Nucleic acid technology has assumed an essential role invarious areas ofin vitrodiagnostics ranging from infectious diseasediagnosis to human genetics. An important requirement of suchmolecular methods is that they achieve high sensitivity andspecificity with a fast turnaround time in a cost-effectivemanner. To this end, in this thesis we have focused on thedevelopment of sensitive nucleic acid strategies thatfacilitate automation and high-throughput analysis. The success of nucleic acid diagnostics in the clinicalsetting depends heavily on the method used for purification ofthe nucleic acid target from biological samples. Here we havefocused on developing strategies for hybridisation capture ofsuch templates. Using biosensor technology we observed that thehybridisation efficiency could be improved using contiguousoligonucleotide probes which acted co-operatively. Byimmobilising one of the probes and annealing the second probein solution, we achieved a marked increase in target capturedue to a base stacking effect between nicked oligonucleotidesand/or due to the opening up of secondary structure. Suchco-operatively interacting modular probes were then combinedwith bio-magnetic bead technology to develop a capture systemfor the extraction of hepatitis C RNA from serum. Viral capturewith such co-operatively interacting probes extracted 2-foldmore target as capture with only a single probe achieving asimilar sensitivity to the conventional extraction protocol. Ananalogous strategy was designed to enrich for sequencingproducts prior to gel electrophoresis removing sequencingreagents and template DNA which interfere with the separationand detection of sequencing ladders, especially in the case ofcapillary gel electrophoresis. This protocol facilitates highthroughput clean-up of cycle sequencing reactions resulting inaccurate sequence data at a low cost, which is a pre-requisitefor large-scale genome sequencing products. Currently, a large effort is directed towards differentialsequencing to identify mutations or polymorphisms both in theclinical laboratory and in medical genetics. Inexpensive, highthroughput methods are therefore required to rapidly screen atarget nucleic acid for sequence based changes. In the clinicalsetting, sequence analysis of human immunodeficiency virus(HIV-1) is used to determine the presence of drug resistancemutations. Here we describe a bioluminometric pyrosequencingapproach to rapidly screen for the presence of drug resistancemutations in the protease gene of HIV-1. This sequencingstrategy can analyse the protease gene of HIV-1 from eightpatients in less than an hour and such non-gel based approachesshould be useful in the future in a clinical setting for rapid,robust mutation detection. Microarray technology facilitates large-scalemutation/polymorphism detection and here we developed amicroarray based single nucleotide polymorphism (SNP)genotyping strategy based on apyrase mediated allele specificextension (AMASE). AMASE exploits the fact that mismatchedprimers exhibit slower reaction kinetics than perfectly matchedprimers by including a nucleotide degrading enzyme (apyrase)which results in degradation of the nucleotides before themismatched primer can be extended. We have successfully typed200 genotypes (14% were incorrect without apyrase) by AMASEwhich cluster into three distinct groups representing the threepossible genotypes. In the future, AMASE on DNA microarraysshould facilitate association studies where an accuracy&gt;99%is required. <b>Keywords:</b>nucleic acid capture, modular probes,biosensor, bio-magnetic separation, hepatitis C, sequencing,pyrosequencing, mutation detection, HIV-1, drug resistance,SNP, allele-specific extension, apyrase, genotyping.

