic hydrophobic compounds such as PCBs2,7 and tin organic compounds8 in the SML. The focus of this work was a detailed chemical characterization of dissolved organic material of three SML samples and the corresponding bulk water taken in the southern Baltic Sea in summer 2006 and in winter and spring 2008. Although a total of three paired samples is quite a low number, first indications of trends regarding the concentrations and enrichment factors for different organic substances can still be obtained from this limited data basis. Besides the determination
INTRODUCTION Exchange processes between air and sea play an essential role as oceans cover a substantial area of the planet. In this context, the sea surface microlayer (SML) represents a physical boundary layer between air and seawater formed due to different physicochemical properties of the two layers.1 The SML has been described as a structured marine compartment that is very important for atmospheric processes; however, its characteristics and dynamics remain largely enigmatic.1,2 Recent investigations suggest that the SML is stable up to a wind speed of 10 m s−1 and is therefore existent at the global average wind speed of 6.6 m s−1.3 One of the main interests in a better understanding of the SML is attributed to the fact that enrichment of inorganic and organic matter can occur in the SML and how this might affect biogeochemical cycles.1 As the SML is the interface for all gaseous, liquid, and particulate mass transfer between the ocean and the atmosphere, the SML may play a key role in the export of organic material from the ocean to the atmosphere due to processes such as bubble bursting. Although important processes near the air-sea interface are not limited to a strictly defined microlayer but rather take place over gradients of varying thicknesses, the SML is often operationally defined as the first 1000 μm of the ocean surface.1 The SML is either described as a series of sublayers of © 2012 American Chemical Society
Special Issue: Marine Boundary Layer: Ocean Atmospheric Interactions Received: Revised: Accepted: Published: 10455
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Slow rotation of the cylinder results in adhesion of the SML on the cylinder surface. The SML is then collected in a box by passing a Teflon wiper and pumped into collecting bottles. This technique provides sampling of a constant film thickness. The second sampling technique used in this work consisted of a glass plate (sampling area 2000 cm2) that is vertically put in the water and slowly removed (Figure 1b). The film adheres to the surface of the glass and is removed by framed Teflon wipers.14 This sampling technique requires only simple equipment but is more time-consuming compared to the rotating drum. Both sampling techniques allowed the sampling of a film thickness of 50−70 μm. Due to the comparable mode of operation and due to a previous study which reported no significant differences in analyte enrichment among different sampling devices,2 a comparability of the results is assumed for this work. Bulk water from 1 m depth was collected using a self-made device consisting of a glass bottle mounted on a telescopic rod that regulates sampling depth. For organic analyses, all samples were filtered directly after sampling (whatman GF/F filters, pore size: 0.7 μm, Whatman International Ltd., Maidstone, UK), and the filtrate was stored at −20 °C until analysis. Each sample was analyzed twice. Enrichment Factor. To describe the enrichment of matter in the SML, an enrichment factor (EF) is defined as the quotient of the concentration (c) of the compound in the surface layer and the bulk phase
of dissolved organic carbon (DOC) and total dissolved nitrogen (TN) the emphasis was put on the analysis of single organic compounds. Proteinogenic amino acids and carbohydrates, which belong to the most important classes of marine biota,9−11 were analyzed and discussed in terms of concentration and analyte enrichment. Moreover, and for the first time, aliphatic amines were studied in SML and bulk water samples, as these compounds were recently found in atmospheric aerosols and are expected to originate from the seawater.12,13
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EXPERIMENTAL PART Reagents and Chemicals. All standard compounds used in this work were of the highest grade commercially available. Methylamine hydrochloride (≥98%), ethylamine hydrochloride (≥98%), dimethylamine hydrochloride (≥99%), diethylamine hydrochloride (≥99%), mannosan (>98%), galactosan (≥98%), meso-erythritol (≥98%), and an L-amino acid kit providing all compounds of a purity >98% were purchased from SigmaAldrich (St. Louis, MO, USA). Ethanolamine (≥99%), Lnorleucine (>98%), the monosaccharides (≥98%), and the sugar alcohols arabitol (≥98%) and mannitol (≥98%) as well as sodium hydroxide solution (50% in H2O, low carbonate ion chromatography grade) were obtained were from Fluka (St. Louis, MO, USA). Morpholine (≥99%) was from Riedel de Haën (St. Louis, MO, USA). Ultrapure (UP) water was prepared using a Milli-Q Gradient A10 with a UV lamp, with resistivity and TOC values of 18.2 MΩ cm−1 and 4 ppb, respectively (Millipore, Billerica, MA, USA). For each compound class (amino acids, amines, and carbohydrates) stock solutions were prepared in UP water and diluted to give six to eleven calibration levels for external calibration. Sampling. Sampling Location. Three samples from the SML and the corresponding bulk water (1 m depth) were taken in the southern Baltic Sea (salinity ≈10 PSU), offshore Warnemuende (54°19′ N, 12°05′ E). Sampling Techniques. A critical step in all SML investigations is the sampling technique. Often only small amounts of the SML can be sampled when dilution of the bulk water is kept to a minimum. The thickness of the sampled layer is depending on the construction, the handling, and the selectivity of the sampler.1 Among the three main techniques (glass plate, metal screen, rotating drum (4)), two sampling methods were chosen in the present work. The first one was the rotating drum, which consists of a ceramic-coated cylinder (sampling area 5750 cm2) placed on a catamaran (Figure 1 a).
