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Viral Loop Dynamics in Temperate and Polar Freshwaters

Säwström, Christin

2006

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Citation for published version (APA):Säwström, C. (2006). Viral Loop Dynamics in Temperate and Polar Freshwaters. Department of Ecology, LundUniversity.

Total number of authors:1

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Viral loop dynamics in temperate and polar freshwaters

Viral loop dynamics in temperate and polarfreshwaters

Christin Säwström

Akademisk avhandling, som för avläggande av filosofie doktorsexamen vid Naturvetenskapligafakulteten, Lunds Universitet, kommer att offentligen försvaras vid Ekologiska institutionen,

Avdelningen för limnologi, fredagen den 28 april 2006, kl 10.00.

Plats: Blå hallen, Ekologihuset, Sölvegatan 37, Lund

Fakultetsopponent: Associate Professor Mathias Middelboe, Marine Biological Laboratory,University of Copenhagen, Denmark.

Avhandlingen kommer att försvaras på engelska.

A doctoral thesis at a university in Sweden is produced either as a monograph or as a collection of papers. In thelatter case, the introductory part constitutes the formal thesis, which summarizes the accompanying papers.These have either already been published or are manuscripts at various stages (in press, submitted or in ms).

Layout: Gunilla Andersson/ZooBoTech & GrafikGruppenProofreading: Christin Säwström

Printed by Xanto Grafiska AB, S Sandby

ISBN 91-7105-235-6SE-LUNBDS/NBLI-06/1060+76 pp

Front cover and chapter separator photos by Christin Säwström

Contents Page

Viral loop dynamics in temperate and polar freshwaters 7

Virusinfekterade vatten – Populärvetenskaplig sammanfattning av avhandlingen 17

Acknowledgments 19

I Anesio, A. M., Hollas, C., Granéli, W. and Laybourn-Parry, J. 2004. 23Influence of humic substances on bacterial and viral dynamics infreshwaters. – Appl. Environ. Microbiol. 70: 4848–4854.

II Säwström, C., Laybourn-Parry, J., Granéli, W. and Anesio, A. M. 2006. 33Nutrient and temperature control of bacterioplankton and virioplanktonin the Arctic and Antarctic freshwater environments. – Submitted.

III Säwström, C., Granéli, W., Laybourn-Parry, J. and Anesio, A. 2006. 49High viral infection rates in Antarctic and Arctic bacterioplankton.– Submitted.

IV Säwström, C., Anesio, A. M., Granéli, W. and Laybourn-Parry, J. 2006. 59Seasonal viral loop dynamics in two large ultra-oligotrophic Antarcticfreshwater lakes. – Submitted.

Paper I is reprinted with permission from the publisher.

91-7105-235-6

76

7

Introduction

Viruses can be found wherever life is presentand are the most abundant biological entitieson our planet. Viruses have caused major devas-tation in human societies and continue to do soas new viral diseases are frequently discovered,like HIV, SARS and avian flu. This has led to anin depth knowledge of the general propertiesand biology of viruses which has in turn greatlycontributed to the discipline of molecular biol-ogy where viruses are used as tools for the mi-crobial geneticist.

Virology is today a well-studied field. On theother hand viral ecology is still a largely un-known field, where the natural occurrence andactivities of viruses in ecosystems are just begin-ning to be unravelled. The presence of viruses inaquatic ecosystems was first discovered in the1950’s [40] and high estimates of virus abun-dance were later reported in marine systems byTorella and Morita in 1979 [45]. Viruses role inthe aquatic food web and their ecological signif-icance was not questioned until 10 years laterwhen Bergh et al. [2] published a letter in Na-ture showing high natural abundance of viruses,the majority classified as bacteriophages (bacte-rial viruses), in unpolluted waters. This impliedthat viruses could be a significant mortality fac-tor in the microbial plankton world that had sofar been overlooked. Seventeen years later and itis now clear that viruses are a crucial componentof the aquatic food web capable of infecting two

