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GEOPltYSICAI, RESI;At•,Ct I I,I¾I"I'I:!RS, V()I,. 28, NO. 2• I•AGI•-œS 239-242, ,IANUARY 15, 2001 Bacterial growth in supercooledcloud droplets Birgit Sattier Instituteof ZoologyandLimnology,Universityof Innsbruck,Austria Hans Puxbaum Institutefor AnalyticalChemistry,ViennaUniversityof Technology,Austria Roland Psenner Instituteof ZoologyandLimnology,Universityof Innsbruck,Austria Abstract. It is well known that the atmosphereis a conveyorof microorganisms,and that bacteria can act as ice or cloud 47ø03'N). The observatoryitself is suppliedwith electricityand condensation nuclei, but clouds have not been considered as a Cloud water sampleswere taken with an active collectorfor hydrometeorsconsistingof a single stage impactorbacked by a largewind shield.Cloud watersampleswere kept frozenin sterile polyethylenebagsuntil analysis. Aerosols were sampled with a Quartz fiber filter (47mm, Pallflex TISSUQUARTZ 2500QAT-UP). The collected air site whereorganismscan live and reproduce.Here we showthat bacteriain clouddropletscollectedat high altitudesare actively growingand reproducingat temperatures at or below 0øC. Since -60% of the earth surface is covered by clouds, cloud water should be considered as a microbial habitat. thus not a source of exhausts. volumeswereabout30 to 32m',the usedflow ratesabout45 i min-•. To excludeanycontamination, fieldblanksweretaken 1. Introduction which were sampledin the same way as aerosol samplesbut samplingtime was limited to 30 seconds.Filters were storedin Dispersalof bacteriaandevenlargerorganisms by windhas petri-disheswith a parafilm-capat 4øC. been known since long time (Gisldn, 1948, Gregory, 1967, Surfacesnowsampleswere takenby collectinga snow layer Schnell and Vali, 1972). Living microorganismshave been of approximately10cm thicknesswhich was put in a sterile collectedevenin the stratosphere (Imshenetsky et al., 1978), and polyethylene bag. Samples were handled with a sterilized there is one observationthat bacteriamay divide on airborne polyethyleneshoveland kept frozen until analysis.All materials particles (Dimmicket al., 1979).Studies onbiological particles in which cameinto contactwith clouddroplets,snow,hail and rime cloud water, however, are scarce(Casareto et al., 1996) and have been carefully washed with hydrochloricacid and rinsed bacteriain the atmosphere havebeencollectedmostlyby the use with sterileparticlefree distilledwater (PFDW). of air samplers (Matthias-Maser anddaenicke,1992).Recently, Bacterial abundance,morphologyand biomass.Thirty ml of Hamiltonand Lenton(1998) hypothesized that marinealgaeand samplewere fixed with prefiltered(0.2[tm) formalin to a final bacteria induce water condensation and ice nucleation in order to concentrationof 2% v/v. Twenty ml were filtered onto Poretics improvetheir dispersaland transportby the atmosphere, thus filters (0.2[tm pore size), stainedwith the fluorochromeDAPI influencing not only distanceanddestination of thejourneybut (4' 6-diamidino-2phenylindole)at a final concentrationof 0.2% also the world climate(Gislen, 1948). Airbornemicroorganisms v/v accordingto Porter and Feig (1980) and countedin a Zeiss were termed "spora",assumingthey were inert or inactive, Axioplan epifluorescence microscope.Image analysiswas