Dimethyl sulfide dynamics in first-year sea ice melt ponds in the Canadian Arctic Archipelago

20 Pages·2017·1.34 MB·English

Biogeosciences,15,3169–3188,2018
https://doi.org/10.5194/bg-15-3169-2018
©Author(s)2018.Thisworkisdistributedunder
theCreativeCommonsAttribution4.0License.
Dimethyl sulfide dynamics in first-year sea ice melt ponds in the
Canadian Arctic Archipelago
MargauxGourdal1,MartineLizotte1,GuillaumeMassé1,MichelGosselin2,MichelPoulin3,MichaelScarratt4,
JoannieCharette2,andMauriceLevasseur1
1Départementdebiologie,Québec-OcéanandUnitéMixteInternationale3376TAKUVIK,CNRS-UniversitéLaval,
1045AvenuedelaMédecine,VilledeQuébec,QuébecG1V0A6,Canada
2InstitutdesSciencesdelaMerdeRimouski(ISMER),UniversitéduQuébecàRimouski,310AlléedesUrsulines,
Rimouski,QuébecG5L3A1,Canada
3ResearchandCollections,CanadianMuseumofNature,P.O.Box3443,StationD,Ottawa,OntarioK1P6P4,Canada
4MauriceLamontagneInstitute,FisheriesandOceansCanada,P.O.Box1000,Mont-Joli,QuébecG5H3Z4,Canada
Correspondence:MargauxGourdal([email protected])
Received:15October2017–Discussionstarted:21November2017
Revised:4April2018–Accepted:7May2018–Published:29May2018
Abstract.Meltpondformationisaseasonalpan-Arcticpro- 1 Introduction
cess. During the thawing season, melt ponds may cover up
to90%oftheArcticfirst-yearseaice(FYI)and15to25% Meltpondsrepresentanimportantbutunderstudiedcompo-
of the multi-year sea ice (MYI). These pools of water ly- nentoftheArcticseaicesystem.Snowdepositedatthesur-
ing at the surface of the sea ice cover are habitats for mi- face of the sea ice progressively melts during the thawing
croorganisms and represent a potential source of the bio- season and may accumulate above sea level in depressions
genic gas dimethyl sulfide (DMS) for the atmosphere. Here at the surface of the ice to form melt ponds (Lüthje et al.,
we report on the concentrations and dynamics of DMS in 2006),likelythrougharecentlyidentifiedprocessofperco-
ninemeltpondssampledinJuly2014intheCanadianArc- lationblockage(Polashenskietal.,2017).IntheArctic,melt
ticArchipelago.DMSconcentrationswereunderthedetec- pond fraction over first-year sea ice (FYI) in late spring–
tion limit (<0.01nmolL−1) in freshwater melt ponds and summer usually ranges from 50 to 60%, locally reaching
increased linearly with salinity (rs=0.84, p≤0.05) from 90% (Fetterer and Untersteiner, 1998; Eicken et al., 2004;
∼3upto∼6nmolL−1(avg.3.7±1.6nmolL−1)inbrackish Lüthjeetal.,2006;Perovichetal.,2011).Röseletal.(2012)
melt ponds. This relationship suggests that the intrusion of reported a 15% increase of the relative melt pond fraction
seawaterinmeltpondsisakeyphysicalmechanismrespon- forthemonthofJuneduringthelastdecade(2001–2011)in
sibleforthepresenceofDMS.Experimentswereconducted theArctic,mostlikelyattributabletoglobalclimatechange.
withwaterfromthreemeltpondsincubatedfor24hwithand Thispartlyreflectstheprogressivereplacementofmulti-year
without the addition of two stable isotope-labelled precur- sea ice (MYI) by FYI observed since the 1980s (National
sorsofDMS(dimethylsulfoniopropionate),(D6-DMSP)and SnowandIceDataCenter,NSIDC,http://nsidc.org,lastac-
dimethylsulfoxide (13C-DMSO). Results show that de novo cess: June 2017), favouring the formation of shallow melt
biological production of DMS can take place within brack- ponds that spread over increasingly large areas (Agarwal et
ish melt ponds through bacterial DMSP uptake and cleav- al.,2011;Ehnetal.,2011).Theimportanceofmeltpondsin
age. Our data suggest that FYI melt ponds could represent the Arctic, as a water–air interface involved in heat and gas
a reservoir of DMS available for potential flux to the atmo- exchanges,isthusexpectedtoincreaseinthefuture.
sphere.Theimportanceofthisice-relatedsourceofDMSfor Dimethylsulfide(DMS)isthemainnaturalsourceofre-
theArcticatmosphereisexpectedtoincreaseasaresponseto duced sulfur for the atmosphere (Bates et al., 1992). Be-
thethinningofseaiceandthearealandtemporalexpansion tween 17.6 and 34.4Tg of sulfur is released annually from
ofmeltpondsonArcticFYI. the ocean to the atmosphere (Lana et al., 2011), accounting
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
3170 M.Gourdaletal.:Dimethylsulfidedynamicsinfirst-yearseaicemeltponds
for50–60%ofthenaturalreducedsulfuremitted(Stefelset viral lysis, and zooplankton grazing. DMSP is then read-
d
al.,2007).DMSisalsoaclimate-relevantgaspotentiallyin- ily available to heterotrophic bacteria as carbon and sulfur
volved in a feedback loop known as the CLAW hypothesis sources (Kiene et al., 2000; Simó, 2001; Vila-Costa et al.,
(Charlson et al., 1987) linking biology and climate through 2006). The fraction of DMSP consumed by heterotrophic
d
the production of DMS-derived sulfate aerosols. According bacteria and enzymatically cleaved by DMSP lyases into
to CLAW, DMS emissions may affect the global radiation DMS(DMSyield)mayvarydependingonthecomposition
budgetdirectlythroughthescatteringofincomingsolarradi- ofmicrobialcommunities,theirsulfurrequirements,andthe
ationandindirectlyviatheproductionofcloudcondensation availability of other reduced forms of sulfur (Kiene et al.,
nuclei (CCN), leading to the genesis of longer-lived clouds 2000;Stefelsetal.,2007).DMSPlyasesarealsopresentin
withhigheralbedo(Twomey,1974;Albrecht,1989).Inspir- severalmembersofthemicroalgalgroupsHaptophyceaeand
ing three decades of research and hundreds of publications, Dinophyceaeand,toalesserextent,Chrysophyceae(Nikiet
thefeedbackmechanismproposedbyCharlsonetal.(1987) al., 2000). Ultimately, between ∼1 and 40% of the DMSP
remains to be demonstrated in its entirety (e.g. Ayers and producedbyalgaereachestheatmosphereasDMS(Stefelset
Cainey, 2008). Although modelling results show that DMS al., 2007; Simó and Pedros-Alio, 1999a). In addition to the
emissionsmayhaveanegativeradiativeeffect(e.g.Boppet DMSP enzymatic cleavage pathway, DMS production may
al., 2004; Gunson et al., 2006; Thomas et al., 2010), CCN arise from dimethylsulfoxide (DMSO) reduction by various