large important groups of the microbial plank-ton; phytoplankton and bacterioplankton. Bac-terioplankton recycle dissolved organic matter(DOM) in the aquatic food web through themicrobial loop [1] (Fig. 1). In the microbialloop, bacteria consume dissolved organic mate-rial that cannot directly be ingested by largerorganisms. The bacteria are in turn grazed uponby protozoa, which are then ingested by zoo-plankton and then ultimately recycled back tohigher trophic levels. Viral lysis of bacterio-plankton disrupts this flow of energy and or-ganic matter by forming a “viral loop” amongbacteria, viruses and the DOM pool (Fig. 1). Ithas been suggested that as much as 30% of bac-terial production can be channelled through theviral loop in aquatic systems [7]. The net effectof this is an increase in respiration and recyclingin the lower parts of the food web and a lessefficient organic matter transfer to highertrophic levels [41]. There is also evidence thatviruses can influence bacterial community andgenetic diversity structure by killing of the mostdominant species and allowing for co-existenceof less competitive species, and through viral-mediated transfer of genes between species [10,7, 48, 54]. Most of the studies on aquatic virus-es have been conducted in marine environ-ments, whereas studies in freshwaters are stilllagging behind. This thesis looks at viruses infreshwater environments from both temperateand polar regions, with the largest emphasis be-ing on viruses in Antarctic and Arctic freshwa-

Viral loop dynamics in temperate and polarfreshwaters

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ters. The thesis describes interactions betweenviruses and bacteria and the factors that may in-fluence these interactions in freshwaters. In ad-dition, it examines what proportion of the bac-terioplankton in Arctic and Antarctic freshwa-ters is infected with viruses and the seasonal rel-evance of the viral loop in ultra-oligotrophicAntarctic lakes.

Viral regulation and replication infreshwaters

The regulation of viruses in aquatic systems isdependent on the release of new viral particlesfrom the host and the survival of free viral parti-cles in the water. The estimation of the abun-dance of free viral particles in natural waters relyon direct counts of virus-like particles (VLP),

using either transmission or epifluorescence mi-croscopy. Direct counting of viruses shows thatthe highest abundance of viruses is found infreshwater rather than marine systems and viralabundance typically exceeds bacterial abun-dance in both systems [25, 61].

Free viral particles are vulnerable and ex-posed in the water media and their survival isdependent on encountering a suitable host. In-formation on the removal and inactivationprocesses of viruses in natural waters is limited.However, studies have shown that solar radia-tion can have a significant negative impact onthe number of infective viral particles in waters[56, 62]. On the other hand, grazing on viralparticles appears to be of minor importance inviral removal [16, 42]. Decay processes of freeviral particles in natural waters can occur viaadsorption to large particles and high molecularweight dissolved organic matter (HMW

Fig. 1. The microbial loop including the viral loop which is formed between bacteria, viruses and dissolvedorganic matter.


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DOM) [32, 35]. Inorganic and organic parti-cles present in the freshwater medium can havea large impact on viral processes [32]. For in-stance, brown-water lakes or humic lakes con-tain high concentrations of humic substanceswhich are complex mixtures containing aliphat-ic and aromatic carboxyl and hydroxyl func-tional groups that have important bindingproperties which may inactivate viruses in fresh-waters [34]. Humic-like substances have beenfound to have a negative effect on viral infectiv-ity and replication in both the human immuno-deficiency virus and the influenza virus [24, 39,49]. However, humic substances have a net pos-itive effect on bacterial growth and several stud-ies [46, 47] have shown that bacteria can utilisehumic substances as an additional/alternativeenergy source. Thus, one would predict that thepositive effects of humic substances on bacteriacould reflect positively on their parasite; viruses.A field and laboratory study was conducted todetermine the effects of humic substances onthe interactions between viruses and bacteria infreshwaters (Paper I). The results from Paper Ishowed that humic substances had a positive ef-fect on bacterial growth but a negative impacton the abundance of viruses. This suggests thatinfection and/or replication rates of viruses infreshwaters with high concentrations of humicsubstances may be significantly different rela-tive to clear water lakes.

There are two principal mechanisms of virusreplication; lytic and lysogenic. In the lytic cy-cle, the virus infects its host and takes over thehost nucleic acid and protein synthetic machin-ery and turns the host cell into an efficient fac-tory for the production of new viral particles.After a latent period during which the new viralparticles are formed and assembled, the host cellis destroyed (lysis) resulting in the release of theviral particles into the environment [5, 60]. Inthe lysogenic cycle the viral nucleic acid is incor-porated into the host genome (lysogens) but re-mains inactive until some external factor induc-es the lytic cycle. The factors that induces thelytic cycle are largely unknown in aquatic sys-tems but studies have shown that environmen-tal factors such as UV irradiation, chemicalsand nutrients – in particular phosphate – maytrigger the lytic cycle [60]. Viruses that are able