done consequently,the atmosphericenvironmentwas considered with an Optotronics ZVS-47EC camera and LUCIA_D primarilyasa mereconveyor, although rainandfogwaterrichin (LaboratoryImaging, Prague, Czech Republic) accordingto nutrientsmay providea good substratum for microorganismsPosch et al. (1997). Biomasshas been estimatedas describedin (Herlihyet al., 1987,Fuzziet al., 1997).The relativelycleanand Loferer-KrOfibacher et al. (1998). coldatmosphere of highaltitudes,however,wasnot regardedasa Bacterial production. Cell multiplicationrates and protein suitable place for bacterial growth. We were, therefore, synthesiswere measuredaccording to Simon (1990) with astonishedto find actively growing bacteria suspendedin supercooled clouddroplets. [methyl-3H]thymidine (specific activity70-90Ci mmol -•) and •4 [ C]leucine(specificactivity 310 mCi mmol -• - both 2. Methods RadiochemicalCentre,Amersham,England). Cloud water was diluted (1:7) with sterileparticlefree distilledwater in order to measuretriplicatesamplesand 1 blank. Twenty ml were marked Sampleswere collectedin late April and early May 1997 at the Sonnblick Observatory(SBO), a meteorologicalstation located on top of mount Sonnblick (3106m a.s.l., Austria, Salzburg) in the main ridge of the Austrian Alps (12ø57'E, with[3H]thymidine (5 nmol1-1) and[14C]leucine (20nmol 1-1) and Copyright 2001bytheAmericanGeophysical Union. 3. Results incubatedfor 20 hours at 0øC in a high precisionwaterbath (HAAKEm). Bacterial carbon production was calculated accordingto SimonandAzam (1989). and Discussion Clouddropletscollectedat SBO containedca. 1.500 bacteria Papernumber2000GL011684. ml-• (Table1), whereas the abundances in snowandgraupel 0094-8276/01/2000GL011684505.00 239 240 Table1. Abundance, meancellsize,biomass, 3H-thymidine and•4C-leucine uptakerates,carbon production andgeneration t•mesof bacteriafrom cloudwater (CW), snow and graupel samples collectedat SonnblickObservatory(3.106m a.s.I.) in April and May 1997; all rates have been measured at 0øC; n.d. = not detectable. CW ... cloud water Sample Date BacteriaCellsize Biomass 3H-thymidine 3H-thymidine•4C-Leucine Carbon Generation uptake rate uptake rate per uptake rate production time cell 1997 (Nml'•) (IJm 3) (ngC I-•) (fmolI'• h'•) (10-24molh'• cell-1) CW 1 29 April 1,180 0.159 CW 2 30 April 2,470 CW 3 1 May 920 CW 4 29 April CW 5 (fmolI-• h-•) (ngC I-• h-•) (days) 37 4.8 4.07 263 0.61 0.030 32 4.2 1.70 391 0.91 8.5 0.025 13 1.2 n.d. 126 0.29 19.5 790 0.029 9 1.1 1.39 153 0.36 10.4 29 April 1,090 0.038 14 1.1 1.01 505 1.17 14.3 CW 6 29 April 1,490 0.048 25 1.9 1.28 278 0.64 11.8 CW 7 29 April 2,060 0.037 25 2.4 1.17 139 0.32 12.4 3.6 CW 8 30 April 1,640 0.068 30 n.d. n.d. 104 0.24 n.d. CW 9 1 May 1,060 0.110 23 3.0 2.83 46 0.11 5.1 CW 10 4 May 1,620 0.036 28 n.d. 7.41 131 0.30 n.d. CW 11 5 May 1,160 0.019 13 1.6 1.38 289 0.67 10.5 CW 12 6 May 2,080 0.026 33 2.4 1.15 n.d. n.d. 12.5 Mean (CW) 1,463 0.052 24 2.4 2.34 221 0.51 10.9 Snow 1 27 April 11,200 0.027 168 9.2 0.82 76 0.18 17.6 Snow 2 27 April 10,900 0.018 120 7.8 0.72 113 0.26 20.2 Snow 3 29 April 11,100 0.033 233 2.6 0.23 n.d. n.d. 61.7 Snow 4 29 April 12,400 0.046 273 1.2 0.097 n.d. n.d. 107.5 Snow 5 29 April 9,500 0.029 143 4.5 0.47 n.d. n.d. 30.5 Snow 6 30 April 10,500 0.051 263 2.2 0.21 n.d. n.d. 68.9 Snow 7 