EF =
c(SML) c(bulkwater )
(I)
Analytical Techniques. Chlorophyll-a. Chlorophyll-a (Chl-a), a commonly used parameter for marine biomass production, was determined according to ref 15. The water samples were filtrated over Whatman GF/F filters applying a pressure of 0.3 atm. Filters were stored at −80 °C until analysis. Chl-a was extracted with 10 mL of ethanol (96%) at darkness and room temperature. To separate the particulate components from the Chl-a extract the solution was centrifuged. Chl-a concentration was determined with a fluorometer (Turner 10AU-005, Turner Designs, Sunnyvale, CA, USA) at a wavelength of 650 nm. Dissolved Organic Carbon and Dissolved Total Nitrogen. DOC and TN (as the sum of dissolved inorganic ammonium, nitrite and nitrate and dissolved organic nitrogen-containing compounds) were determined using a total organic carbon analyzer (TOC-Vcph) which was equipped with a TNM-1 device (Shimadzu, Kyoto, Japan). To convert inorganic carbonates to carbon dioxide (CO2), 1.5 vol% of a hydrochloric acid solution (2 M) was added to the seawater samples, resulting in a pH value of 2. Afterward, synthetic particle free air was passed through the acidified sample to remove the formed CO2. The oxidation of the carbon and nitrogen within the sample was performed by adding 300 μL of the carbonate-free solutions to a catalyst (aluminum oxide/platinum on quartz wool) held at a temperature of 720 °C. The formed carbon dioxide was detected by NDIR (nondispersive infrared), while the formed nitrogen monoxide was further oxidized to nitrogen dioxide by ozone and subsequently detected by chemiluminescence. DOC and TN contents were determined by using an external calibration with potassium hydrogen phthalate and potassium nitrate, respectively. Aliphatic Amines and Dissolved Free Amino Acids. Reasonable analytical procedures for the determination of
Figure 1. Operation mode of the two employed sampling devices, a) glass plate sampling (dimensions of the glass plate: 500 × 250 × 4 mm), b) rotating drum. 10456
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Table 1. DOC and TN Concentrations and Corresponding EF for the Three Sampling Pairs As Well As Sampling Techniques and Meteorological Parameters DOC sampling date 13.07.2006 (summer sample) 20.02.2008 (winter sample) 09.05.2008 (spring sample) a
SML (μg L‑1)
BW (μg L‑1)
TN EF DOC
SML (μg L‑1)
BW (μg L‑1)
EF TN
sampling technique
wind speed (m s −1)
water temp (°C)
Chl-a (μg L−1)
-a
3450
-a
-a
388
-a
rotating drum
1.13
18.5
-
4113
3778
1.09
355
304
1.10
glass plate
2.90
4.70
2.1
3958
3792
1.04
310
257
1.21
glass plate
2.60
15.5
1.4
No data for SML due to instrument problems, therefore EF calculation was not possible.
