Cyanobacteria are prokaryotes that combine narrowly defined metabolic system with extraordinary structural diversity and as such are excellent model organisms to study the patterns of microbial diversification.

This approach will be applied to compare microbial populations of the North Sea Tidal Flats with ecologically similar, geographically distant (tropical, polar and arid) habitats.

The discovery, documentation and characterization of genetically distinct ecotype-populations will broadly impact the thinking in microbial evolution, and promote a common approach to the systematics, ecology and physiology of these organisms.

This work concentrates on assessment of tropical benthic cyanobacteria associated with coral reef ecosystems as well as tropical lagoons, that have been neglected in comparison with numerous studies of planktonic and picoplanktonic forms.

We have discovered very close genetic relationship between dominant benthic cyanobacteria of the genus Hydrocoleumand well known planktonic nitrogen-fixing species of Trichodesmium, suggesting that the main constituents of cyanobacterial benthos and plankton have an early common origin and represent major contributors to nitrogen budget of tropical oceans today as in the distant geological past.

Further studies on benthic cyanobacteria include phylogeny, nitrogen fixantion, chromatic adaptation and possible toxicity of main representatives of cyanobacteria.

We have applied polyphasic taxonomy integrating genotypic and phenotypic characterisations to filamentous, non-heterocystous, Phormidium-like cyanobacteria originating from different places all over the world.

Observations on Phormidium-like strains showed that their distribution follows patterns that correlate rather with ecological determinants then with the organisms determined as "Phormidium". Phormidium has cosmopolitan distribution, but only in areas that meet their ecological requirements. Our results, demonstrate that ubiquitous dispersal is not a universal trait of free-living cyanobacteria. Species which are found in less extreme biotopes are more widely distributed. Further, we also suggest that ecological adaptation must be considered as factor influencing their speciation.

We have analyzed and characterized dried herbarium specimens of cyanobacteria (exsiccata) deposited over 100 years ago, using combined morphological and molecular approaches. Morphological features observed by light and electron microscopy were correlated with the results of 16S rRNA gene sequencing.

The results of this study open the possibility to provide genotypic characterization to botanical type specimens, thus to reconcile the botanical and bacteriological approaches in taxonomic treatment of these microorganisms.

It was tested for the first time whether microorganisms identified by phenotypic characters indeed correspond to the same genotype as traditionally assumed. These results also contribute to discussions on traditional and newly emerging concepts of species and speciation in prokaryotes.