mayexhibitalowsensitivitytochangesinDMSonaglobal groupsofmarinebacteriaincludingproteobacteria(e.g.Vogt
scale (Woodhouse et al., 2010). Recent studies questioning etal.,1997),membersoftheRoseobactergroup(Gonzálezet
the relative importance of DMS in new particle formation al.,1999),andmat-formingcyanobacteria(vanBergeijkand
have emerged, suggesting that the global CLAW feedback Stal,1996).However,theubiquityofthisDMSO-to-DMSre-
maybeweak(e.g.QuinnandBates,2011;GreenandHatton, ductionpathwayamongstbacterialassemblageshasnotbeen
2014). On a regional scale, however, the response of CCN established (Hatton et al., 2012). A limited number of phy-
production to change in DMS may vary by a factor of 20 toplankton species could also be involved in the reduction
(Woodhouseetal.,2010).TheimpactofDMSemissionson of DMSO into DMS (e.g. Fuse et al., 1995; Spiese et al.,
cloud properties (through the production of CCN) could be 2009). Increasing evidence suggests that particulate DMSO
particularly important in remote pristine marine areas such (DMSO )maybedirectlysynthesizedbyapotentiallywide
p
as the polar regions (Carslaw et al., 2013). In the Southern rangeofmarinephytoplanktonandcouldbeinvolvedinos-
Ocean,DMSmayhavecontributedupto33%oftheincrease moprotection,cryoprotection(LeeanddeMora,1999),and
in CCN observed south of 65◦S as a response of increased antioxidant protective mechanisms (Sunda et al., 2002). As
wind speed since the early 1980s (Korhonen et al., 2010). for dissolved DMSO (DMSO ), it is ubiquitous in seawater
d
The summertime Arctic marine boundary layer (MBL) is and continuous improvements in analytical techniques sug-
leftrelativelycleanafterseasonalwetdepositionofparticles gestthatDMSO maybeasabundantasDMSinsurfacewa-
d
and reduced atmospheric transport of aerosols from anthro- ters(e.g.Simóetal.,2000).DMSOisalsoaknownsinkfor
pogenicsourcesatlowerlatitudes(Stohl,2006;Browseetal., DMS(Hattonetal.,2004)viabacterialandphoto-oxidation
2012;Croftetal.,2016).Suchpristineconditions,combined of DMS to DMSO. Vertical mixing and ventilation are also
withthermallystableMBL,aretypicaloftheArcticsummer- majorremovalprocessesinfluencingDMSconcentrationsin
time(e.g.Aliabadietal.,2016).CleanArcticairmassesal- surfacemixedlayers(Batesetal.,1994;Kieberetal.,1996;
lowultrafine(5–20nmdiameter)particleformation(Burkart SimóandPedrós-Alió,1999b;delValleetal.,2007,2009).
etal.,2017)andthepotentialgrowthofsecondarymarineor- Ice-associatedenvironmentssuchasbottomseaice,brine
ganic aerosols (including DMS-derived particles) into CCN channels, melt ponds, under-ice surface waters, and leads
(Willisetal.,2016).Hence,theArcticisafavourableterrain provide complex and dynamic habitats to diverse microor-
fornewparticleformationfrombiogenicDMS(Changetal., ganism communities involved in sulfur cycling (Levasseur,
2011;Rempilloetal.,2011;Collinsetal.,2017;Giamarelou 2013). In the Arctic, the highest microalgal biomasses are
etal.,2016;Mungalletal.,2016;Willisetal.,2016). found in the bottom ∼0.1m of sea ice, with chlorophyll a
DMS stems mainly from the enzymatic cleavage of (Chl a) concentrations several orders of magnitude above
dimethylsulfoniopropionate (DMSP) by algal and bacterial values for under-ice waters values (e.g. Legendre et al.,
DMSPlyases.DMSPisacellularmetabolitefoundinseveral 1992). A similar pattern of DMSP, DMSO and DMS build-
phytoplankton species as particulate DMSP (DMSP ) (see up in bottom ice has been reported both in the Arctic and
p
the review of Green and Hatton, 2014). DMSP plays vari- Antarctica(Kirstetal.,1991;Levasseuretal.,1994;Turner
p
ousrolesinphytoplankton,includingosmoregulation(Lyon et al., 1995; DiTullio and Smith Jr., 1995; Lee et al., 2001;
et al., 2016), cryoprotection (Karsten et al., 1996), and pre- Trevenaetal.,2003;TrevenaandJones,2006;Delilleetal.,
ventionofcellularoxidation(Sundaetal.,2002).Partofthe 2007; Tison et al., 2010; Asher et al., 2011; Nomura et al.,
DMSP produced by algae is released in the water column 2012; Galindo et al., 2015). For example, DMSP concen-
p p
asdissolvedDMSP(DMSP )viaseveralpathwaysreviewed trations up to 15000nmolL−1 have been documented dur-
d
inStefelsetal.(2007),includingactiveexudation,celllysis, ing spring in bottom FYI of the eastern Arctic (Galindo et
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M.Gourdaletal.:Dimethylsulfidedynamicsinfirst-yearseaicemeltponds 3171
al.,2014)whilesurfacewaterconcentrationsgenerallyrange may originate from algal and bacterial metabolism. How-
between 1±0.2 and 50nmolL−1 (Vila-Costa et al., 2008; ever, in situ DMS production had never been measured nor
MatraiandVernet,1997).DMSP,DMSO,andDMSarealso hadkeymechanismsbeenidentified.Here,wereportonthe
present throughout the ice column within the brine network DMSconcentrationsinninemeltpondslocatedintheeastern
(Levasseur et al., 1994; Trevena and Jones, 2006; Asher et CanadianArcticArchipelago(CAA)andontheprerequisites
al.,2011).Giventhatprimaryproducersarethesolesource and processes responsible for the presence of this climate-
ofDMSP,veryhighiceconcentrationsofChla canbecor- active gas. This is the first attempt to assess the dynamics
related with DMSP through a first-order relationship (Lev- of DMS in Arctic melt ponds. We identified sea ice perme-
asseur,2013).ThisChlatoDMSPrelationshipmaynothold abilityasamajorcontrolofDMSproductioninmeltponds,
forlowerbiomassconcentrations(Tisonetal.,2010).Inad- mediating the transport of both DMS and DMS-producing
ditiontothevariabilityinducedbyinter-specificdifferences communitiestowardthesurfaceofseaice.Wealsoprovide
in DMSP cellular contents (e.g. Keller et al., 1989; Stefels, the first evidence for direct in situ DMS production in Arc-
2000), environmental forcing are known to control DMSP, tic melt ponds. We propose that seasonally melting sea ice
DMSO,andDMSconcentrations.Inice-associatedenviron- mightbecomeincreasinglypronetoDMSproductionasFYI
ments,brinevolumefractionmightalsobekeyinexplaining becomelargelypredominantontheregionalscale.