histidin) showed recoveries of only about 30% that could not be improved by varying extraction conditions. All concentration data were corrected for the observed recoveries. LOD for amino acids ranged between 5 ng L−1 (proline) and 50 ng L−1 (asparagine) with an exception of 145 ng L−1 for glycine. Dissolved Free Carbohydrates. Dissolved free monosaccharides, monosaccharide anhydrates, and sugar alcohols were analyzed with high performance anion-exchange chromatography (HPAEC) with pulsed amperometric detection (ICS 3000, Dionex, Sunnyvale, CA, USA) according to the method of Iinuma et al.16 Separation was achieved using a CarboPac MA 1 column (Dionex, 4 × 250 mm) and a CarboPac MA1 precolumn (Dionex, 4 × 50 mm) using gradient elution (solvent A: Milli-Q-water, solvent B: 1 M sodium hydroxide solution). Quantitation was performed using an external calibration (R2 better than 0.999). Before analysis, the samples underwent a desalting step using a 1:1 mixture of a strong cation exchange (C100H) and weak anion exchange (A100) granulate (Purolite, Bala Cynwyd, PA, USA). 100 mL sample and 100 g of the precleaned polymer mixture were stirred for 15 min and then filtered. For complete recovery, the granulate was washed again with a methanol−water mixture (8:2). The unified salt free aqueous phases were evaporated to dryness using a rotary evaporator (100 mbar, 60 °C). The residue was redissolved in 3 × 2 mL methanol−water mixture (8:2), which was again evaporated to dryness and redissolved in 700 μL of water. The extract was finally filtered with a syringe filter (PTFE, 0.2 μm pore size) and kept at −20 °C until analysis. This extraction procedure led to 100% (±10%) recovery for synthetic seawater and spiked real samples resulting in LODs between 15 ng L−1 (erythritol) and 35 ng L−1 (mannose) with two exceptions (galactosan: 70 ng L−1 and fructose: 560 ng L−1) and RSD values better than 12%.
single organic compounds with respect to desalting and enrichment were established. Analysis of aliphatic amines and dissolved free amino acids was performed by high performance liquid chromatography with electrospray ionization and ion trap mass spectrometry (HPLC/ESI-ITMS). An HPLC system (series 1100, Agilent, Santa Clara, CA, USA) was coupled to an ESI-ITMS (ESQUIRE 3000 plus, Bruker Daltonics, Billerica, MA, USA) applying detection in positive mode. The separation was carried out on a Phenomenex Gemini C6 Phenyl 110A column (Phenomenex, Torrance, CA, USA) with the following dimensions: 5 μm, 2 × 100 mm at 25 °C. The eluent composition was (A) 2 vol % acetic acid in Milli-Q water and (B) 2 vol % acetic acid in acetonitrile, and the separation was performed at a flow rate of 0.5 mL min−1. The eluent gradient program was as follows: 2% of B for 2 min, 2% B to 50% B in 53 min, 50% B to 80% B in 5 min, and held constant for 5 min. Before analysis, the sample underwent a sample preparation procedure comprising a derivatization and enrichment step. Ten μL of an internal standard solution (50 μM, L-norleucine) was added to a 100 mL water sample as well as 250 μL of sodium hydroxide (NaOH, 10 M) and 2 mL of benzenesulfonyl chloride solution (BSCl, 1 M in acetonitrile) to perform the derivatization of the amines and amino acids. The mixture was stirred intensively for 1 h at room temperature. To hydrolyze the excess of BSCl, 1 mL of NaOH solution (10 M) was added, and the sample was stirred again for 1 h at 80 °C. In the next step, enrichment of the derivates was achieved by solid phase extraction (Strata-X, 200 mg sorbent, 3 mL cartridge, Phenomenex, Torrance, CA, USA). Briefly, the aqueous derivates were acidified with 1.1 mL of concentrated acetic acid and passed the cartridge at a flow rate of about 0.8 mL min−1. Elution was performed with a mixture of methanol (3 mL) and acetonitrile (1 mL). Afterward, the