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Cyanobacteria are the only prokaryotes with the ability to conduct oxygenic photosynthesis,
therefore having major influence on the evolution of life on earth. Their diverse morphology
was traditionally the basis for taxonomy and classification. For example, the genus
Chroococcidiopsis has been classified within the order Pleurocapsales, based on a unique
reproduction modus by baeocytes. Recent phylogenetic results suggested a closer
relationship of this genus to the order Nostocales. However, these studies were based
mostly on the highly conserved 16S rRNA and a small selection of Chroococcidiopsis
strains. One aim of this present thesis was to investigate the evolutionary relationships of
the genus Chroococcidiopsis, the Pleurocapsales and remaining cyanobacteria using
16S rRNA, rpoC1 and gyrB gene. Including the single gene, as the multigene analyses of
97 strains clearly showed a separation of the genus Chroococcidiopsis from the
Pleurocapsales. Furthermore, a sister relationship between the genus Chroococcidiopsis
and the order Nostocales was confirmed. Consequently, the monogeneric family
Chroococcidiopsidaceae Geitler ex. Büdel, Donner & Kauff familia nova is justified. The
phylogenetic analyses also revealed the polyphyly of the remaining Pleurocapsales, due to
the fact that the strain Pleurocapsa PCC 7327 was always separated from other strains.
This is supported by differences in their metabolism, ecology and physiology.
A second aim of this study was to investigate the thylakoid arrangement of
Chroococcidiopsis and a selection of cyanobacterial strains. The investigation of 13 strains
with Low Temperature Scanning Electron Microscopy revealed two unknown thylakoidal
arrangements within Chroococcidiopsis (parietal and stacked). This result revised the
knowledge of the thylakoid arrangement in this genus. Previously, only a coiled
arrangement was known for three strains. Based on the data of 66 strains, the feature
thylakoid arrangement was tested as a potential feature for morphological identification of
cyanobacteria. The results showed a strong relationship between the group assignment of
cyanobacteria and their thylakoid arrangements. Hence, it is in general possible to
conclude from this certain phenotypic character the affiliation to a particular family, order
or genus.
The third aim of this study was to investigate biogeographical patterns of the worldwide
distributed genus Chroococcidiopsis. The phylogenetic analysis suggested that the genus do not have biogeographical patterns, which is in contrast with a recent study on hypolithic
living Chroococcidiopsis strains and the majority of phylogeographic analysis of
microorganisms. Further analysis showed no separation of different life-strategies within
the genus. These results could be related to the genetic markers utilized, which may not
contain biogeographical information. Hence the present study can neither exclude nor
prove the possibility of biogeographic and life-strategy patterns in the genus
Chroococcidiopsis.
Future research should be focused on finding appropriate genetic markers investigate of
evolutionary relationships and biogeographical patterns within Chroococcidiopsis.
About 2.4 Ga ago the Great Oxygenation Event (GOE) started the permanent oxygenation of Earth’s anoxic atmosphere. The oxygen was most likely produced by oxygenic photosynthesis in Cyanobacteria. However, hints for local occurrences of Cyanobacterial life and free oxygen exists for at least 300 Ma prior to the GOE. Different hypotheses were proposed to explain this delay between the evolution of oxygen producers and the start of the GOE. For this thesis, theoretic predictions made by two of those hypotheses were tested in laboratory experiments using ancestral, basal clade Cyanobacteria grown under simulated Archean like conditions.
Cyanobacteria might have evolved in freshwater environments and subsequently had to adapt to the higher salinity of the Archean ocean. In turn, this would have delayed their global expansion required for the GOE. Experiments with the most primitive freshwater Cyanobacterium Gloeobacter violaceus PCC 7421, showed its ability to tolerate and slowly grow in brackish water, thereby providing a route for the evolution of open ocean dwelling, salt tolerant species. The Archean ocean may have presented another hurdle to Cyanobacterial expansion as it contained large amounts of Fe(II), which is presumed to be toxic to Cyanobacteria. This thesis shows that the localised activity of Cyanobacteria could have formed marine oxygen oases in shallow coastal regions. This would have negated the toxicity of Fe(II) and could have produced more net O2 then modern oxic systems. Additionally, the formation of green rust was observed, which seemed to have a toxic effect on Cyanobacterial growth and could be an important factor for the genesis of banded iron formations.
In conclusion, this thesis could show the viability of both, the “freshwater-origin” and “Fe(II)-toxicity”, hypothesis. Nevertheless, how long it took for Cyanobacteria to overcome the restrictions described above to expand into the open ocean is uncertain and needs to be further studied.