DMS cycling variability via the control of ice permeability
(Carnat et al., 2014). Structural changes within sea ice dur-
ingthemeltseason,namelyincreasesinbrinevolumefrac- 2 Materialsandmethods
tionandicedesalination,resultinincreasedconnectivityand
permeabilityinthewarmingseaice(Willisetal.,2016;Po- 2.1 Studysitesandenvironmentalmeasurements
lashenskietal.,2012)andinfluenceDMSPandDMScycling
(Tisonetal.,2010;Carnatetal.,2014).Also,phytoplankton Nine melt ponds distributed between four stations located
blooms developing under the ice during the melting period in Navy Board Inlet (Ice1–MP1 and MP2–18 July), Bar-
havebeenshowntoproducelargequantitiesofDMSP ,po- row Strait (Ice2–MP1 to MP3–20 July and Ice3–MP1 and
p
tentiallyleadingtoabuild-upofDMSconcentrations(Lev- MP2–21July),andResolutePassage(Ice4–MP1andMP2–
asseuretal.,1994).Inspiteofthespatialimportanceofmelt 23July)weresampledduringthejointNETCARE/ArcticNet
ponds, only a few studies have investigated their role as a research cruise conducted in 2014 on board the Canadian
source of DMS for the Arctic atmosphere (e.g. Levasseur, CoastGuardShip(CCGS)Amundsen(Fig.1).
2013). At each station except for Ice2 (logistical constraints as-
Considerable efforts have been dedicated to the under- sociatedwithshiptimeline),measurementsofseaicethick-
standing of underlying process controlling the physics of ness,snowdepth,andseaicefreeboard(theheightofseaice
meltpondsandtheirfeedbacksonclimatethroughthecon- above the ocean surface) were conducted within a 3m dis-
trolofsurfaceenergybalanceoftheice(Lüthjeetal.,2006; tance of the melt ponds using a gauge (Kovacs Enterprise,
Polashenskietal.,2017).However,littleisknownabouttheir Roseburg,OR,USA)(Table1).The3mdistancewasacom-
biogeochemistry. Four studies have specifically reported on promise between maximizing the proximity of ice and melt
DMS in melt ponds so far. They reveal negligible DMS pondsamplesandminimizingmeltponddisturbanceduring
concentrations in MYI ice melt ponds in the central Arctic sampling operations. Ice and freeboard thickness presented
Ocean and concentrations up to 2.2nmolL−1 in the High in Table 1 are averaged values of the seven (Ice1) to eight
Arctic (Leck and Persson, 1996; Sharma et al., 1999). In (Ice3 and Ice4) ice cores sampled at each station between
Antarctica, DMS concentrations ranging between 1.1 and the team members for their respective projects. In order to
3.7nmolL−1andbetweenbelowthedetectionlimit(d.l.)and estimate the permeability of the ponded ice, sea ice tem-
250nmolL−1 weremeasuredintwostudies(Nomuraetal., perature and bulk salinity were measured following Miller
2012,andAsheretal.,2011,respectively).Inthelatterstudy, et al. (2015) at stations Ice1, Ice3, and Ice4. Two ice cores
bacterialDMSOreductionwassuggestedasapossiblemech- for sea ice temperature and salinity measurements were ex-
anismresponsibleforthehighDMSconcentrationsobserved tracted using a 0.09m core barrel (Kovacs Mark II, Kovacs
although no actual rates of DMS production, either from Enterprise, Roseburg, OR, USA). In situ sea ice tempera-
DMSOorDMSP,weremeasured.HighDMSconcentrations ture profiles were measured directly, at 0.1m intervals, us-
reportedintheAntarcticaremostlikelyrelatedtothedevel- ing a high-precision thermometer (Testo® 720; precision of
opmentofasurfaceicecommunityfollowingflooding,apro- ±0.1◦C). Corresponding sea ice salinity profiles were also
cesswherebyheavysnowloadpushestheicebelowthewater determined at 0.1m intervals. Each 0.1m section was cut
level.FloodingiscommonintheAntarcticandresultsinthe with a handsaw, stored in a plastic container, and allowed
formation of snow ice (Hunke et al., 2011). Several studies tomeltatroomtemperature.Bulksalinityofthemeltedice
documentmeltpondcolonizationbymicro-,nano-,andpico- section was determined using a conductivity probe (Cond
sizedalgaeaswellasbacteria(Bursa,1963;Gradingeretal., 330i, WTW™; precision of ±0.1%). Permeability to fluid
2005;Elliottetal.,2015),suggestingthatDMSinmeltponds transport was assessed with brine volume profile calcula-
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3172 M.Gourdaletal.:Dimethylsulfidedynamicsinfirst-yearseaicemeltponds
Figure1.(a)Regionalmapshowingthelocationofthefoursamplingstations(Ice1toIce4)(redcircles)duringtheNETCARE/ArcticNet
2014campaign.(b)MODISimageryabovethefoursamplingstation(redcircles)showingtheiceconditionson18July2014inthesampling
area.(c)Lefttoright,picturesofstationsIce1,Ice2,Ice3andIce4withsizescale.MPFstandsforthemeltpondfractionvisuallyestimated
fromthebridgeforstationsIce1,Ice2,Ice3,andIce4.
tionsfrombulksalinitiesandseaicetemperaturesfollowing theextractedpigmentswasmeasuredonboardwithaTurner
equationsfromLeppärantaandManninen(1988)forseaice Designsfluorometer(model10-005R;TurnerDesigns,Inc.)