to use the lysogenic cycle are called temperateand over 90% of known bacteriophages aretemperate [14]. Temperate phages have beenfound in both marine and freshwater environ-ments [44, 55, 58] and it has been hypothesisedthat temperate phages may have an advantageover virulent phages (viruses only using the lyticcycle) in aquatic ecosystems. A virulent phagein an oligotrophic environment with a slow-growing host present in small numbers couldrun the chance of being eliminated as it wouldliterally run out of host cells to infect. On thecontrary, a temperate phage could “survive”harsh periods of low host abundance and bideits time until better conditions prevail. The pre-diction would then be that temperate phagesshould predominate in oligotrophic aquatic sys-tems [60].

Paper I, II, IV used a direct approach to esti-mate the occurrence of lysogens in the naturalbacterioplankton community by monitoringthe viral abundance in water samples treatedwith a potent inducing agent (the antibioticMitomycin C) and comparing it with untreatedcontrol samples [33]. The susceptibility of natu-ral bacterioplankton communities to Mitomy-cin C may vary between environments and bac-terial species. Thus there is the possibility that

Table 1. Percentage of lysogens in the total bacterialpopulation in temperate and polar freshwaterenvironments.

Environment Lysogeny (%)

Lake Skärlen NDLake Feresjön 31.8Lake Homehultsjön 13.5Lake Stråken 23.1Lake Skärshultsjön 18.1Arctic Lake 1 NDArctic Lake 2 NDCryoconite hole NDLake Druzhby (17/12/03) 22.3Crooked Lake (11/08/04) 73.0

In Crooked Lake and Lake Druzhby lysogens weredetected on 3 out of 10 sampling occasions and thevalue presented in the table is the highest percentageof lysogens detected during season 2003/2004. ND =Not detected.


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only a part of the lysogenic bacterial populationis affected, thereby resulting in an underestima-tion of the occurrence of lysogens. Four of fivelakes in Småland, Sweden (Paper I) showedpresence of lysogens, indicating that lysogenymight be an important ecological strategy inhumic lakes (Table 1). One may speculate thatlysogeny is a strategy for viruses to survive inhumic lakes, where high rates of destructionand inactivation of viruses is likely to take placedue to the presence of HMW DOM. On thecontrary, findings from the Arctic and the Ant-arctic freshwater environment (Paper II, IV)showed that lysogens were uncommon in polarfreshwaters and that lysogenic viral productionwas of minor importance (Table 1).

Virus and bacteria interactions infreshwaters

Viruses are parasites and their survival is de-pendent on the existence of a suitable host.Host encounter occurs via passive diffusion(Brownian motion) and hence, at low host den-sity the opportunity for a virus to make contactwith a bacterial cell is reduced. In most marineand freshwater environments the microbialplankton, in this case referring to the bacterio-plankton, are living under less than optimalconditions, where low nutrient levels and tem-peratures restricts their growth, often resultingin a slow-growing population at low density. Inthe extremes of the Arctic and Antarctic fresh-waters microbial life is pushed to its limits inregards to temperatures, light- and nutrient lev-els [23]. These freshwater ecosystems have lowtrophic complexity and are mainly made up ofthe microbial loop, where the bacterioplanktonplay a crucial role as recyclers of the availablenutrients [22] (Fig. 1). The bacteria in polar sys-tems most likely live in a state of starvation witha large proportion of the bacteria being inactiveor/and dormant, which will ultimately have anegative effect on the production of viruses.Studies have shown that the metabolic state ofthe host cell and its surrounding environmentcan affect the number of virus particles releasedupon lysis (burst size) and the length of the la-

tent period [18, 27, 36]. Often there is a posi-tive correlation between bacterial and viralabundance in natural aquatic systems (PaperII), indicating that these two components areclosely linked and that parameters which con-trol bacterial production may also influence vi-ral processes [61]. Several studies have shownthat freshwater bacterioplankton growth is pri-marily limited by phosphorus [8, 9, 11, 12, 30,37, 51] and that carbon may on occasion act asa secondary limiting nutrient [17]. Paper II in-vestigated how nutrient addition and tempera-ture influenced virus-bacteria interactions inthe Arctic and Antarctic freshwater environ-ment. The results of Paper II adds to a growingset of evidence that freshwater bacterioplanktongrowth is primarily limited by phosphorus, inboth tropical, temperate and polar environ-ments. Furthermore, Arctic bacteria seemed tohave a temperature-substrate dependent growthwith quicker response to nutrient amendmentsat high temperatures relative to low tempera-tures. Whereas, Antarctic bacteria were welladapted to low temperatures. Even though anincrease in phosphorus concentration had apositive effect on bacteria this was not reflectedin the virus abundance, which stayed fairly sta-ble throughout the study. Results from bothPaper I and II indicate that viral abundances arenot necessarily affected by the same parametersthat control bacterial production in freshwaters.