30 April 13,100 0.026 183 2.2 0.17 45 0.10 86.0 Mean (snow) 11,243 0.033 198 4.2 0.39 78 0.18 56.1 Graupel 1 29 April 6,900 0.025 90 3.2 0.46 n.d. n.d. 31.1 Graupel 2 29 April 11,500 0.035 173 2.1 0.18 79 0.18 79.1 Mean (graupel) 9,200 0.030 132 2.7 0.33 79 0.18 55.1 collectedat the same occasionwere five to ten times higher. Nonetheless,theseare amongthe lowest abundancesfound in aquaticenvironmentsand comparableto unpollutedgroundwater (Al)qeider et al., 1997). The uptake rates of tritium labelled thymidineinto DNA, which is generallyused to measurecell Which naturalfactorscouldlimit bacterialproductivityin the atmosphere.'? Air temperatures duringsamplingwere between-4 oceans. temperatures. and-9øC, and it is well known that cloud water can remain supercooled at temperatures of-15øC or even less(Pruppacher and Klett, 1978; Rosenfeldand Woodley,2000). We have no growth, showed average valuesof 1.1to 4.8 10-ismol1-1h'l. direct measurementsof bacterial uptake rates at in situ Although measured at 0øC,uptake ratespercell(2.34-10-2]mol temperaturebecausethe labelledsubstratescan only be usedin thymidine h'l cell'l) in cloudwaterwereas highas in typical liquid water. Snow and ice bacteriafrom a high altitudesite in lakewater samples(Fig. 1). Accordingly, generationtimes in Tyrol showeda 1.5 fold increasein growth rates if samplesare cloudwaterrangedfrom 3.6 to 19.5 days,comparableto thoseof gradually warmed from ambient temperaturesof 0øC to 5øC phytoplankton in the ocean,i.e. aboutoneweek(Falkowskiet al., (Sattier, unpublisheddata). Thus, in situ growth rates in 1998).Theincorparation of •4C-leucine showed valuesof upto supercooleddropletsmay be only 50-70% of thosemeasuredat 0.5 pmol1-1h'l (Table1). Bacteria in cloudwaterproduced on 0øC(Table 1). Priscuet al. (1998) who studiedprokaryotesliving average 12ngC 1-1d-1,witha maximum of 28 ngC 1-• d'l. They at temperaturesaround 0øC in the ice of permanentlyfrozen were generallysmallerthan 0.5 [tm in length,the averagecell Antarctic lakes, found that low temperaturesalone and freezevolume was0.052!am 3,corresponding to 17fg C cell-1.Thisisa thaw cyclesdo not seemto inhibit microbialgrowth.As shown typical value for marineand freshwaterecosystems, whereasthe by Felip et al. (1995) for the winter coverof alpine lakes,high biomasswas up to threeordersbelow valuesfound in lakesand substrateconcentrations can partly compensatethe effect of low 241 surfaceof a highmountainlakewerequitedifferentfromthosein slush layers and in lakewater(Alfreider et al., 1996). It has lOO • lO become evident that much of the microflora in remote environ- ments is transportedby air currents(Marshall, 1996, 1997, Psennerand Sattler, 1998) providingan importantway of •' Ol colonization .• oOl • oOOl o OOOl , 10 2 10 3 10 4 10 5 10 6 , 10 ? Bacterial abundance IN ml'1] ß O ß ? ß O e • • of otherwise isolated environments. Taxonomic studiesaboutairborneorganisms are very scarce- Sandset al. (1982) identifiedPseudomonas syringaewhichis probablythe most abundantice nucleatingbacteriain air, rain and hail. An ._ V•nter cover of Gossenk011esee,Austna, 2 417m Lake water of Gossenk011esee (Sattier 1996) Lake water of P•burgerSee, Austria,913m (Sattier 1993) Lake water of Ta• Hu, China (Sattier 1993) Cloud droplets of SonnbhckObservatory, Austria (this