temperatures>−2◦C(Fig.3).Duetologisticalconstraints before and after acidification with 5% HCl. The fluorome-
mentioned above, neither ice nor snow measurements were terwascalibratedwithacommerciallyavailableChla stan-
conductedatstationIce2. dard(Anacystisnidulans,Sigma).Chlaconcentrationswere
Melt pond depth, length, and width were determined us- calculated using the equation provided by Holm-Hansen et
ingagraduatedstickandataperuler.Meltpondwatertem- al.(1965).
perature was measured using a high-precision thermometer Microscopicidentificationandenumerationofeukaryotic
(61220-601digitaldatalogger,VWR)andwatersalinitywas cells>2µmwereconductedineachmeltpond.Samplesof
measuredusingtheconductivityprobementionedinthepre- 250mL were collected and preserved with acidic Lugol so-
vious paragraph (Table 2). For each sampling location, two lution (0.4% final concentration; Parsons et al., 1984), then
tothreemembersoftheresearchteamvisuallyassessedthe storedinthedarkat4◦Cuntilanalysiswasconductedbyin-
pond fraction based on pictures taken from the bridge (see verted microscopy (Lund et al., 1958; Parsons et al., 1984).
Fig.1cforexamples)andameanvaluewascalculated. Foreachsample,aminimumof400cells(accuracy±10%)
and three transects of 20mm were counted at a magnifica-
2.2 Phytoplanktonbiomassandenumeration, tionof400×.Themaintaxonomicreferencesusedtoiden-
bacterialcount tifytheeukaryoticcellsareTomasandHasle(1997),Bérard-
Therriaultetal.(1999),andThrondsenetal.(2003).
For Chl a quantification, 1000 to 1500mL duplicates of in Theabundanceofbacteriawasdeterminedbyflowcytom-
situ pond water were filtered onto Whatman® GF/F 25mm etry (Marie et al., 2005). Duplicate 4mL subsamples were
filters. Pigments were extracted in 90% acetone for 18 to fixed with 20µL of 25% glutaraldehyde Grade I (0.1% fi-
24hinthedarkat4◦C(Parsonsetal.,1984).Fluorescenceof
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M.Gourdaletal.:Dimethylsulfidedynamicsinfirst-yearseaicemeltponds 3173
Table1.Physicalcharacteristicsoftheseaicesurroundingthemelt Table2.Physicalcharacteristicsofthemeltpondwater.Formelt
ponds.Notethatonlymeltpondsampling(i.e.noicesampling)was ponddepth,mean±standarddeviationvaluesarepresented.
conductedatstationIce2duetoship-relatedlogisticalconstraints.
A negative freeboard height indicates that the ice surface was lo- Station Meltpondno. Meltpond Meltpond Meltpond
callybelowthemeansealevel.NAstandsfornon-availabledata. depth(m) salinity Temperature(◦C)
Icethicknessandfreeboardvaluesareaveragesof7(Ice1)to8(Ice3 Ice1 MP1 0.18±0.01 5.2 1.9
andIce4)icecoressampledateachstation. Ice1 MP2 0.18±0.04 4.1 1.8
Ice2 MP1 0.29±0.05 0.7 0.4
Ice2 MP2 0.19±0.03 0.4 0.3
Station Sampling Snowand Ice Freeboard
Ice2 MP3 0.12±0.01 0.2 0.2
date frozensnow thickness (m)
Ice3 MP1 0.07±0.01 1.1 0.2
depth(m) (m)
Ice3 MP2 0.10±0.00 0.9 0.2
Ice1 18Jul2014 0 1.21±0.02 −0.01±0.01 Ice4 MP1 0.12±0.01 8.1 0.2
Ice2 20Jul2014 0 NA NA Ice4 MP2 0.11±0.02 8.5 0.3
Ice3 21Jul2014 0+0.07∗ 1.13±0.07 0.10±0.02
Ice4 23Jul2014 0 1.27±0.01 0.07±0.04
al. (2012). Briefly, DMS was stripped from liquid samples
usingheliumgas(Praxair™ He,purity99.999%)flowingat
nalconcentration;Sigma-AldrichG5882),thensubjectedto 50±5mLmin−1 inthePnTsystem.Oneto5mLofsample
quick-freeze in liquid nitrogen for 24h, and finally stored wasinjectedinthePnT.FivemLofMilli-Q™ water(Milli-
at −80◦C until analysis. Samples were analyzed using a pore filter system, Millipore Co., Bedford, MA, USA) was
FACS Calibur FCB3 flow cytometer (Becton Dickinson). subsequentlypushedintothesystemtocompletelyflushthe
Heterotrophic bacteria samples were stained with SYBR sample into the glass bubbling chamber. The outer walls of
Green I and measured at 525nm to quantify bacteria with the bubbling chamber were heated at 70◦C with a circu-
lownucleicacid(LNA;potentiallylessactive)andhighnu- lating bath. Humidity in the gas sample downstream of the
cleicacid(HNA;potentiallymoreactive)content(Gasoland bubbling step was minimized using a 4◦C circulating bath
delGiorgio,2000;Lebaronetal.,2001).Analysiswereper- totriggercondensation.ANafion® membraneseparatedthe
formed on an Epics Altra flow cytometer (Beckman Coul- gas sample and He-carrier gas from a drying He counter-
ter), fitted with a 488nm laser (15mW output; blue), using flowsetat100mLmin−1tofurtherdesiccatethegassample.
Expo32v1.2bsoftware(BeckmanCoulter). FluxesinthePnTsystemweremonitoredusingaflowmeter
(Varian™).