eluate was concentrated to dryness under a gentle nitrogen stream at 10 °C. The residue was dissolved in 50 μL of an acetonitrile water mixture (8:2) and stored at −20 °C until analysis that was performed within one week. The recoveries of each derivatized amine (determined as the ratio of analyte mass in the extract to original analyte mass) was close to 100% (±15%) with good precision values resulting in relative standard deviation (RSD) of the peak areas better that 10% (n = 2). The recoveries were not significantly influenced by the seawater matrix (tested with synthetic and spiked seawater samples). Limits of detection (LOD), determined at the signal-to-noise ratio of 3, were between 5 ng L−1 (morpholine) and 58 ng L−1 (dimethylamine) for the derivatized amines. Also for less polar amino acids recovery was close to 100% (±12%) using the selected extraction conditions. For two polar amino acids (glutamine, asparagine) slightly lower recovery was found but still above 60%. Amino acids with a basic side chain (lysine, arginin, and
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RESULTS AND DISCUSSION Sum Parameters: DOC and TN. Concentrations and enrichment factors of DOC and TN as well as meteorological parameters and Chl-a concentrations for the three sampling events are presented in Table 1. The average DOC concentration of 3.5 ± 0.5 mg L−1 and average TN concentration of 0.4 ± 0.1 mg L−1 are consistent with previously reported data of the spring sample14 and are overall comparable to values found in several marine environments.2,17−23 Slightly higher DOC and TN concentrations in the winter sample can be attributed to the higher Chl-a concentration on this date, which was the beginning of the spring phytoplankton bloom. Slight DOC enrichment in the SML (EF of 1.04 and 1.09) was found. In several previous studies, only small DOC enrichment was reported as well7,19,20,22 with enrichment factors between 0.94 (sampling 10457
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Table 2. Concentrations of Amino Acids, Amines, and Carbohydrates in the Three Samplesa 13.07.2006 (summer sample) Amino Acids glycine alanine valine leucine iso leucine proline phenylalanine methionin glutamic acid asparagic acid arginine histidine serine threonine glutamine asparagine tyrosine tryptophane average sum Amines methylamine ethylamine dimethylamine diethylamine ethanolamine morpholine average sum Carbohydrates mannose/arabinose glucose/xylose fructose galactose ribose arabitol erythritol mannitol levoglucosan mannosan galactosan average sum a
BW (ng L‑1)
SML (ng L‑1)
n.d. 98 146 69 118 240 109 n.d. n.d. n.d. 266 15 15921 284 n.d. n.d. 50 n.d. 1574 17315
n.d. 168 283 142 205 272 290 n.d. n.d. n.d. 139 89 4403 205 n.d. n.d. 135 n.d. 575 6329
57 8.8 274 627 297 642 318 1906 560 3411 8396 1269 637 650 30 n.d. n.d. n.d. 318 1909 15270
20.02.2008 (winter sample) BW (ng L‑1)
09.05.2008 (spring sample)
SML (ng L‑1)
BW (ng L‑1)
SML (ng L‑1)
n.d. 47 73 35 40 57 44 n.d. n.d. n.d. 59 15 121 41 n.d. n.d. n.d. n.d. 53 530
n.d. 237 382 194 300 533 263 n.d. n.d. n.d. 217 484 1371 201 n.d. n.d. 120 n.d. 391 4302
n.d. n.d. 866 439 686 497 597 n.d. n.d. n.d. 120 284 281 208 n.d. n.d. 405 n.d. 438 4383
n.d. n.d. 543 237 445 844 272 n.d. n.d. n.d. 498 279 996 219 n.d. n.d. 213 n.d. 455 4547
127 42 450 4264 272 278 905 5433
108 60 135 619 64 37 170 1022
159 25 206 493 151 181 202 1214
126 71 64 439 82 16 133 798
210 86 139 640 158 53 214 1285
624 2187 11245 1113 386 1320 62 n.d. n.d. n.d. n.d. 2419 16936
1228 4007 17202 3366 763 4254 62 n.d. n.d. n.d. n.d. 4412 30882
n.d. 2126 18022 1346 396 2725 49 n.d. n.d. n.d. 3015 3954 27679
1046 1803 9629 1181 491 2121 42 n.d. n.d. n.d. n.d. 2331 16314
638 2149 12203 939 394 1329 36 n.d. n.d. 107 2762 2284 20557
BW = bulk water, SML = surface microlayer, n.d. = not detected.