The ability to determine nucleic acid sequences is one ofthe most important platforms for the detailed study ofbiological systems. Pyrosequencing technology is a relativelynovel DNA sequencing technique with multifaceted uniquecharacteristics, adjustable to different strategies, formatsand instrumentations. The aims of this thesis were to improvethe chemistry of the Pyrosequencing technique for increasedread-length, enhance the general sequence quality and improvethe sequencing performance for challenging templates. Improvedchemistry would enable Pyrosequencing technique to be used fornumerous applications with inherent advantages in accuracy,flexibility and parallel processing. Pyrosequencing technology, at its advent, was restricted tosequencing short stretches of DNA. The major limiting factorwas presence of an isomer of dATPaS, a substitute for thenatural dATP, which inhibited enzyme activity in thePyrosequencing chemistry. By removing this non-functionalnucleotide, we were able to achieve DNA read-lengths of up toone hundred bases, which has been a substantial accomplishmentfor performance of different applications. Furthermore, the useof a new polymerase, called Sequenase, has enabled sequencingof homopolymeric T-regions, which are challenging for thetraditional Klenow polymerase. Sequenase has markedly madepossible sequencing of such templates with synchronizedextension. The improved read-length and chemistry has enabledadditional applications, which were not possible previously.DNA sequencing is the gold standard method for microbial andvial typing. We have utilized Pyrosequencing technology foraccurate typing ofhuman papillomaviruses, and bacterial andfungal identification with promising results. Furthermore, DNA sequencing technologies are not capable oftyping of a sample harboring a multitude of species/types orunspecific amplification products. We have addressed theproblem of multiple infections/variants present in a clinicalsample by a new versatile method. The multiple sequencingprimer method is suited for detection and typing of samplesharboring different clinically important types/species(multiple infections) and unspecific amplifications, whicheliminates the need for nested PCR, stringent PCR conditionsand cloning. Furthermore, the method has proved to be usefulfor samples containing subdominant types/species, and sampleswith low PCR yield, which avoids reperforming unsuccessfulPCRs. We also introduce the sequence pattern recognition whenthere is a plurality of genotypes in the sample, whichfacilitates typing of more than one target DNA in the sample.Moreover, target specific sequencing primers could be easilytailored and adapted according to the desired applications orclinical settings based on regional prevalence ofmicroorganisms and viruses. Pyrosequencing technology has also been used forclone-checking by using preprogrammed nucleotide additionorder, EST sequencing and SNP analysis, yielding accurate andreliable results. <b>Keywords:</b>apyrase, bacterial identification, dATPaS, ESTsequencing, fungal identification, human papillomavirus (HPV),microbial and viral typing, multiple sequencing primer method,Pyrosequencing technology, Sequenase, single-strandedDNA-binding protein (SSB), SNP analysis

Genetic profiling is commonly performed on the autosomes using multiple DNA markers. Although routine forensic DNA analysis is robust and based on reliable technologies, samples with degraded or limited amounts of DNA often fail. In these cases, the analysis of mitochondrial DNA (mtDNA) can be very valuable due to the high copy number per cell. This thesis describes evaluation and modifications of existing technologies that are useful in forensic DNA typing, mainly focusing on mtDNA. DNA quantities isolated from common evidence materials such as hairs, fingerprints and accessories were estimated using a real-time quantification assay. Knowledge of quantitative differences between materials can guide forensic scientists to perform the best analysis (Paper I). The current mtDNA analysis is based on hypervariable region (HVI/HVII) sequencing, which is the most rigorous and time-consuming forensic DNA analysis. Therefore, we evaluated the possibility to exclude individuals by screening for non-matching samples using the rapid and easy mtDNA Linear Array Assay (Paper II). The major disadvantage using mtDNA is the lower discrimination power compared to multiple nuclear DNA markers. In contrast to the nuclear genome, due to the uniparental (maternal) mode of inheritance, no individual has unique mtDNA. We investigated the possibility of increasing the discrimination power by using pyrosequencing technology to analyse parts of the coding region in addition to HVI/HVII (Paper III). Furthermore, the addition of coding mtDNA information was evaluated by comparing several recently published mtDNA coding region assays (Paper IV). Mixtures of DNA are common in forensic genetics due to contribution of DNA from several individuals, contamination or heteroplasmy. To resolve mixtures we have developed a pyrosequencing-based assay for the accurate quantification of the mtDNA mixture components (Paper V). In conclusion, this thesis describes several assays that are valuable in forensic genetics for DNA quantification, improved mtDNA analysis, and mtDNA mixture interpretation.

Pancreatic endocrine tumors (PETs) may cause typical syndromes of hormone excess, or appear clinically non-functioning without hormonal symptoms. PETs occur sporadically, in association with the multiple endocrine neoplasia type 1 (MEN1) syndrome, or rarely the von Hippel-Lindau syndrome. Molecular genetic investigations may reveal pathways important for tumor development, and be of clinical use. The aim of this thesis was to investigate regulation of different genes involved in cell proliferation, and relate findings to signs of malignancy in PETs. The MEN1 gene on chromosome 11q13 was mutated in three out of eleven sporadic malignant PETs. Two nonsense mutations, causing truncation of the protein, and one missense mutation were found. Relation of allelic loss at 11q13 and 3p25 to malignant behavior was observed in sporadic PETs. Allelic loss at 18q21 was found in a subset of sporadic and MEN1-associated PETs, and mutation analysis of Smad4 excluded a tumor suppressor gene function. In PETs with allelic loss on chromosome 3p25, mutation analysis of WNT7A and HDAC11 excluded function as tumor suppressor genes. Menin, encoded by the MEN1 gene, was reported to regulate expression of the cyclin-dependent kinase inhibitors CDKN2C/p18, CDKN1B/p27, and CDKN2B/p15 in mouse pancreatic islet tumor models. Here, the mRNA expression of these genes was not related to MEN1 gene mutations in human PETs. Cyclin-dependent kinase 4 (CDK4) and the protooncogene c-Myc were found to be overexpressed regardless of MEN1 gene mutational status of the PETs. The CDK4 gene was neither amplified nor mutated. Targeting of CDK4 may present an alternative to traditional chemotherapy of PETs in the future.