Results from Paper II showed that the bacte-rioplankton growth was severely restricted bythe in situ environment in polar aquatic ecosys-tems. With a host living under low nutrient andtemperature constraints the question arises as tohow an active virus population can be main-tained. To answer this question a bi-polar inves-tigation was conducted (Paper III) in the Ant-arctic and Arctic freshwaters. Transmission elec-tron microscopy (TEM) was used to visualisevirus-infected bacterioplankton in water sam-ples taken from two ultra-oligotrophic Antarc-tic freshwater lakes (Crooked Lake and LakeDruzhby Fig. 2A, B) and from cryoconite holes(small, water-filled melt holes that form on thesurface of the glacier) on the Midre Løvenbreenglacier in the high Arctic (Fig. 2C). TEM hasbeen used extensively to study the morphologi-cal diversity of aquatic viruses [20, 52] as well as


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the frequency of visibly infected bacterial cells(FVIB) and burst sizes [3, 13, 19, 20, 21, 26,50, 52, 59]. In temperate freshwaters FVIBranges between 0.6 to 5% with an average of 26phages per cell [3, 13, 19, 20, 21, 26, 50, 52,59]. Virus particles within the host cell becomevisible with TEM only towards the end of thelatent period, thus the total fraction of infectedbacterial cells (FIB) will be much greater thanthe number of visibly infected bacterial cellsviewed under the microscope. Correction fac-tors to convert FVIB to FIB vary and requireestimates of what portion of the latent periodcells are visibly infected [4, 57]. As mentionedearlier, the length of the latent period can varybut is often assumed to be equal to the bacterialgeneration time [36]. Results from Paper IIIindicate that virus-bacteria interactions in polarregions differ significantly from those in tem-perate regions, as the study reveals the highestFVIB (range 8.1% to 66.7%) and the lowestburst sizes (average 4) ever reported in the liter-ature. Long generation times in the polar bacte-rioplankton, in particularly in the Antarctic ul-

tra-oligotrophic lakes where the generationtime can be as high as 2.5 days (Paper III), mayresult in low burst sizes and long latent periods.For instance, results from studies of a marinevirus-host system showed that an increase in thebacterial growth rate (i.e. shorter generationtimes) resulted in a higher burst size and a short-er latent period [27]. Paper III, shows that evenin ultra-oligotrophic environments, where hostdensity is low and often slow growing with ex-tremely long generation times, it is possible tofind an active viral community. Furthermore,high bacterial infection rates in the Arctic andAntarctic indicate that the lytic cycle may pre-vail in polar freshwater environments. Thisstatement is further strengthened by the factthat lysogens were infrequently detected in po-lar freshwaters (Paper II, IV). One may furtherspeculate that a high frequency of visibly infect-ed cells may offer a novel way for the viral pop-ulation to survive when there are only a few vi-ruses released per infected cells. Moreover, highinfection rates means that there is frequent vi-rus-bacteria encounter and for this to occur in

Fig. 2. Photographs of the two study lakes in the Ant-arctic and of cryoconite holes in the Arctic. A:Crooked Lake. B: Lake Druzhby C: Cryoconiteholes on Midre Løvenbreen glacier in the Arctic.


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an environment with low bacterial density thevirus must be either capable of infecting multi-ple bacterial species or bacterial diversity is lowwith only a few virus-host systems.

Impact of viruses on the microbialloop

Viruses are contributors to the element cyclingwithin the microbial loop as they act as catalyststhat accelerate the transformation of particulateorganic matter into dissolved organic matter[41]. Both experimental and theoretical studies[6, 15, 29, 31] have shown that viruses mayhave a significant impact on the nutrient andcarbon cycling in microbial communities.When bacteria or phytoplankton are destroyedby viruses new viral particles are released alongwith dissolved and colloidal organic matter, re-ferred to as lysate products [29]. These lysateproducts contain both labile and refractoryproducts which can potentially be used by theuninfected microbial community. Fischer andVelimirov [13] estimated that viral lysis of bac-terial cells in an eutrophic lake could potentiallyrelease 15.2 µg C l–1 d–1 which corresponded to46% of the bacterial production in the watercolumn. Other studies conducted by Middel-boe et al. [27, 28] indicated that lysate productscould be used by the uninfected bacterial com-munity but required more energy for assimila-tion into bacterial biomass, which was indicatedby the decrease in bacterial growth efficiency (theratio of biomass produced to substrate utilised).