in the Mediterranean sea using glass plate sampling7) and 1.5 (sampling in a Norwegian fjord with a screen sampler).22 Higher DOC enrichment was reported by other authors (maximum EFs of 1.84,18 1.87,21 2.4,24 and 2.617) sampling in different marine compartments. Carlson evaluated a large number of bulk water and SML samples and observed that enrichment diminished with increasing bulk water DOC concentrations.20 In detail, the author found low enrichment (EFs around 1) in coastal water with DOC concentration above 2 mg L −1 and higher enrichment in oceanic samples with DOC concentrations of about 1 mg L−1. Furthermore, for oceanic samples the author observed a high variability in enrichment with EFs between 1 and 4. Also in a later field study, the trend
of higher enrichment for lower DOC concentrations in bulk water (DOC below 2.4 mg L−1) and lower enrichment for higher DOC concentrations (DOC above 2.4 mg L−1) was reported.17 Samples of the present study were taken offshore Warnemuende and clearly are coastal samples. This is also reflected by relatively high DOC concentrations above 3 mg/L. Therefore, the assumption of low enrichment in regions with high DOC concentrations (typically coastal water) is supported by our data. The lower enrichment of DOC can be caused by biological, physical, and chemical DOC removal from the SML.20 Such effects (for example the formation of particulate organic matter or its polymeric precursors by DOC) might be more effective in coastal areas with higher DOC than in oceanic 10458
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areas with lower DOC.20 Consistently, a much stronger enrichment of particulate compared to dissolved organic material was found during spring time at the sampling site.14 Enrichment of TN in the SML was slightly higher (EF: 1.1− 1.2) compared to DOC enrichment (Table 1). TN enrichment given in the literature is similar to our values (EFs: 1.0−1.918) when sampling with a screen in the Gulf of California, but also significantly higher values were reported at other places (EFs: 1.7−6.624). For a detailed interpretation of TN enrichment regarding analyte concentration, a larger sample number needs to be studied. Regarding physical dependency, no relationship between DOC and TN enrichment and meteorological conditions (wind speed, water temperature) was found in our data which are, for DOC, also reported in the literature.20 Interestingly, even during slick formation at the sampling site, DOC concentrations were relatively stable.14 Nonetheless, the data of our case study and literature data suggest a general DOC and TN enrichment in the SML of the oceans but with large variability in time and space. Single Compound Analysis. Dissolved Free Amino Acids. In Table 2 the concentrations of the analyzed dissolved free amino acids are summarized. On average, they were between 50 and 1600 ng L −1 with particularly high concentration of serine (15 μg L−1) in the bulk water of the summer sample. For the SML, the concentrations of amino acids were in the same order of magnitude for the three sampling dates which were collected under different meteorological conditions with two different sampling devices (listed in Table 1). This is consistent with other publications observing amino acid concentration being unrelated to environmental parameters such as wind, humidity, and light.25 Thus, it can be said that there was no significant impact of the analyte concentrations on the sampling technique (glass plate, rotating drum). However, amino acid concentrations in the bulk water differed strongly among the three sampling dates. The most abundant amino acids were, besides serine, proline, valine, and iso-leucine. Mean amino acid composition was not significantly different from bulk water and SML. Amino acids with an acidic side chain (glutamic acid, aspartic acid) were not detected in the samples. Mopper and Lindroth23 reported similar concentrations for amino acids found in the Baltic Sea water column with serine and glycine as well as acidic and basic amino acids as major constituents. Slightly higher concentrations of amino acids (dominated by serine, glycine and alanine) in water and SML samples, ranging in the low μmol range, were found by Kuznetsova et al.6,25 who sampled among others in the North Atlantic Ocean and in the Caribbean Sea. The EFs of the single compounds are presented in Figure 2. In all samples investigated in this work, concentrations of amino acids were on average higher in the SML than in the bulk water indicating their enrichment at the surface. However, there was a high variability between the three sampling dates. Low enrichment was found for the summer and spring samples. Especially in the spring sample the majority of less polar amino acids were slightly depleted, while two polar amino acids (serine and arginine) were clearly enriched, resulting in a positive average enrichment factor. One possible explanation for seasonally different accumulation of amino acids might be attributed to seasonal changes of bacterioneuston activity. On the sampling days, bacterioneuston 3H-hymidine uptake was lower in the winter than in the spring/summer samples.14 The overall low bacterial activity in the winter SML might have caused the strong enrichment of amino acids due to decreased
Figure 2. EF of free dissolved amino acids for the three investigated samples, RSD < 12%, n = 2. (1): compounds were detected in the SML but not in the bulk water, legend: ■ - 13.07.2006 (summer sample), ○ - 20.02.2008 (winter sample), ▲ - 09.05.2008 (spring sample).