study) Snow from SonnbhckObservatory(th•sstudy) Snow from Gossenk011esee(Sattier 1996) Ice commud•es•n East Lobe Bonney, Antarcbca(Sattier et al unpubhsheddata) Average Lake water Thy uptakerate per cell Figure1. Bacterial abundance andSH-thymidine uptake ratesin cloudwater,freshlyfallen snow,and slushlayersof lake ice measuredat 0øC, and in lakewater measuredat in situ temperatures(Sattier, 1993, 1996). Solidline:meanthymidine uptakeratepercell(1.96 '10-2] molh-])in lakewater atinsitutemperatures. earlier studydealingwith microorganisms in hailstonesby Mandrioliet al. (1973)identifiedvariouscolonies of fungiand pollen - the remainderwas a singletype of gram-positive streptococcus. The number of bacteria observed in snow was around 10.000 ml-], Table1, andan estimate for a typicalnumberof snow crystalsforming 1 ml water is around 10.000 (Pruppacherand Klett, 1978, Mishima and Stanley, 1998). From this and current views aboutthe role of bacteriain the atmosphere(Lenton, 1998, Gisldn, 1948, Hamilton and Lenton, 1998) it appearslikely that the observedbacteriawere actingas ice nuclei. Schnell statedin 1976 that bacteriaattachedto mineralparticleswere responsible for ice nucleation rather than the mineral particles itself as Tscherwenkaet al. (1998) argue.A closeup look on our bacterial observations,however, revealed that bacteria are frequently attachedto particles- which indicatesthat there is possiblyno contradictionbetweenour andthe previousinterpretations.Since thenumber of bacteria in cloudwater(-1500ml-], Tab.1) is far Thenutrient conditions of cloudandrainwaterandfleshly lower than the averagenumberof cloud dropletsforming 1 ml fallen snow(Brantnetet al., 1994) can be evenbetterthan in water(-2.10•) it appears thatbacteria-although growingin many freshwaterlakes. Cloud water at the SBO containedrather highamounts of organic acids,i.e. upto 0.3 mg1-] of formiate cloud droplets - are not an important source of cloud condensation nuclei. and2.1mg1'] of acetate (Brantnet etal.,1994),butalsoseveral The consistentlydifferent numbers of bacteria in cloud [tg1-] of alcohols, suchasdodecanol, tetradecanol, hexadecanol dropletsand snow seemto dependon the conditionsin "mixed" and octadecanol (Lirabeckand Puxbaum,2000). CI 2-C18 supercooledclouds- where supercooledcloud dropletsand ice alcohols havebeenascribed to bacterialsources (Simoneit and crystalscompetefor the available water. In this process- the Mazurek, 1982), and bacterial utilization of formic and acetic seeder/feedermechanism- the higher vapor pressureof liquid acidin rainwaterwasshownby Herlihyet al. (1987).It is not cloudwateras comparedto the vaporpressureoverthe ice phase clear, however, whether alcoholsin cloud water and snow are fayourscrystalgrowthon the expenseof droplets.Later, during produced or usedby microorganisms. Hexadecane, for instance, the fall of the ice crystals,cloud dropletsare scavengedby the canbe usedby Arthrobacter sp. whichproduces hexadecanol,crystalsformingaccretions(Pruppacherand Klett, 1978, Bryant, andArthrobacter sp.isoneofthemostabundant andactive types 1997). Magonoand Lee (1973) observedin snowstormsin Japan of bacteriain dry soilswhichcouldbe suspended by wind. that ice water concentrations were around 10 times lower than Another possible candidate isRhodococcus sp.,a