In microbially dominated Antarctic freshwa-ter lakes the major pathway of energy and car-bon flow is through the microbial loop. Virallysis of bacteria can disrupt this flow and shuntorganic carbon back into the dissolved organiccarbon (DOC) pool (Fig. 1), having the net ef-fect of increasing the DOC pool and making itavailable for incorporation into new bacterialbiomass. The two study lakes, Lake Druzhbyand Crooked Lake (Paper IV) are ultra-oligo-trophic with a small DOC pool where virtuallyall the carbon is said to originate from authoch-tonous sources, such as primary production[22]. Efficient recycling of the available DOC

within the lakes would thus be crucial for sus-taining an active bacterial population through-out the season. The results from Paper IVshowed that viral activity contributed signifi-cantly to the recycling of DOC in the studylakes with over 60% of the estimated DOCpool being derived from viral activity in thedark winter months. The findings presented inboth Paper III and Paper IV add to a growingset of evidence that the greatest viral impact oc-curs in low productivity lake ecosystems, rangingfrom temperate to polar environments [3, 31].

Summary of papers I–IV

Objectives

The overall objective of this thesis was to exam-ine virus-bacteria interactions in temperate andpolar freshwaters and what factors may influ-ence the temporal and seasonal dynamics of vi-ruses and their bacterial host in freshwaters. Spe-cifically the thesis addresses the following issues:

� The influence of inorganic and organicnutrients on virus-bacteria interactions intemperate and oligotrophic polar fresh-waters (Paper I, II).

� The role of lysogeny in temperate and ol-igotrophic polar freshwaters (Paper I, II,IV).

� The frequency of visibly infected bacteri-al cells and the average number of virus-like particles per cell in the Arctic andAntarctic freshwaters (Paper III).

� The role of the viral loop in microbiallydominated Antarctic freshwaters and therelevance of viral-induced carbon transferin Antarctic ultra-oligotrophic lakes (Pa-per IV).

Study sites and methods

Fieldwork was carried out in temperate and po-lar freshwater environments. In Paper I, a lakesurvey of 24 lakes in southern Sweden


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(57°10Ń) was conducted to identify any rela-tionships between viruses, bacteria and DOCconcentrations. The lakes were chosen to repre-sent a DOC gradient from clear water to humicwater and the DOC concentrations rangedfrom 3 to 19 mg of C liter–1. Water sampleswere analysed for viral and bacterial abundance,DOC concentrations, Chl a levels and for totalnitrogen and phosphorus. The occurrence oflysogeny was determined in samples from fivelakes using Mitomycin C as an inducing agentof cells containing prophages. In addition, graz-er-free water from Lake Skärlen (with relativelow DOC concentration of 3.3 mg of C liter–1)was used in a short-term (15 days) laboratoryexperiment to look at the effects of differentsources of carbon (i.e. glucose and fulvic acids)and nutrients on virus-bacteria interactions.Microbial parameters, including viral and bac-terial abundance and bacterial production, weremonitored throughout the experiment whereaslysogeny was only analysed on a subset of sam-ples.

In Paper II, fieldwork was carried out nearNy-Ålesund, northwest Svalbard (78°56Ń,11°56É) and the Antarctic (Vestfold Hills,Eastern Antarctica (68°35´S, 78°20É). Waterwas collected from a variety of freshwater envi-ronments, including crycoconite holes, glacierrunoffs, melt pools and freshwater lakes. I con-ducted a field survey of several Arctic freshwaterenvironments to examine the virus-bacteria in-teractions in these systems. Water samples wereanalysed for bacterial and viral abundance, bac-terial production, DOC and dissolved nitrogenconcentrations, chl a levels and occurrence oflysogeny. In addition, I set up several nutrientenrichment experiments with water from boththe Arctic and the Antarctic to elucidate whatfactors (i.e., nutrients and temperature) limitsbacterial growth and how these factors influ-ence virus-bacteria interactions in polar fresh-waters. The changes in bacterial and viral abun-dance and bacterial production were monitoredthroughout the experiments.