2.3 DMS(P)sampling,conservationandanalysis ForDMSP samples,3.5mLofmeltpondwaterwascol-
t
lected in duplicate into a 5mL Falcon™ tube. DMSP sam-
d
Duplicate samples for total DMSP (DMSPt), dissolved ples were obtained using the less disruptive small-volume
DMSP (DMSPd) and DMS measurements were collected gravity drip filtration (SVDF) method (Kiene and Slezak,
from the melt ponds using a submersible pump (Cyclone – 2006).ParticulateDMSP(DMSP )concentrationswerecal-
p
Aquameric™) connected to a sealed lead–acid battery and culatedbysubtractingDMSP fromDMSP.DMSPsamples
d t
fittedwithLDPEtubing.Thepumpwasplacedclosetothe were preserved with 50µL of 50% sulfuric acid (H SO )
2 4
pondbottom,withouttouchingtheice.Stratificationwasre- to prevent DMSP transformation and remove pre-existing
portedinopenmeltponds(i.e.meltpondsthathavemelted DMS. Samples were analyzed using the same methods as
all the way to the sea surface) in Arctic FYI (Jung et al., described above for DMS samples, following mole-to-mole
2015). However, closed FYI melt pond, such as those sam- conversionofDMSPintoDMSviaNaOH(5M)hydrolysis
pledduringthisstudy,arenotpronetoverticalstratification (DaceyandBlough,1987).
duetoconvective-andwind-drivenmixing(Skyllingstadand
Paulson,2007).Giventheirshallowdepths(lessthan0.3m), 2.4 Processstudies
meltpondstratificationwasmostprobablyinexistentormin-
imal during our study. Glass serum bottles were filled with InordertoexaminethepathwaysofinsituDMSproduction
sampled water, temporarily sealed with a butyl cap and an in melt ponds, three 24h incubation experiments were con-
aluminum lid, and kept in the dark in a cooler until analy- ducted with water from the MP1 sampled at stations Ice1,
sisuponreturntotheship.Analysiswereperformedusinga Ice3,andIce4.Waterfromthemeltpondswascollectedus-
purgeandtrap(PnT)systemcoupledtoaVarian™ 3800gas ing the pump described in Sect. 2.3, pooled in clean 19L
chromatograph (GC), equipped with a pulsed flame photo- Coleman™coolerjugsonsite,andthentransferredintogas-
metric detector (PFPD). Analytical precision of the method tight3LpolyvinylfluorideTedlar®bags.Lighttransmittance
was better than 5%. Analytical d.l. was 0.01nmolL−1 for throughtheincubationbagmaterialdiminishedwithdecreas-
all sulfur compounds. The protocol is a modified version inglightwavelength.Between99and92%ofthephotosyn-
of the method of Leck and Bågander (1988) as described thetically active radiations (PAR, 400–700nm) were trans-
in Scarratt et al. (2000) and further revised in Lizotte et mitted through the bag material. Transmittances of ultravi-
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3174 M.Gourdaletal.:Dimethylsulfidedynamicsinfirst-yearseaicemeltponds
olet A radiations (UVA, 315–400nm) and ultraviolet B ra- DMS versus time in the controls; (3) net potential DMSP
d
diations(UVB,290–315nm)rangedbetween92and82and changesinnaturallightandinthedarkderivedfromthere-
between82and38%,respectively.Theincubationbagswere gressionslopeofDMSP versustimeinL-DMSP/OandD-
d
rinsedoncewith∼10%HCl,threetimeswithMilli-Q™wa- DMSP/O; (4) net potential DMS production rate in natural
ter,andtwicewithmeltpondwatertoavoidcontamination. light and in the dark derived from the regression slope of
The bags were custom-built and pre-closed on three sides DMSversustimeinL-DMSP/OandD-DMSP/O.Thedaily
(Dalian Delin Gas Packaging Co., Ltd.). After the addition rates were obtained from the slopes between final and ini-
of the melt pond water, the bags were sealed with Clip-n- tialconcentrationsover24h.Ourexperimentalsetupalsoal-
seal™ Teflonclosuredevices.Avalvewasfittedtoeachbag lowstheestimationoftherelativecontributionofDMSPand
toallowtheremovalofanyremainingbubbles. DMSO to the production of DMS, using the discrimination
Thesamplesweresubjectedtothreeduplicatetreatments ofthedifferentisotopesofDMS(seeSect.2.5).
(total of six bags): (1) two bags of unaltered melt pond
water incubated under natural light (control), (2) two bags 2.5 DMSisotopicsignatures
amendedwithD6-DMSPand13C-DMSO(100nmolL−1,fi-
nalconcentrationeach)incubatedundernaturallight(light- The discrimination of the different isotopic forms of DMS,
DMSP/O or L-DMSP/O), and (3) two bags amended with including D6-DMS and 13C-DMS stemming from D6-
D6-DMSP and 13C-DMSO (100nmolL−1, final concen- DMSPcleavageand13C-DMSOreduction,respectively,was
tration each) incubated in the dark (dark-DMSP/O or D- performed using GC-MS analysis following purging as de-
DMSP/O). L- and D-DMSP/O bags were amended with scribed hereafter. Two sets of DMS sample duplicates were
∼100µL of freshly thawed aliquots of two D6-DMSP and taken for the incubation experiments. The first set of du-
13C-DMSO stock solutions (high purity >99%, Sigma- plicates was measured directly on board using the Varian™
Aldrich®).Thehighconcentrationsofisotopesaddedaimed 3800GCdescribedinSect.2.2.ThesecondsetofDMSdu-
totriggerarapidandclearbiologicalresponse(i.e.potential plicateswaspreservedthroughcryo-trapping.Cryo-trapping
DMS production rates) measurable during our 24h incuba- of DMS was conducted using glass GC liners filled with
tions.DMSPandDMSOuptakearenotexpectedtobemu- Tenax-TA polymer (high sulfur affinity) (Pio et al., 1996;
tuallyexclusiveandhavebeenobservedconcomitantlyboth Zemmelink et al., 2002; Pandey and Kim, 2009) kept at
inlivecultures(Spieseetal.,2009)andinsitu(Asheretal., −80◦Cpriortotheiruseandmaintainedbelow−10◦Cdur-
2011). ingthe5minpurgingandtrappingprocess.TheTenax-filled
Bagswereincubatedontheforedeckoftheship.Thetem- deactivatedlinersweremounteddownstreamofthePnTsys-
peraturewaskeptasneartoinsituwatertemperatureaspos- tem described earlier. After gas extraction from the liquid
siblebycontinuouslyflowingsurfaceseawaterintheincuba- samples,TenaxlinersandtheirDMScontentwerewrapped
tor.ThetemperaturesoftheincubationwaterforIce1-MP1, individuallyinaluminumfoil,placedinaPyrex™glasstube
Ice3-MP1, and Ice4-MP1 were 1.29±1.75, −0.28±0.26, sealed with a Teflon lid, and returned to the −80◦C freezer
and −0.73±0.09◦C, respectively. These mean values were forseveralweeksuntilanalysisonaland-basedGC-MS.