psychrotrophic liquid cloud water. Thus a liquid water content(LWC) of 0.5 bacterium whichis ableto degrade variable chain-length alkanes g/m3 would imply-750 bacteriaper m3 incorporatedin cloud at low temperatures (Whyteet al., 1998). The concentration of water and an ice water concentrationof 0.05 g/m3 would imply alcohols, however, largelyexceeded bacterial cloudbiomass, thus -550 bacteriaper m3 incorporatedin ice particles(i.e. snow). suggesting a terrestrialsourcefor alcohols,but it indicatesthat Cloud systemsbearingdropletsof 10 to 100 [tm diametercan persistfor hoursor longerin the atmospherealthoughthe lifetime farlarger thantheamount needed tosupport bacterial growth of individual cloud droplets is normally much lower. If we ThepH, whichcouldseverlyimpedebacterial metabolism, assumea residencetime of one day and an ambienttemperature wasratherfavourable in all samples, withvalues between pH 5.4 of 0øC,bacterialbiomassin clouddropletscan increaseby up to the concentration of dissolved organiccarbonin cloudwateris and6.8, and dissolved organiccarbonconcentrations canreach 20%(Tab.1). A meancloudwatercontent of 0.3 g m-3anda several mg1-], astypicalforlakewater conditions. Naturalsolar cloudthickness of 2 km yields•600 g waterm'2, although radiation athighaltitudes maystrongly inhibitmicrobial activity Rosenfeld and Woodley(2000) foundnearly2 g m-3 of (Sommaruga et al., 1997),but cloudsthemselves canprovide supercooledliquid cloud water in deep convectivecloudsof a shading fromdamaging UV radiation. continentalstorm.If our measurements of bacterialproductionin Thesource of airborne organisms is stilluncertain butthey clouddropletswererepresentative andconsidering the percentage mayoriginate fromplantsurfaces (Lenton,1998),watersurfaces of cloudedsky,i.e. 60% of the earthsurface(Bryant, 1997), we or soilsandfromreplicating bacteria withinclouds. Transport of air masses fromthewell mixedboundarylayerto the highAlpine siteshasbeenfrequentlyobserved,in particularduringthe warm season(Tscherwenka et al., 1998). A previousstudydoneby in situ hybridisationwith RNA probesclearlyshowedthat bacterial communities in freshly fallen snow collected on the frozen caninfer a bacterialproductionof organiccarbonin the rangeof 1-10 Tg carbonper year.This is a smallnumbercomparedto the primary productionin the oceans(Falkowski et al., 1998) of ca. 45-50PgC yr-] ortotheemissions of organic carbon (ca.1 PgC yr-]).An interesting point,however, is whether airborne bacteria carryout or affectkey processes in cloudchemistryor physics. Concluding,we considerthe atmospherenot only as a conveyorof organisms but as a sitewheresignificantmicrobial processes cantakeplaceduringtransport.How stronglybiologic processes influencetransformations of organicand inorganic components of the atmosphere has still to be explored,but bacteriagrowanddivide,andproduceproteinsanda plethoraof Magono,C. and C.W. Lee, d. Meteorol.Soc.Japan.,51,176, 1973. Mandrioli, P., G.L. Puppi, and N. Bagni, Distributionof microorganisms in hailstones, Nature,246, 416-417,1973. Marshall, W.A., Biologicalparticlesover Antarctica,Nature, 383, 680, 1996 Marshall, W.A and M.O. Chalmers,Airborne dispersalof antarcticterrestrialalgaeand cyanobacteria, Ecography,20, metaboliteswithin the atmosphere,even at temperaturesof or 