In Paper III, I used electron microscopy toestimate what percentage of the Arctic and Ant-arctic freshwater bacteria population was visiblyinfected by viruses and the average number ofvirus-like particles within each bacterial cell. Vi-

rus-infected bacterial cells were investigatedwith a slightly modified method relative to theone previously described by Suttle in 1993 [42].In brief, water samples were centrifuged ontoelectron microscope grids at a relative low cen-trifugation speed (9000 × g) which only al-lowed for bacterial cells to be harvested, henceimproving the estimation of infected bacterialcells as all viruses found were within the bacteri-al cells.

To investigate the importance of viruses andtheir role in carbon cycling in polar freshwa-ters, I conducted a seasonal study in the Ant-arctic on two ultra-oligotrophic lakes (Fig. 2A,B). The two lakes were sampled fortnightly andwater samples were analysed for DOC, inor-ganic nutrients, chl a, bacterial and viral abun-dance and bacterial production whereas occur-rence of lysogeny in each lake was analysed on amonthly basis. Electron microscopy data fromPaper III was used in combination with theseasonal data set from Paper IV to work out thefraction of bacterial mortality caused by virallysis and the contribution of viral-inducedDOC release in Lake Druzhby and CrookedLake.

Results and conclusions

� Parameters that had a positive net effecton bacterial growth and abundance, suchas phosphorus, humic substances andtemperature, did not cause a positive re-sponse in viral abundance. This showsthat viral and bacterial abundances arenot always positively correlated and thatthe presence of a healthy and high densityhost cannot be used as a sole indicator ofviral success in freshwaters.

� Humic substances influenced viral abun-dance and virus to bacteria ratio negative-ly. This demonstrates that virus-bacteriainteractions in humic lakes may be signif-icant different relative to clear water lakes.Factors such as binding to HMW sub-stances, destruction, lower infectivity orlower replication rates are likely scenariostaking place in humic lakes.


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� The incidence of lysogeny was high in thehumic lakes of Småland, which showsthat lysogeny might be an importantmechanism in these aquatic systems. Onthe contrary, lysogeny was uncommon inpolar freshwaters where the lytic cycleseemed to prevail.

� The frequency of visibly infected bacteri-al cells was exceptionally high in both theArctic and Antarctic freshwater environ-ment and the average number of virus-like particles per cell was very low relativeto previous reported data from temperatefreshwaters. It is evident that virus-bacte-ria interactions in microbially dominatedpolar freshwaters differ significantly totemperate freshwaters.

� Viruses were important players in theAntarctic ultra-oligotrophic lakes. A largeproportion of bacterial mortality was dueto viral lysis and viral induced carbontransfer seemed to be of quantitative sig-nificance in both Lake Druzhby andCrooked Lake, particularly during thewinter months.

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Alla organismer i våra sjöar är mottagliga för vi-rusattacker. Virus är de minsta men vanligastebiologiska enheterna i sjöar. En klunk sjövatteninnehåller till exempel miljardtals virus! De ärväldigt små och för att begripa hur små de ärkan man föreställa sig ett knappnålshuvud däröver 20 miljarder virus kan få plats.

Då de är så små behöver man speciell utrust-ning för att kunna se dem och det var inte för-rän i mitten av 50-talet som man fick in denförsta rapporten om bakteriofager (virus somattackerar bakterier) i havet. Den ekologiska be-tydelsen av virus började inte diskuteras förrän islutet av 80-talet, då en norsk forskargrupp an-vände sig av elektronmikroskopi och hittadeöver 10 miljoner virus per ml i havsvatten. Desjälvklara frågor som dök upp var: ”varför finnsde så många virus i våra hav och vad gör deegentligen?” Detta sporrade forskningen inomakvatisk virusekologi och nu 17 år senare vet viganska mycket mer, men långt ifrån tillräckligt,om virusens betydelse i sjöar och hav. Virus kantill exempel infektera två mycket viktiga orga-nismgrupper i sjöar och hav, växtplankton ochbakterier. Bakterier är efter virus de biologiskaenheter som förekommer i högst antal och deär viktiga i sjöar och hav då de omvandlar löstorganiskt kol till partikulärt som sedan blir till-gängligt till organismer högre upp i födoked-jan.