bacterial uptake (average amino acid EF = 9.4). Moreover, Figure 2 shows that less polar amino acids (alanine to phenylalanine) as well as polar ones and amino acids with basic side chain are clearly enriched in the SML in the winter sample. Especially histidine (polar amino acid with basic side chain) reached a 32 fold enrichment. A literature survey generally indicates an enrichment of amino acids in the SML with no significant dependency of the EF concerning polarity.6,10,25,26 Amines. As a further nitrogen containing class, aliphatic amines were analyzed in the samples. The concentrations of amines in bulk water and SML are listed in Table 2. Diethylamine was found in a relatively high concentration of 4 μg L−1 in the summer SML, the other amines were in the range of a few hundred to several hundred ng L−1. As average concentrations between the three samples were in the same order of magnitude, no relation of analyte concentration to water temperature, wind speed, or sampling technique can be inferred. The corresponding EFs are presented in Figure 3. As for the amino acids, in general, amine concentrations were higher in the SML than in the bulk water, indicating an enrichment of these compounds with average EFs between 2 and 3. The higher analyte enrichment in the winter sample that was found for the amino acids (described in the previous chapter) was not observed for the amines, except for morpholine (EF: 4.8 in the winter sample). Contrarily, the summer sample showed a generally high enrichment of the amines (average EF: 2.8). In this sample a trend of enhanced enrichment with longer chain length (decreased polarity) of the amines can be clearly seen for methylamine, ethylamine, and diethylamine, resulting in particularly strong enrichment for ethylamine (EF: 4.8) and diethylamine (EF: 6.8). Further polarity dependency for amine enrichment was not observed. In contrast to the summer sample, a similar pattern of the EF for the different amines is found for the winter and the spring sample, leading to the same average EF for the two samples. However, lower enrichment or even depletion of the volatile amines (methyl-, ethyl-, dimethyl-, diethylamine) in the winter 10459
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about possible origins in the ocean is needed, though, to better explain these findings. The EFs for the carbohydrates are reported in Figure 4. The EFs of the monosaccharides
Figure 3. EF of amines for the three investigated samples, RSD < 12%, n = 2, legend: ■ - 13.07.2006 (summer sample), ○ - 20.02.2008 (winter sample), ▲ - 09.05.2008 (spring sample).
sample was observed and can possibly be explained by the slightly higher wind speed on the sampling day. Higher wind speeds lead to enhanced wave formation, which for the volatile amines is likely to result in an enhanced transfer to the atmosphere due to their high vapor pressure. This is consistent with the observation that morpholine and ethanolamine, the two amines with the lowest vapor pressure, do not seem to be affected by the wind strength and are significantly enriched in the winter SML. In summary amines tend to be enriched in the SML, but clearly a larger data set is needed in future studies, especially to differentiate amine production and decomposition dynamics. Dissolved Free Carbohydrates. The third compound group analyzed in the samples comprised dissolved free sugar alcohols, monosaccharides, and monosaccharide anhydrates. The observed concentrations were on average between 2 and 4 μg L−1 (Table 2). Compared to the concentration of amino acids and amines, the carbohydrates were slightly more abundant. Again, as average concentrations between the three samples were in the same order of magnitude, no significant dependence on water temperature, wind speed, or sampling technique was found for the carbohydrates. For aerosols, it has been reported that the concentrations of monosaccharides and sugar alcohols, which are markers for biological activity, are typically higher in summer than in winter.27 This was, however, not the case for our water samples. The concentrations for these compounds were similar in the summer and spring sample and only about half of the concentrations in the winter sample. Whether this was due to autotrophic activity (onset of spring bloom) or low bacterial degradation (as outlined above) remains unclear. Regarding the monosaccharide anhydrates, galactosan was found in 1 order of magnitude higher concentration in the winter and in the spring sample compared to the summer sample. Mannosan was exclusively found in the spring sample (SML) but in a relatively low concentration. In the atmosphere, monosaccharide anhydrates are markers for biomass burning and therefore more abundant in winter.27 Notably, in our samples the monosaccharide anhydrates were nearly always detected in the SML only (except galactosan in summer). The seasonal trend of the monosaccharide anhydrides in the water samples suggests an import from atmospheric particles to the ocean, especially at a coastal site close to anthropogenic emissions. A more detailed knowledge
Figure 4. EF of free dissolved carbohydrates for the three investigated samples, RSD < 12%, n = 2. (1): compounds were detected in the SML but not in the bulk water, (2): compounds were detected in the bulk water but not in the SML, legend: ■ - 13.07.2006 (summer sample), ○ - 20.02.2008 (winter sample), ▲ - 09.05.2008 (spring sample).