585-594, 1997. below 0øC. The limiting stepfor microbialprocesses in cloud Matthias-Maser, S. and R. Jaenicke, in Nucleation and droplets may thus be neither temperaturenor nutrient Atmospheric Aerosols,editedby N. Fukuta,P.E. Wagner,pp. 413-415,DeepakPublishing, Hampton,Virginia,1992. concentrations but their residencetime in the atmosphere. After betweenliquid, deposition,airbornebacteriawill influencethe composition of Mishima,O. andH.E. Stanley,The relationship supercooled andglassy water,Nature,396,329-335,1998. microbialassemblages in snow,soils,lakewaterandoceans,and Porter, K.G. andY.S. Feig,The useof DAPI for identifyingand metabolicpathwaysandfoodwebsin thoseenvironments. countingaquaticmicroflora,Limnol.Oceanogr.,25, 943948, 1980. Acknowledgements.We wish to thank Andy Limbeck,Christina Gr011ert and Werner Tscherwenka for field work and discussions. Heidi Bauer earnsa thanksfor intensiveliteratureinvestigations. Furthermore we are in debt of Alois Halswanter,Anton Wille, Tom Battin, Thomas Posch,andReinholdP0derfor helpfulideas.The studywassupported by the AustrianScienceFoundation(FWF Project11685-MOB). References Posch,T., J. Pernthaler,A. Alfreider,R. Psenner,Cell-specific respiratory activityof aquaticbacteriastudiedwith thetetrazolium reduction method, cyto-clear slides, and image analysis, Appl.Environ. Microbiol., 63,867-873,1997. Priscu,J.C., C.H. Fritsen,E.E. Adams,S.J. Giovannoni,H.W Paerl,C.P. McKay,P.T. Doran,D.A. Gordon,B.D. Lanoil, J.L.Pickney, Perennial antarctic lakes:an oasisfor life in a polardesert, Science, 280,2095-2098,1998. Pruppacher, H.R. andJ.D.Klett,Microphysics of Cloudsand Precipitation, pp.714,D. Reidel,Dordrecht, 1978. Alfreider, A., J. Pernthaler,R. Amann, B. Sattler,F.O. G16ckner, Psenner,R. and B. Sattler,Life at the freezingpoint,Science, 280, 2073-2074, 1998. A. Wille, R. Psenner,Communityanalysisof the bacterial D. andW.L. Woodley,Deepconvective cloudswith assemblages in the winter cover,Appl. Environ.Microbiol., Rosenfeld, 62, 2138-2144, 1996. Alfreider, A., M. Kr613bacher, R. Psenner,Groundwatersamples do not reflect bacterial densities and activities of subsurface sustained supercooled liquidwaterdownto-37.5øC,Nature, 405, 440-442, 2000. SandsD.C., V.E. Langhans, A.L. Scharen,G.de Smet,The association betweenbacteriaand rain and possibleresultant systems,Wat.Res.,31,832-840, 1997. meteorological implications, IdOjdrds, 86, 148-152,1982. Brantner,B., H. Fierlinger,H. Puxbaum,A. Berner,Cloudwater Sattler, B., MessungbakteriellerSekundiirproduktion und chemistry in the supercooleddroplet regime at mount Grazing-Ratenmittels radiochemischer Methoden im Sonnblick(3.106m a.s.l.,Salzburg,Austria),Water,Air, and Soil Pollution, 74, 363-384, 1994. Bryant, E., Climate processand change,pp. 225, Cambridge University Press,1997. Casareto,B.E., Y. Suzuki, K. Okada, M. Morita, Biological particlesin rain water, Geophys.Res. Letters,23, 173-176, 1996. Piburger See(Otztal,Tirol).MSc.University of Innsbruck, pp. 221, 1993. Sattler, B., Microorganisms in High Mountain Lakes. PhD Universityof Innsbruck,pp.96, 1996. Schnell, R.C. and G. Vali, Atmosphericice nuclei from decomposing vegetation, Nature,236, 163-165,1972. Schnell,R.C.,BiogenicIce Nuclei:PartI. Terrestrial andmarine Dimmick, R.L., H. Wolochow, M.A. Chatigny, Evidence