The meals from many oilseed crops contain biocidal chemicals that are known to inhibit the growth and activity of several soil pathogens, though little is known concerning impacts on whole soil microbial communities. We investigated the effect of oilseed meals (SMs) from both brassicaceous plants, including mustard and camelina, as well as non-brassicaceous plants, including jatropha and flax, on P. omnivora (the casual agent of cotton root rot) in Branyon clay soil (at 1 and 5% application rates). We also investigated the effect of SMs from camelina, jatropha, flax, and wheat straw on microbial communities in Weswood loam soil. We also used four types of isothiocyanates (ITCs) including allyl, butyl, phenyl, and benzyl ITC to test their effects on P. omnivora growth on potato dextrose agar (PDA), as well as on soil microbial communities in a microcosm study. Community qPCR assays were used to evaluate relative abundances of soil microbial populations. Soil microbial community composition was determined through tag-pyrosequencing using 454 GS FLX titanium technology, targeting ITS and 16S rRNA gene regions for fungal and bacterial communities, respectively. The results showed that all tested brassicaceous and jatropha SMs were able to inhibit P. omnivora sclerotial germination and hyphal growth, with mustard SM being the most effective. Flax didn't show any inhibitory effects on sclerotial germination. All tested ITCs inhibited P. omnivora OKAlf8 hyphal growth, and the level of inhibition varied with concentration and ITC type. Total soil fungal populations were reduced by ITC addition, and microbial community compositions were changed following SM and ITC application. These changes varied according to the type of SM or ITC added. Our results indicated that SMs of several brassicaceous species as well as jatropha may have potential for reducing cotton root rot as well as some other pathogens. Different SMs releasing varied ITCs may result in differential impacts on soil microorganisms including some pathogens.

Pathogenic organisms are transmitted to the host organism through all possible connected pathways, and cause a myriad of diseases states. Commonly occurring curable infectious diseases still impose the greatest health impacts on a worldwide perspective. The Bill &amp; Melinda Gates Foundation partnered with RAND Corporation to form the Global Health Diagnostics Forum, with the goal of establishing and interpreting mathematical models for what effects a newly introduced point-of-care pathogen diagnostic would have in developing countries. The results were astonishing, with potentially millions of lives to be saved on an annual basis. Golden standard for diagnostics of pathogenic bacteria has long been cultureable medias. Environmental biologists have estimated that less than 1% of all bacteria are cultureable. Genomic-based approaches offer the potential to identify all microbes from all the biological kingdoms. Nucleic acid based pathogen diagnostics has evolved significantly over the past decades. Novel technologies offer increased potential in sensitivity, specificity, decreased costs and parallel sample management. However, most methods are confined to core laboratory facilities. To construct an ultimate nucleic acid based diagnostic for use in areas of need, potential frontline techniques need to be identified and combined. The research focus of this doctoral thesis work has been to develop and apply nucleic acid based methods for pathogen diagnostics. Methods and assays were applied to the two distinct systems i) screening for antibiotic resistance mutations in the bacterial pathogen Neisseria gonorrhoeae, and ii) genotype determination of the cancer causative Human Papillomavirus (HPV). The first part of the study included development of rapid, direct and multiplex Pyrosequencing nucleic acid screenings. With improved methodology in the sample preparation process, we could detect an existence of multiple co-infecting HPV genotypes at greater sensitivities than previously described, when using the same type of methodology. The second part of the study focused on multiplex nucleic acid amplification strategies using Molecular Inversion Probes with end-step Pyrosequencing screening. The PathogenMip assay presents a complete detection schematic for virtually any known pathogenic organism. We also introduce the novel Connector Inversion Probe, a padlock probe capable of complete gap-fill reactions for multiplex nucleic acid amplifications. / Patogena organismer smittas till värd organismen genom alla möjliga kontaktnätverk och skapar en mångfald olika sjukdomstillstånd. Dock är det fortfarande vanligt förekommande behandlingsbara infektiösa sjukdomar som orsakar den största hälsoförlusten, sett från ett globalt perspektiv. Bill och Melinda Gates Stiftelsen samarbetade med RAND kooperation