Cyanobacteria are an important and a diverse group of photo synthetic prokaryotes found in marine, freshwater and terrestrial environments. Over the past two decades, research has shown that cyanobacterial community structures are influenced through infection by cyanobacterial viruses (cyanophages). In marine systems cyanophages have been shown to play roles in the mortality of their hosts, as well as in driving the succession and evolution of phenotypes.

Database searches revealed that some species of cyanobacteria, such as Synechocystis sp. PCC 6803 and Nostoc sp. PCC 7120, contain homologues of the Fet3p-Ftrlp high-affinity ferrous-iron uptake system, defined originally in the yeast Saccharomyces cerevisiae. These homologues include the ftr (ferric permease homologue, encoding a protein similar to Ftr 1 p) gene in both Synechocystis and Nostoc, and mco (a1l3942 a multicopper oxidase homologue- encoding a protein somewhat similar to Fet3p) found only in Nostoc where it is close to ftr. RT-PCR analysis indicated that ftr and mco in Nostoc are induced upon iron restriction as is fir in Synechocystis, thus supporting a role for these genes in iron uptake. Phenotypic studies in E. coli JC28 (a strain deficient in iron uptake) revealed that Ftr from Synechocystis is not functional in compensating for lack of iron-uptake capacity in E. coli. However, phenotypic studies revealed that the mco-all3941-bfr-ftr fragment of Nostoc causes a major enhancement of growth under aerobic iron restriction in E. coli JC28. However, the mco gene was not required for this growth promotion effect although inclusion of bfr (encoding the iron-storage protein, bacterioferritin) upstream of fir enhanced the effect of fir, and the inclusion of all3941-bfr upstream of fir raised growth still further. Thus, the Ftr of Nostoc does not require a Mco protein for its activity, but may instead use a bacterioferritin. Ftr-mediated growth enhancement was promoted by low pH and by the reductant ascobate, suggesting ferrous iron is the preferred Ftr substrate. In addition, the Ftr system had a weaker iron- restriction growth benefit anaerobically than that seen aerobically. Bioinformatics analysis indicated that All3942 (Mco) is a Tat exported periplasmic protein containing two cupredoxin domains with the blue copper and tri-nuclear inter-domain copper binding sites characteristic of multi copper oxidases. Overproduced and purified MalE-Mco was able to bind 5.8, 1.2 and 3.1 atoms of Cu, Zn and Fe, respectively, per subunit which is consistent with its Mco designation. UV-visible and EPR spectroscopy revealed the likely presence of Tl, T2, and T3 copper centres. Upon addition of ferrous iron to the copper-containing Mal-Mco, Tl signals were lost, presumably due to reduction of the centre, suggesting ferroxidase activity. Copper- soaked MalE-Mco showed phenoloxidase activity, as found for other Mco proteins, as well as limited ferroxidase activity indicating that it may, like Fet3p, oxidise ferrous iron in vivo. However, no requirement for the All3942/Mco protein in Ftr-mediated iron uptake was found in E. coli JC28 which suggests that the Nostoc Ftr protein may transport ferrous iron using a mechanism distinct from that of the yeast Ftr1p.

Marine Synechococcus spp. are ubiquitous in the marine environment and are known to contribute significantly to primary production. The ecological success of these organisms may be due to the evolution of various genetic lineages physiologically adapted to specific ocean niches. This study focused on one specific physiological attribute, the utilisation and genetic regulation of nitrogen (N) nutrition. Growth data showed no discernable difference in N source utilisation capabilities between marine Synechococcus lineages, but rather a broad utilisation of a range of N sources including ammonium, nitrate, nitrite, urea, and some amino acids. The specific inability of individual strains to utilise urea or nitrate correlated with a lack of genetic capacity to acquire and metabolise these substrates. Such gene loss is likely a function then, of very local environmental conditions 'acting on' a specific strain, rather than a much broader environmental influence on the lineage as a whole.