Virus är parasiter som är totalt beroende avsin värd för sin överlevnad och då bakterierfinns i så stort antal är de ett bra byte för virus.Virus reproducerar sig genom att infektera sin

värdcell och sedan tvingar de värdcellen att pro-ducera nya viruspartiklar vilka släpps ut fråncellen när den exploderar (lytisk virusinfek-tion). Men virus kan även använda sig av enannan typ av reproduktion som innebär attvärdcellen hålls vid liv och att virusarvsmassankopieras tillsammans med bakteriearvsmassannär nya bakterier tillverkas (icke-lytisk virusin-fektion). Den icke-lytiska virusinfektionen an-vänds oftast när värdcellen inte lever under opti-mala förhållanden.

Syftet med avhandling har varit att undersö-ka interaktioner mellan virus och bakterier i oli-ka typer av näringsfattiga sötvatten. Jag har un-dersökt bruna skogssjöar i Småland, glaciäreroch vattendrag i Arktis och extremt näringsfat-tiga sjöar i Antarktis.

Bakterier i bruna sjöar kan använda sig avdet organiska materialet, som finns i höga kon-centrationer. Då bakterier trivs i bruna sjöarmed mycket organiskt material (humöst mate-rial) skulle man kunna tänka sig att virusen ock-så skulle gilla att leva där, eftersom det då finnsgott om värdbakterier. Till min förvåning visadesig detta vara osant och höga koncentrationer avorganiskt material påverkade viruspartiklarnanegativt då de stora komplexa molekylerna ver-kade binda eller inaktivera virusen. Detta visaratt fria viruspartiklar i vatten är ganska utsattaoch sårbara och deras överlevnadstid i sjöar på-verkas av hur snabbt de hittar en bra värd menockså av miljön i sjön. Jag konstaterade även atti bruna skogssjöar var den icke-lytiska virusin-fektionen vanlig vilket kan vara ett sätt för viru-

Virusinfekterade vattenPopulärvetenskaplig sammanfattning av avhandlingen

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sen att kompensera för de negativa effekterna avatt leva i en miljö med hög virusförstörelse.

Sjöbakterier lever oftast under väldigt svåraförhållanden och får nöja sig med lite näringoch oftast låga temperaturer. I extrema akvatis-ka miljöer som i Arktis och Antarktis lever mik-roorganismer på gränsen till vad som är möjligtmed konstant låga temperaturer, ljus- och nä-ringshalter. Jag undersökte vad det var som be-gränsade tillväxten av bakterier i olika sötvatteni både Arktis och Antarktis. Det visade sig attbakterierna främst var begränsade av tillgångenpå fosfor medan temperaturen hade en mindrebetydelse för tillväxten. Jag ställde mig alltså frå-gan hur en aktiv viruspopulation kan överlevadå deras värd lever under sådana svåra förhållan-den. Det visade sig att andelen virusinfekteradebakterier i Antarktis och Arktis sötvatten varmycket hög jämfört med vanliga sjöar, men åandra sidan innehöll varje cell bara ett fåtal vi-ruspartiklar. Jag konstaterade att interaktionermellan virus och bakterier i sötvatten från po-larområden skiljer sig signifikant från sötvatteni t ex sjöar i södra Sverige. Det är möjligt attpolarvirus har utvecklat unika strategier för attöverleva i en extrem miljö. Då bara ett fåtal vi-ruspartiklar släpps ut per cell måste tex viruseninfektera flera bakterier för att upprätthålla enkonstant viruspopulation. Dessutom betyderen hög infektionsfrekvens att fria viruspartiklar

måste stöta ihop ofta med bakterier, vilket resul-terar i infektion. Men i en miljö med få bakte-rier tyder det på att virusen antingen kan infek-tera mer än en bakterieart eller att det bara finnsett fåtal bakteriearter. Jag hade förväntat mig atticke-lytiska virusinfektioner skulle vara vanliga ibåde Arktis och Antarktis då bakterier lever un-der extrema förhållanden men detta antagandevisade sig vara fel då lytiska virusinfektioner ver-kade dominera i både Arktis och Antarktis.

Ett annat syfte med min avhandling har varitatt undersöka hur viktiga virus är i Antarktiskasjöar där födokjedjan är kort och domineras avmikroorganismer. En studie jag gjorde i två sjö-ar under ett år i Antarktis visade att virus varväsentliga för näringsåtervinning. Virusaktivite-ten bidrog med mycket näring till de icke infek-terade bakterierna, speciellt under de kalla ochmörka vintermånaderna då det inte sker någonfotosyntes p g a totalt mörker.