within1◦Coftheinsitumeltpondwatertemperatures(Ta- Quantification of D6-DMS and 13C-DMS was conducted
ble2). via GC-MS analysis (6978 GC coupled to a 7000B Triple-
DMSP, DMSP , and DMS concentrations were mea- Quad MS from Agilent). Mass spectra were collected both
t d
sured in duplicate every 6h during the incubation period in full scan (m/z45–100) and in selected ion monitoring
as described above. DMS production from DMSP cleav- (m/z62, 63, and 68) modes. Final concentrations were cal-
age and DMSO reduction were determined through gas culatedfromstandardcurvesusingknownconcentrationsof
chromatograph–mass spectrometry (GC-MS) analysis as an both unlabelled DMS and labelled DMS carrying the D6-
increase of D6-DMS and 13C-DMS, respectively, in the L- DMS and 13C-DMS signatures. The comparison between
DMSP/OandD-DMSP/Otreatments.Discriminationbythe fresh DMS samples measured directly on board during the
microorganisms toward lighter (natural) isotopes of DMSP NETCARE/ArcticNet campaign and cryo-preserved DMS
andDMSOisexpectedtobeminimal(<10%)accordingto samples shows excellent agreement between the two meth-
Elizabeth Colleen Asher (unpublished data). The observed ods(r2=0.96,Fig.2).
ratesofchangeintheconcentrationofDMSstableisotopes
are thus assumed to be representative of the potential for 2.6 Satellitedata
DMScyclinginthesemeltponds.
This experimental setup allows the measurement of the Distancesbetweeneachstationsandtheopenoceanwereas-
following rates over 6 and 24h: (1) net changes of in situ sessed using scaled NASA’s Earth Observing System Data
DMSP andDMSP innaturallightderivedfromthediffer- andInformationSystem(EOSDIS)imagery.Mapsoftheice
d p
ence of DMSP and DMSP concentrations versus time in cover were accessed for the sampling dates in July 2014
d p
thecontrols,respectively;(2)netinsitumicrobialDMSpro- through the MODIS (Terra/Aqua) Corrected Reflectance
ductioninnaturallightderivedfromtheregressionslopeof (True Color) layer combined with MODIS (Terra) Cor-
Biogeosciences,15,3169–3188,2018 www.biogeosciences.net/15/3169/2018/
M.Gourdaletal.:Dimethylsulfidedynamicsinfirst-yearseaicemeltponds 3175
each treatment L-DMSP/O and D-DMSP/O and (2) assess
thepotentialeffectoflightontheconcentrationsandchange
rates of the reduced-sulfur compounds under study (DMS,
DMSP ,andDMSP )bycomparingpaireddependentsam-
d p
ples(repeatedmeasures)fromL-DMSP/OandD-DMSP/O.
3 Results
3.1 Pondedseaiceandsnowproperties
The physical characteristics of the sea ice surrounding the
melt ponds are presented in Table 1 and in Fig. 3. All the
samplingsiteswerecharacterizedbyFYI,whichwasthepre-
dominant ice type throughout the region under study. Aver-
agedseaicethicknessaroundthemeltpondswererelatively
Figure 2. Relationship between the concentrations of fresh DMS
uniform, varying between 1.13±0.07 and 1.27±0.01m at
samplesmeasuredonboardtheshipviagaschromatographyduring
the different sites. Average freeboard values were relatively
thecampaignandtheconcentrationsofthecorrespondingpreserved
morevariable.StationIce1wascharacterizedbylowicefree-
duplicate samples measured via coupled gas chromatography and
boards −0.01±0.01m. Station Ice3 had the highest posi-
massspectrometryinalaboratorysetting.Theconcentrationsofthe
preservedDMSsamplesplottedarethesumofthethreeisotopesof tive freeboards with 0.10±0.02m. Station Ice4 freeboards
DMSinvestigatedinthisstudy(m/zof62,63,and68;seeMaterials were also positive and showed the greatest variability, with
andMethods). 0.07± 0.04m.
Brine volume fraction was calculated using sea ice salin-
ity and temperature values and used as a proxy of sea ice
rected Reflectance (bands 3, 6, 7). These open-source data permeability(Fig.3).Averagedvaluesforbulkseaicesalin-
are accessible through the Global Imagery Browse Ser- ityoverthefullthicknessoftheicewere1.73,2.83,and3.75
vices (GIBS) (https://worldview.earthdata.nasa.gov, last ac- atstationsIce1,Ice3,andIce4,respectively.Maximumbulk
cess: August 2017). The imagery had a resolution of 250m salinity never exceeded 5.00 (Ice4, 1.2–1.3m section). In
onadailyscale. situtemperatures,averagedoverthefullthicknessoftheice,
were−0.54,−0.52,and−0.98◦CatstationsIce1,Ice3,and
2.7 Statisticalanalysis Ice4,respectively,andreachedaminimumvalueof−1.39◦C
(Ice4, 0.8–0.9m section). Brine volume fraction constantly
Normality of the data was assessed using the Shapiro–Wilk
exceeded 10% in the ice profiles except in the upper 0.1m
test with a 0.05 significance level (R statistical software, R
sectionoftheIce3station,wherewelikelyobservedtheef-
Core Team, 2016), which revealed that most variables were
fectsofrefreezingmetamorphosisofsnowand/orseaicere-
non-normally distributed (n=9, df=8, α=0.05). Non-
crystallization. Snow meltwater percolation and refreezing
parametricSpearman’srankcorrelationtest(r )witha0.05
s can form such superimposed ice layers as observed at sta-
significancelevelwasusedtoassesscorrelationbetweenkey
tion Ice3. The resulting impermeable layer at the top of the
variables since normality could not be achieved uniformly
icecontributedtothehighfreeboard(0.1m)measuredatthis
through standard normalization methods. Model I linear re-
station,representing6%ofthetotalicethickness.Visuales-
gressions(r2)wereusedtodeterminebiologicalratesduring
timates of the pond fraction ranged from 30 to 60% (see
theincubationexperiments(SokalandRohlf,1995).