The cyanobacteria are a unique and important phylum of bacteria. They are photosynthetic, thought to be responsible for up to half of all atmospheric carbon dioxide fixation, and are capable of fixation of atmospheric nitrogen, thus contributing to the nitrogen cycle. Many species readily form symbiotic relationships with a variety of other organisms, from marine animals, through mosses and lichens, to large land plants. The precursor to the modern chloroplast is thought to have been a cyanobacterium. They have played, and continue to play, a huge part in the formation of the environment of the Earth. Using a wide array of monoclonal antibodies to a variety of plant cell wall components, along with a specific reagent (~-glucosyl Yariv reagent), this study has shown that free- living cyanobacteria have components expressed on the outer surfaces of cells with characteristics very much like those of arabinogalactan-proteins (AGPs). AGPs are a class of plant cell wall glycoproteins with a large number of proposed actions in plant growth, development and signalling (including symbiotic signalling). Bioinformatic analysis of the model symbiotic cyanobacterial proteome of Nostoc punctiforme 29133 has suggested that it contains several proteins which share critical characteristics with known plant AGPs. Wider bioinformatic analysis hints at the presence of AGP-like proteins in a wide variety of ; other cyanobacterial species. The study concludes that free-living cyanobacteria produce AGPs and proposes a role for these in extracellular signalling. The study also uses an array of plant cell wall-specific monoclonal antibodies to investigate the potential interactions between Nostoc, a symbiotic cyanobacterial genus, and two of their symbionts, the angiosperm Gunnera and the liverwort B/asia. The presence of AGP- specific epitopes, along with pectins and hemicelluloses at the symbiotic surface is demonstrated. Other than AGPs, analysis suggests that symbiotic cyanobacteria lack the genetic capability to produce these components, and hence it is hvpotheslsed that the plant produces these at the symbiotic surface. Potential roles for these are discussed, including roles in symbiotic signalling and helping the cyanobiont evade plant immune responses.

No description available.

Les picocyanobactéries marines Synechococcus sont les seconds organismes photosynthétiques les plus abondants sur Terre. Elles présentent une grande diversité pigmentaire du fait de différences dans la composition de leur antenne collectrice de lumière (phycobilisome), ce qui leur permet d'utiliser efficacement une grande partie du spectre lumineux. Cependant, l'évolution, l'écologie et les bases moléculaires de cette diversité restent mal comprises. La comparaison d'une région génomique impliquée dans la synthèse des phycobilisomes de 54 souches et de populations naturelles m'a permis de proposer un scénario pour l'évolution des différents types pigmentaires et de montrer que cette diversité pigmentaire précède la diversification des Synechococcus marins. J'ai ensuite développé une procédure bioinformatique pour quantifier l'abondance relative de tous les types pigmentaires connus à partir de métagénomes. Appliquée aux données de Tara Oceans, cela m'a permis de décrire leur répartition à l'échelle mondiale, révélant que l'acclimatation chromatique de type IV, qui permet aux cellules de modifier leur spectre d'absorption en fonction de la couleur de la lumière, domine les populations naturelles de Synechococcus, et que des mutants naturels de l'acclimatation chromatique prévalent dans les étendues oligotrophes de l'océan Pacifique sud. Enfin, la caractérisation génétique de deux membres d'une famille d'enzymes liant les pigments à la phycoérythrine II, constituant majeur des phycobilisomes, a apporté de nouvelles perspectives sur les bases moléculaires de l'acclimatation chromatique et révélé l'importance des variations alléliques dans la diversité des types pigmentaires. / Marine Synechococcus are the second most abundant photosynthetic organisms on the planet. These picocyanobacteria present very diverse pigmentations due to differences in the composition of their light-harvesting antenna (phycobilisome), allowing them to efficiently exploit a wide range of spectral niches. Yet, the evolution, ecology and molecular bases of the different Synechococcus pigment types are not well understood. By comparing the genomic regions involved in the synthesis of phycobilisome rods from 54 sequenced isolates spanning all cultured pigment types and from natural Synechococcus populations, I proposed a scenario for the evolution of the different pigment types and showed that the pigment diversity of marine Synechococcus predates the diversification of this genus. Then, I developed a bioinformatic pipeline for reliably quantifying all known Synechococcus pigment types from metagenomes. Applying it to the Tara Oceans dataset allowed me to describe for the first time their distribution in the global ocean and revealed that type IV chromatic acclimation, a process by which cells can match their absorption properties to the ambient light colour, is widespread and constitutes the dominant pigmentation in Synechococcus populations. It also showed that natural chromatic acclimation mutants prevail in wide oligotrophic areas of the southern Pacific Ocean. Finally, I genetically characterized two members of an enzyme family binding chromophores to phycoerythrin-II, a major component of phycobilisomes. This provided new insights into the molecular bases of chromatic acclimation and revealed the importance of allelic variation for the diversity of pigment types.