för att forma “The Global Health Diagnostics Forum”. Deras mål var att etablera och analysera matematiska modeller för vilka effekter en ny diagnostisk metod utrustat för fältarbete skulle ha i utvecklingsländer. Resultaten var häpnadsveckande, med potentiellt miljoner av liv som skulle kunna räddas på en årlig basis. Den etablerade standarden för diagnostik av patogena bakterier har länge varit kultiveringsmedia baserad. Miljö specialiserade biologer har estimerat att mindre än 1 % av alla bakterie arter går att kultivera. Dock erbjuder genetiska analyser potentialen att kunna identifiera alla mikrober från alla de biologiska rikena. Nukleinsyrebaserade diagnostiska metoder har märkbart förbättrats över de senaste årtionden. Nya tekniker erbjuder utökad sensitivitet, selektivitet, sänkta kostnader och parallella analyser av patient prover. Dock är de flesta metoderna begränsade till standardiserade laboratoriemiljöer. För att konstruera en väl fungerande diagnostisk fältutrustning för användning i problem områden, behöver världsledande tekniker identifieras och kombineras. Fokuseringsområdet för denna doktorsavhandling har varit att utveckla och utföra nukleinsyrebaserade metoder för patogen diagnostik. Metoder och experimentella utförande applicerades på två distinkta system i) sökning av antibiotika resistens relaterade mutationer i den patogena bakterien Neisseria gonorrhoeae och ii) genotypning av det cancer orsakande Humana Papillomaviruset (HPV). Den första delen av studien inriktade sig mot utveckling av snabba, direkta och multiplexa Pyrosekvenserings baserade nukleinsyreanalyser. Med förbättrad provprepareringsmetodologi kunde vi detektera multipla HPV infektioner med högre sensitivitet än vad tidigare beskrivits med liknande metodologi. Den andra delen av studien fokuserades på multiplexa nukleinsyre amplifikationer med “Molecular Inversion Probe” tekniken med sista steg Pyrosekvenserings analys. “PathogenMip assay” erbjuder ett komplett detektionsprotokoll för alla kända patogena organismer. Vi introducerar även den nya “Connector Inversion Probe”, en “Padlock Probe” kapabel att genomföra kompletta gap fyllningar för multiplex nukleinsyre amplifiering. / QC 20100624

<p>Nucleic acid technology has assumed an essential role invarious areas of<i>in vitro</i>diagnostics ranging from infectious diseasediagnosis to human genetics. An important requirement of suchmolecular methods is that they achieve high sensitivity andspecificity with a fast turnaround time in a cost-effectivemanner. To this end, in this thesis we have focused on thedevelopment of sensitive nucleic acid strategies thatfacilitate automation and high-throughput analysis.</p><p>The success of nucleic acid diagnostics in the clinicalsetting depends heavily on the method used for purification ofthe nucleic acid target from biological samples. Here we havefocused on developing strategies for hybridisation capture ofsuch templates. Using biosensor technology we observed that thehybridisation efficiency could be improved using contiguousoligonucleotide probes which acted co-operatively. Byimmobilising one of the probes and annealing the second probein solution, we achieved a marked increase in target capturedue to a base stacking effect between nicked oligonucleotidesand/or due to the opening up of secondary structure. Suchco-operatively interacting modular probes were then combinedwith bio-magnetic bead technology to develop a capture systemfor the extraction of hepatitis C RNA from serum. Viral capturewith such co-operatively interacting probes extracted 2-foldmore target as capture with only a single probe achieving asimilar sensitivity to the conventional extraction protocol. Ananalogous strategy was designed to enrich for sequencingproducts prior to gel electrophoresis removing sequencingreagents and template DNA which interfere with the separationand detection of sequencing ladders, especially in the case ofcapillary gel electrophoresis. This protocol facilitates highthroughput clean-up of cycle sequencing reactions resulting inaccurate sequence data at a low cost, which is a pre-requisitefor large-scale genome sequencing products.</p><p>Currently, a large effort is directed towards differentialsequencing to identify mutations or polymorphisms both in theclinical laboratory and in medical genetics. Inexpensive, highthroughput methods are therefore required to rapidly screen atarget nucleic acid for sequence based changes. In the clinicalsetting, sequence analysis of human immunodeficiency virus(HIV-1) is used to determine the presence of drug resistancemutations. Here we describe a bioluminometric pyrosequencingapproach to rapidly screen for the presence of drug resistancemutations in the protease gene of HIV-1. This sequencingstrategy can analyse the protease gene of HIV-1 from eightpatients in less than an hour and such non-gel based approachesshould be useful in the future in a clinical setting for rapid,robust mutation detection.