Fig. 1c) and the remaining surface of sea ice was bare ice
A non-parametric Mann–Whitney U test was used to de-
atstationsIce1,Ice2,andIce4.
termine whether the distributions of reduced-sulfur com-
pounds (i.e. DMS, DMSPp, and DMSPd) in the Ice1-MP1 3.2 Physical,chemical,andbiologicalcharacteristicsof
andIce4-MP1incubationsexperimentswerestatisticallydif- themeltpondwater
ferent from one another. The difference in reduced-sulfur
compoundconcentrationsbetweenthetwoincubationexper- Thephysicalandchemicalcharacteristicsofthemeltponds
iments was not found to be statistically significant (n=45, are presented in Table 2. All melt ponds were closed melt
df=16,α=0.05). ponds,i.e.notdirectlyconnectedwiththewatercolumn(Lee
Based on the results of the Mann–Whitney U test, a se- et al., 2012). The mean depth of the individual melt ponds
ries of Wilcoxon signed-rank tests with a significance level ranged from 0.07 to 0.29m, with length and width varying
α=0.05 were conducted on the combined data sets of sta- between 1.00 and 25.00m (Fig. 1). Melt pond water tem-
tions Ice1-MP1 and Ice4-MP1 in order to (1) assess the peraturesandsalinitiesvariedbetween0.21and1.86◦Cand
presence of statistical differences between the controls and between0.2and8.5,respectively.Chlaconcentrationswere
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3176 M.Gourdaletal.:Dimethylsulfidedynamicsinfirst-yearseaicemeltponds
Figure3.Insitutemperature(•)andbulkicesalinity(◦)profilesoftheseaicesurroundingthemeltpondssampledatstationsIce1(a),
Ice3(b),andIce4(c).Temperatureandsalinityvaluesofeach0.1mseaicesectionwereusedtocalculatebrinevolumes(orangebars),an
indicatorofseaicepermeability,throughoutthefulldepthofseaice(CoxandWeeks,1983;PetrichandEicken,2010).
variable, ranging from 0.03 to 0.48µgL−1 with a mean of 3.3 Dynamicsandcyclingofreduced-sulfur
0.20µgL−1 (Table 3). The composition of the algal assem- compoundsinArcticmeltponds
blagepresentinthemeltpondswillbedescribedindetailina
companionpaper(Charetteetal.,2018)butissummarizedin
Results from the Ice1-MP1 and Ice4-MP1 incubation ex-
Table 3. The algal assemblages were dominated by uniden-
periments are presented in Fig. 4 (panels a and c, and pan-
tifiedflagellates,ice-associatedpennatediatoms,andchrys-
elsbandd,respectively).ResultsfromtheIce3-MP1exper-
ophytes. Empty diatom frustules were abundant in all melt
imentsarenotpresentedsinceDMSP andDMSconcentra-
ponds.AbundanceofheterotrophicbacteriawithHNAcon- d
tentvariedbetween0.02and0.24×109cellsL−1(Table3). tionsshowednovariationduringthe24hincubationperiod
in the controls and in the amended treatments. This will be
In situ DMSP and DMSP concentrations ranged from
p d
1.8 to 4.0nmolL−1 and from below d.l. (<0.01nmolL−1) discussedinSect.4.2.2.
to 1.4nmolL−1, respectively. Melt pond DMS concentra- During the Ice1-MP1 incubation, initial DMSPd con-
tions ranged from below d.l. to 6.1nmolL−1 (Table 3). centration was 1.30nmolL−1 in the control and slightly
increased to reach 5.3nmolL−1 during the 24h incuba-
Spearman’srankcorrelationcoefficientsbetweenkeyinsitu
tion period (Fig. 4a). In the light (L-DMSP/O) and dark
variables measured in the melt ponds are presented in Ta-
(D-DMSP/O) amended treatments, DMSP concentrations
ble4.DMSconcentrationssignificantlyco-variedwithsalin- d
started at 102nmolL−1, decreased to ∼35nmolL−1 at
ity (r =0.84, p<0.05) and Chl a (r =0.84, p<0.05).
s s
T , and remained stable (dark treatments) or decreased to
None of the other variables measured displayed significant 6
10nmolL−1 (light treatments) until T (Fig. 4a). Con-
relationshipsbetweeneachother(notshown). 24
centrations of DMS in the control of Ice1-MP1 started at
3.0nmolL−1,increasedto8.8nmolL−1 betweenT andT ,
0 6
andthendecreasedregularlyto4.2nmolL−1atT (Fig.4c).
24
TheadditionoflabelledDMSPandDMSOstimulatedDMS
production. In the L-DMSP/O treatment, DMS concentra-
tionsincreasedto12.6nmolL−1atT ,remainedatthislevel
6
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M.Gourdaletal.:Dimethylsulfidedynamicsinfirst-yearseaicemeltponds 3177
Figure4.TemporalvariationsinDMSPd(a,b)andDMS(c,d)concentrationsduringtheIce1-MP1andIce4-MP1incubationexperiments.
Bothlight(◦)anddark(•)treatmentswereinitiallyamendedwith100nmolL−1ofbothD6-DMSPand13C-DMSO.Controltreatments((cid:52))
mimicnaturalconcentrationchangesovertime.Inpanels(a,b),verticalbarsrepresentstandarderrorsofmeanvaluesbetweenduplicate
samples.
between T and T , increased again between T and T , ment,andamaximalvalueof11.5nmolL−1 wasreachedat
6 12 12 18
and remained stable at ∼19nmolL−1 between T and T T .
18 24 24
(Fig. 4c). DMS concentrations were consistently higher in Insituandpotentialchangeratesofthesulfurcompounds
theD-DMSP/OtreatmentthaninL-DMSP/O(Fig.4c).They duringtheincubationexperimentsarepresentedinTables5
first reached 15.6nmolL−1 at T , increased gradually to and 6, respectively. Changes in DMSP , and to a lesser ex-
6 d
reach a peak value of 24.2nmolL−1 at T , and decreased tentDMSconcentrations,weregenerallynotlinearoverthe
18
slightlyto21.6nmolL−1 atT .NotethatdissolvedDMSO 24hincubationperiod,withmorepronouncedvariationsdur-
24
was not measured during this study due to methodological ingthefirst6h.Totakeintoaccountthisnon-linearity,both
issues. hourlyratesmeasuredbetweenT andT andbetweenT and
0 6 6
In the Ice4-MP1 incubation, DMSP concentrations T ,aswellasdailyrates(T –T ),arepresentedintheseta-
d 24 0 24
startedat3.0nmolL−1 inthecontrolandremainedcloseto bles.