Sammanfattningsvis kunde jag konstateraatt interaktioner mellan virus och bakterierskiljer sig mellan olika typer av sötvatten vilketberodde både på värdcellens tillstånd och påkvalitén på vattnet. I vatten med korta födo-kedjor, dvs i Antarktis och Arktis, var virus ex-tremt viktiga då deras aktivitet kunde hjälpa tillmed återvinning av näringsämnen, som finns iextremt låga koncentrationer i Arktis och An-tarktis.

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Acknowledgments

Four years have passed by quickly and I havemore or less managed to travel around theworld and back. During my PhD I have realisedthat with some help from other people anythingis possible and as long as you show determina-tion and stubbornness you can achieve amazingthings. I have been extremely lucky in beingable to visit two of the most fascinating places inthe world; the Arctic and the Antarctic. Thiscould never have been achieved without thesupport from my two main supervisors Wil-helm Granéli and Johanna Laybourn-Parry. Iam grateful to both of you for all your help withmanuscript preparations, financial and scientif-ic support and most of all for believing in meand encouraging me to become an independentscientist. I learnt incredibly much during myyear in the Antarctic, both good and bad thingsabout science and also life. I met a remarkablebunch of people in the Antarctic you were myfamily for a year and all helped me throughhighs and lows and most importantly made itpossible for me to sample my two lakes consist-ently throughout the year. I’d like to mention afew that were my closest friends; Imojen – forbeing a brilliant scientist and friend whomwithout I would have gone insane in the isolat-ed lab, Tb – for excellent support in the field,delivering thai curry to the doorstep and Fridayg + t’s, what more can you ask for?, Benny – forexcellent support in the field and for a greatfriendship, coffee making in the met buildingand endless of hours talking rubbish on bundy,Winkle – for constantly being happy and al-ways having new plans for exiting things to doon an isolated station, Macca – for entertain-ment on the harmonica and for feeding us allwinter, Watto – for playing “waiting for an an-

gel” with Ben Harper on request, Eddie – forbeing understanding of Friday duff duff fromyour neighbour. In addition I also like to thankBob, our trusty station leader in Antarctic 2004for logistical support and encouragement. Inour Antarctic yearbook of 2004 Bob put in aquote by Len Sales that was published in theNew Zealand Antarctic Society Bulletin in1964 which I think summarise very nicely theimpact of Antarctic on a person. I quote,“Those who have spent an Antarctic night bearan indelible mark on their innermost self, a feel-ing which a person finds hard to define and ex-plain to another”.

The Limnology department in Lund is anamazing scientific place and to be part of thisscientific community has been an honour and Ithank you all for making me feel welcomed.Carina and Emma thank you for letting me askstupid questions and for giving me good scien-tific advice. Kelly and Cesar, thank you both forbeing great friends and listeners. Johanna thankyou for your help on anything to do withmaths. Anders N thanks for help with statistics.My special thanks to Alexandre for your con-structive reviews of manuscripts and scientificdiscussions, you have been a great inspiration tome and your constant positive attitude and sci-entific knowledge has helped me throughoutmy entire PhD. Ramiro, Alice and Astrid,thanks for being good friends and always beingup for going out in Lund.

Thanks are also due to Gerry Nash at the Aus-tralian Antarctic Division and Rita Wallen atthe Zoology department at Lund University forhelp with transmission electron microscopy.

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Thanks to all my friends that kept me in touchwith the real world and allowed me to forgetabout viruses from time to time. Ben King forbeing a great friend and support throughout mywhole PhD, I will always remember your fasci-nation for the Antarctic and I hope you will getto go there eventually. Vicky for being an awe-some friend and for all our girlie talks. To all myparty friends out there, without you therewould have been no party and no party makesChristin sad so thanks for keeping me happy! Abig shout goes out to my old party crew in Not-tingham (Dom, Monica, Peter, Toby, Wendi &

Adam) remember the good old days at theAtomic Jam and the Garvey, priceless memories.

To my mother, you have been a constant sup-port in my life and without you I would neverhave become the person I am today. You are thebest!

If I have forgot to acknowledge someone it wasnot my intention to and to be on the safe side Iwish to express thanks to all the people that Ihave ever met in my entire life as you have allinfluenced me in one way or another.

viral loop dynamics in temperate and polar freshwaters...

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