</p><p>Microarray technology facilitates large-scalemutation/polymorphism detection and here we developed amicroarray based single nucleotide polymorphism (SNP)genotyping strategy based on apyrase mediated allele specificextension (AMASE). AMASE exploits the fact that mismatchedprimers exhibit slower reaction kinetics than perfectly matchedprimers by including a nucleotide degrading enzyme (apyrase)which results in degradation of the nucleotides before themismatched primer can be extended. We have successfully typed200 genotypes (14% were incorrect without apyrase) by AMASEwhich cluster into three distinct groups representing the threepossible genotypes. In the future, AMASE on DNA microarraysshould facilitate association studies where an accuracy>99%is required.</p><p><b>Keywords:</b>nucleic acid capture, modular probes,biosensor, bio-magnetic separation, hepatitis C, sequencing,pyrosequencing, mutation detection, HIV-1, drug resistance,SNP, allele-specific extension, apyrase, genotyping.</p>

<p>The ability to determine nucleic acid sequences is one ofthe most important platforms for the detailed study ofbiological systems. Pyrosequencing technology is a relativelynovel DNA sequencing technique with multifaceted uniquecharacteristics, adjustable to different strategies, formatsand instrumentations. The aims of this thesis were to improvethe chemistry of the Pyrosequencing technique for increasedread-length, enhance the general sequence quality and improvethe sequencing performance for challenging templates. Improvedchemistry would enable Pyrosequencing technique to be used fornumerous applications with inherent advantages in accuracy,flexibility and parallel processing.</p><p>Pyrosequencing technology, at its advent, was restricted tosequencing short stretches of DNA. The major limiting factorwas presence of an isomer of dATPaS, a substitute for thenatural dATP, which inhibited enzyme activity in thePyrosequencing chemistry. By removing this non-functionalnucleotide, we were able to achieve DNA read-lengths of up toone hundred bases, which has been a substantial accomplishmentfor performance of different applications. Furthermore, the useof a new polymerase, called Sequenase, has enabled sequencingof homopolymeric T-regions, which are challenging for thetraditional Klenow polymerase. Sequenase has markedly madepossible sequencing of such templates with synchronizedextension.</p><p>The improved read-length and chemistry has enabledadditional applications, which were not possible previously.DNA sequencing is the gold standard method for microbial andvial typing. We have utilized Pyrosequencing technology foraccurate typing ofhuman papillomaviruses, and bacterial andfungal identification with promising results.</p><p>Furthermore, DNA sequencing technologies are not capable oftyping of a sample harboring a multitude of species/types orunspecific amplification products. We have addressed theproblem of multiple infections/variants present in a clinicalsample by a new versatile method. The multiple sequencingprimer method is suited for detection and typing of samplesharboring different clinically important types/species(multiple infections) and unspecific amplifications, whicheliminates the need for nested PCR, stringent PCR conditionsand cloning. Furthermore, the method has proved to be usefulfor samples containing subdominant types/species, and sampleswith low PCR yield, which avoids reperforming unsuccessfulPCRs. We also introduce the sequence pattern recognition whenthere is a plurality of genotypes in the sample, whichfacilitates typing of more than one target DNA in the sample.Moreover, target specific sequencing primers could be easilytailored and adapted according to the desired applications orclinical settings based on regional prevalence ofmicroorganisms and viruses.</p><p>Pyrosequencing technology has also been used forclone-checking by using preprogrammed nucleotide additionorder, EST sequencing and SNP analysis, yielding accurate andreliable results.</p><p><b>Keywords:</b>apyrase, bacterial identification, dATPaS, ESTsequencing, fungal identification, human papillomavirus (HPV),microbial and viral typing, multiple sequencing primer method,Pyrosequencing technology, Sequenase, single-strandedDNA-binding protein (SSB), SNP analysis</p>