this value during the whole experiment (Fig. 4b). In the L- In Ice1-MP1, the concentrations of DMSP in the con-
p
DMSP/OandD-DMSP/Oamendedtreatments,DMSP con- trol decreased at a rate of 2.2nmolL−1d−1 (Table 5). We
d
centrations started at 87 and 96nmolL−1, respectively. As measured no change in DMSP during the first 6h, but a
d
observed in the previous melt pond, the concentrations de- positive net increase of 4.0nmolL−1 over the full 24h in-
creasedto∼45nmolL−1atT andthenslowlydecreasedto cubation period was observed. In situ DMS changes in-
6
a value of ∼30nmolL−1 at T (Fig. 4b). DMS concentra- creased by 1.0nmolL−1h−1 during the first 6h and by
24
tionsinthecontrolofIce1-MP1startedat2.6nmolL−1 and 1.2nmolL−1d−1 over 24h. Potential net DMSP change
d
remained at this level during the 24h experiment (Fig. 4d). ratesof−11.6and−10.2nmolL−1h−1 weremeasureddur-
IntheL-DMSP/Otreatment,DMSconcentrationsincreased ing the first 6h of incubation in L- and D-DMSP/O treat-
moreorlesslinearlyfrom2.6nmolL−1atT to6.7nmolL−1 ments, respectively (Table 6). These rates became −1.2
0
at T . In the D-DMSP/O treatment, the increase in DMS and −0.6nmolL−1h−1 between T and T in L- and D-
24 6 24
concentrationswasmorepronouncedthaninthelighttreat- DMSP/O, respectively. Over 24h, negative potential net
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3178 M.Gourdaletal.:Dimethylsulfidedynamicsinfirst-yearseaicemeltponds
Table3.Reduced-sulfurcompoundconcentrationsmeasuredinsituinthemeltpondsandtheassociatedbiologicalcharacteristics(abundance
ofhighnucleicacid(HNA)bacteria,Chlaconcentrations,andrelativeabundancesofmajortaxonomicgroups)ofthemeltpondwater.
Station Melt Insitu Insitu Insitu Abundanceof Totalabundance Chla Dominantalgal
pond DMSPp DMSPd DMS bacteria(HNA) ofalgae (µgL−1) group(%)
(nmolL−1) (nmolL−1) (nmolL−1) (×109cellsL−1) (×106cellsL−1)
Ice1 MP1 2.2 1.3 3.0 0.24 2.00 0.48 Unidentifiedflagellates(50%)
Prasinophytes(ca.25%)
MP2 2.0 1.4 3.1 0.40 Unidentifiedflagellates(55%)
Prasinophytes(ca.25%)
Ice2 MP1 1.8 d.l. 0.0 0.04 0.50 0.03 Unidentifiedflagellates(90%)
Pennatediatoms(ca.28%)
MP2 2.4 d.l. 0.0 0.09 Unidentifiedflagellates(50%)
Pennatediatoms(ca.28%)
MP3 2.3 d.l. 0.0 0.06 Unidentifiedflagellates(70%)
Pennatediatoms(ca.28%)
Ice3 MP1 2.0 d.l. 0.0 0.02 0.30 0.05 Unidentifiedflagellates(45%)
Chrysophytes(29%)
MP2 2.3 d.l. 0.0 0.04 Unidentifiedflagellates(55%)
Chrysophytes(23%)
Ice4 MP1 4.0 d.l. 2.6 0.15 1.00 0.18 Unidentifiedflagellates(50%)
Pennatediatoms(20%)
MP2 3.7 1.1 6.1 0.20 Unidentifiedflagellates(60%)
Pennatediatoms(25%)
Table 4. Spearman’s rank correlation coefficients between key in ments. Potential net DMS change rates remained low in
situvariablesmeasuredinthemeltponds. bothL-DMSP/OandD-DMSP/Otreatmentsduringthefirst
6h of incubation with values at 0.1 and 0.3nmolL−1h−1,
DMS Salinity Temperature Chla respectively. For the complete 24h incubation, potential
∗ ∗ net DMS change rates in light and dark reached 4.2 and
DMS 0.84 0.51 0.84
8.9nmolL−1d−1,respectively.
Salinity 0.40 0.56
Temperature 0.60 During both Ice1-MP1 and Ice4-MP1 incubation exper-
iments, the light versus dark treatment had no effect on
∗indicatesa0.05significancelevel. the net changes in DMSP concentrations between the L-
d
DMSP/O and D-DMSP/O treatments (Wilcoxon signed-
ranktest;n=8,df=3,α=0.05),butsignificantlyimpacted
DMSP changeratesof∼−91nmolL−1and−71nmolL−1
d the rates of net accumulation of DMS (Wilcoxon signed-
for the L-DMSP/O and D-DMSP/O treatments were cal-
rank test; n=12, df=5, α=0.05). The accumulation of
culated. Positive potential net DMS change rates of 1.6
DMS over 24h in the L-DMSP/O treatments were con-
and 2.1nmolL−1h−1 were measured during the first 6h
sistently and significantly lower than in the corresponding
of incubation in L-DMSP/O and D-DMSP/O, respectively.
D-DMSP/O treatments (Wilcoxon signed-rank test; n=8,
For the complete 24h incubation, potential net DMS
df=3, α=0.05). Based on the difference between the L-
change rates reached 15.4nmolL−1d−1 in the light and
andD-DMSP/Otreatmentsafter24h,weestimatedthelight-
18.6nmolL−1d−1inthedark. associatedDMSsinksat3.2nmolL−1d−1 inIce1-MP1and
In Ice4-MP1, in situ DMSPp decreased at a rate of at4.7nmolL−1d−1inIce4-MP1(Table6).
1.9nmolL−1d−1 over the course of the incubation (Ta-
ble 5). Meanwhile, in situ DMSPd changes rates were be- 3.4 IsotopicdiscriminationofDMSsources
low the d.l. during the first 6h and almost null over 24h
(Table 5). In situ DMS change rates were close to zero af- Table7showstheconcentrationsofDMSisotopes(m/z62)
ter6handbelowd.l.after24h.PotentialnetDMSP change and(m/z68)after24hincubationinthethreetreatmentsand
d
rates of −8.1nmolL−1h−1 were measured during the first theirrelativecontribution(%)tothetotalDMSmeasuredat
6h of incubation in both L- and D-DMSP/O (Table 6). T .Asexpected,100%ofthetotalDMSinthecontrolsof
24
Theseratessloweddownto−0.5and−0.9nmolL−1h−1be- thetwoexperiments(3.0and2.3nmolL−1)showedtheiso-
tweenT andT ,respectively.Overoneday,averagepoten- topicsignatureofnaturalDMS(m/z62).IntheL-DMSP/O
6 24
tial net DMSP change rates of ∼−59 and −62nmolL−1 treatmentoftheIce1-MP1incubation,78%(14.4nmolL−1)
d
were calculated for the L-DMSP/O and D-DMSP/O treat- of the DMS measured at T derived from D6-DMSP addi-
24
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