(mannose to ribose) are about 1 in all samples with a trend toward depletion. Low enrichments of these compounds might also be due to increased bacterial uptake in the SML as previously reported for glucose.28,29 The monosaccharide anhydrates were mostly enriched, probably due to the coastal sampling site with anthropogenic influences. Significant enrichment for the sugar alcohols arabitol and erythritol (mannitol was below LOD) was found in the summer sample although their concentrations were low. This enrichment might be attributed to a generally higher biological production in summer.27 No Chl-a data were available for this sample; therefore a relationship to biological activity could not be checked. To summarize the carbohydrate characteristics, monosaccharide anhydrates were observed nearly exclusively in the SML samples, with high concentrations in winter and spring, the biological markers arabitol and mannitol showed a high variability of the EF (enrichment in summer, depletion in winter) and the monosaccharides showed slight depletion. In contrast to the behaviors of amines and amino acids, the average EFs of carbohydrates indicate a trend toward depletion. Most studies report enrichment of carbohydrates in the SML; however, mostly the total carbohydrate concentration (after a hydrolysis step) is considered and enrichment of carbohydrates is usually less pronounced compared to amino acids.10,26 Additionally, the results of our case study suggest that different compound groups within the broad chemical class of carbohydrates can show very different behavior with respect to water concentrations and EFs. Contribution to DOC and TN. From the DOC/TN data and the single analyte concentrations, the contribution of the compounds to DOC/TN was calculated (Table S1). Averaging both bulk water and SML, about 0.3% of the dissolved organic carbon and 0.4% of dissolved total nitrogen was identified as carbohydrate carbon or amine/amino acid carbon or nitrogen. The contribution of the analytes to TN in bulk water was higher in summer (0.7%) than in winter (0.1%). Apart from 10460
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that seasonal variations there are small but no significant differences in DOC/TN contribution regarding SML and bulk water. The investigated analytes, especially the dissolved free carbohydrates, are probably contributing to marine exopolymers produced by bacteria and microalgae.30−32 Though, total carbohydrates and combined amino acids are expected to have significantly higher contribution to marine polymer gels30 and also higher accumulation as well as higher contribution to DOC and TN as indicated by other studies.10,26 Investigation of these compound classes is part of our future experiments. Further studies with a larger number of samples are necessary to clarify the processes of SML enrichment of organic material as well as studying the dependence of the enrichment on meteorological and biological parameters. Also investigations of the atmospheric layer above the oceans, such as the measurements of the aerosol and the gaseous phase, are tasks of future concerted experiments. For example Orellana et al. have characterized and quantified marine polymer gels in seawater as well as in clouds, fog, and aerosols.30 Another study focused on the determination of organic carbon in seawater and marine aerosols in the Arctic and North Atlantic ocean.33 Such measurements are important in order to obtain a better understanding of exchange processes between the oceans and the atmosphere and to address the question if there is a significant net influx or efflux of specific organic matter compounds.
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ASSOCIATED CONTENT
S Supporting Information *
A table containing the contribution of the analyte groups to the DOC and TN pool. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +49-341-235-2446. Fax: +49-341-235-2325. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge Klaus Nagel and Wiebke Mohr for sampling, Ina Topp for chlorophyll measurements, and the Leibniz society (WGL-SAW project FILGAS) and the Bundesministerium fuer Bildung und Forschung (BMBF) for funding within the SOPRAN phase II project.
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REFERENCES
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