for sources, dAtmos.Sciences, 33, 1554-1564,1976. more than one division of bacteriawithin airborneparticles, Simon,M. and F. Azam, Proteincontentand proteinsysnthesis Appl. Environ.Microbiol.38, 642-643, 1979. ratesof planktonic marinebacteria,Mar. Ecol.Prog.Set.,5, Falkowski, P.G., R.T. Barber, V. Smetacek,Biogeochemical 201-213, 1989. controls and feedbacks on Ocean primary production, Simon,M., 1990,Improvedassessment of bacterialproduction: combinedmeasurements of proteinsynthesis via leucineand Felip, M., B. Sattier, R. Psenner,J. Catalan, Highly active cell multiplicationvia thymidine incorporation,Arch. microbial communitiesin the ice and snow cover of high Hydrobiol.Beih.Ergebn.Limnol.,34, 151- 155, 1990. mountainlakes,Appl. Environ. Microbiol., 61, 2394-2401, Simoneit, B.R.T. and D.A. Mazurek, Organic matter of the 1995. troposhereII. Naturalbackground of biogeniclipid matter Fuzzi, S., P. Mandrioli, A. Perfetto, Fog droplets- an Science, 281, 200-206, 1998. atmosphericsourceof secondarybiologicalaerosolparticles, Atmos.Environ., 31,287-290, 1997. in aerosols over the rural western United States, Atmos. Environ., 16, 2139-2159, 1982. Sommaruga,R., I. Obernosterer, G.J. Herndl, R. Psenner, Inhibitory effect of solar radiation on thymidineandleucine Reviews, 23, 109-126, 1948. incorporation by freshwaterand marine bacterioplankton, Gregory, P.H., Atmosphericmicrobial cloud systems,Science Appl.Environ.Microbiol.,63, 4178-4184,1997. Progress,Oxford55, 613-628, 1967. Gis16n,T., Aerial planktonand its conditionsof life, Biological Hamilton, W.D. and T.M. Lenton, Sporaand Gala, how microbes fly with their clouds,Ethology,Ecology& Evolution,10, 116, 1998. Herlihy, L.J., J.N. Galloway,A.L. Mills, Bacterialutilizationof formic and acetic acid in rainwater, Atmos. Environ. 21, 2397-2402, 1987. Imshenetsky, A.A., S.V. Lysenko, G.A. Kazakov, Upper boundaryto the biosphere, Appl. Environ.Microbiol.,35, 15, 1978. Lenton, T.M., Gala and natural selection,Nature, 394, 439-447, 1998. TscherwenkaW, P. Seibert,A. Kasper,H. Puxbaum,On-line measurements of sulfur dioxide at the 3 km level over central Europe (SonnblickObservatory,Austria) and statistical trajectorysourceanalysis.Atmos.Environ.32, 3941-3952, 1998. Birgit'Sattier,RolandPsenner,Universityof Innsbruck, Instituteof ZoologyandLimnology, Technikerstrasse 25, 6020 Innsbruck, Austria (e-mail:birgitsattler•uibk.ac.at) Hans Puxbaum,Vienna Universityof Technology,Institute Limbeck A. and H. Puxbaum, Dependence of in-cloud 9/151, 1060 Vienna, scavenging of polar organicaerosolconstituents on the water of AnalyticalChemistry,Getreidemarkt Austria solubility,J.Geophys.Res.,in press. Loferer-KrO13bacher, M., J. Klima, R. Psenner, Determination of bacterial-celldry massby transmissionelectron-microscop•, April 12,2000;revisedJuly17,2000, and densitometric image-analysis, Appl. Environ.Microbiol., Received acceptedJuly25, 2000.) 4, 688-694, 1998.
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WRITING INTRODUCTION TO AN ARTICLE
Introduction
Airborne microorganisms in the stratosphere remain elusive as there is lack of proper sample
collection system. As such, the dispersal of bacteria by the wind is known subject for a long
time. Living microorganisms are co...


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