<p>Pyrosequencing is a new DNA sequencing method relying on thesequencing-by-synthesis principle and bioluminometric detectionof nucleotide incorporation events. The objective of thisthesis was improvement of the Pyrosequencing method byincreasing the thermal stability of firefly luciferase, and byintroducing an alternative DNA polymerase and a new nucleotideanalog. Furthermore, the development of a new bioluminescentassay is described for the detection of inorganicpyrophosphatase activity.</p><p>The wild-type North American firefly<i>(Photinus pyralis)</i>luciferase is a heat-sensitiveenzyme, the catalytic activity of which is rapidly lost attemperatures over 30°C. Two strategies for increasing thethermostability of the enzyme are presented and discussed. Inthe first strategy, the solution thermodynamics of the systemis affected by osmolytes in such a way that heat-mediatedinactivation of the enzyme is prevented. In the secondstrategy, the enzyme is thermostabilized by mutagenesis. Bothstabilizing strategies can be utilized to allow bioluminometricassays to be performed at higher temperatures. For instance,both DNA polymerase and ATP sulfurylase activity could beanalyzed at 37°C.</p><p>The osmolyte strategy was successfully employed forincreasing the reaction temperature for the Pyrosequencingmethod. By increasing the reaction temperature to 37°Cunspecific signals from primer-dimers and 3’-end loopswere reduced. Furthermore, sequencing of a challenging templateat 37°C, which previously yielded poor, non-interpretablesequence signals at lower temperatures was now possible.</p><p>Introduction of a new adenosine nucleotide analog,7-deaza-2’-deoxyadenosine-5’-triphosphate (c⁷dATP) reduced the inhibitory effect on apyraseobserved with the currently used analog,2’-deoxyadenosine-5’-O-(1-thiotriphosphate)(dATPαS).</p><p>Sequencing of homopolymeric T-regions has previously beendifficult with the exonuclease-deficient form of the DNApolymerase I large (Klenow) fragment. By using the DNApolymerase from bacteriophage T7, known as Sequenase, templateswith homopolymeric T-regions were successfully sequenced.Furthermore, it was found that the strand displacement activityfor both polymerases was strongly assisted if the displacedstrand had a 5’-overhang. In contrast, the stranddisplacement activity for both polymerases was inhibitedwithout an overhang, resulting in reduced sequencingperformance in double stranded regions.</p><p>A firefly bioluminescent assay for the real-time detectionof inorganic pyrophosphatase in the hydrolytic direction wasalso developed. The assay is versatile and has a linearresponse in the range between 8 and 500 mU.</p><p><b>Key words:</b>bioluminescence, osmolytes, glycine betaine,thermostability, firefly luciferase, inorganic pyrophosphatase,inorganic pyrophosphate, Pyrosequencing technology, secondaryDNA-structures, Sequenase, Klenow-polymerase, reaction rates,temperature, c⁷dATP, dATPαS.</p>

In aquatic environments the availability of nitrogen (N) generally limits primary production. N2-fixing prokaryotes (diazotrophs) can convert N2 gas into ammonium and provide significant input of N into the oceans. Cyanobacteria are thought to be the main N2-fixers but diazotrophs also include a wide range of heterotrophic bacteria. However, their activity and regulation in the water column is largely unknown. In this thesis the distribution, diversity, abundance, and activity of marine and estuarine heterotrophic diazotrophs was investigated. With molecular methods targeting the nifH gene, encoding the nitrogenase enzyme for N2 fixation, it was shown that diverse nifH genes affiliating with heterotrophic bacteria were ubiquitous in surface waters from ten marine locations world-wide and the estuarine Baltic Sea. Through enrichment cultures of Baltic Sea surface water in anaerobic N-free medium, heterotrophic N2 fixation was induced showing that there was a functional N2-fixing community present and isolates of heterotrophic diazotrophs were obtained. In Sargasso Sea surface waters, transcripts of nifH related to heterotrophic bacteria were detected indicating heterotrophic N2-fixing activity. Nitrogenase expression is thought to be highly regulated by the availability of inorganic N and the presence of oxygen. Low oxygen zones within the water column can be found in association with plankton. The presence of diazotrophs as symbionts of heterotrophic dinoflagellates was investigated and nifH genes related to heterotrophic diazotrophs rather than the cyanobacterial symbionts were found, suggesting that a symbiotic co-existence prevailed. Oxic-anoxic interfaces could also be potential sites for heterotrophic N2 fixation. The Baltic Sea contains large areas of anoxic bottom water. At the chemocline and in anoxic deep water heterotrophic diazotrophs were diverse, abundant and active. These findings extend the currently known regime of N2 fixation to also include ammonium-rich anaerobic waters. The results of this thesis suggest that heterotrophic diazotrophs are diverse and widely distributed in marine and estuarine waters and that they can also be active. However, limits in the knowledge on their physiology and factors which regulate their N2 fixation activity currently prevent an evaluation of their importance in the global marine N budget.