The power of the small

Cyanobacteria are prokaryotic organisms, which belong to the oldest micro-organisms on earth and exist since 3,5 billion years. They can tolerate extreme environmental conditions referring to temperature, pH and salinity. They occur in different habitats all over the world (including hot springs and the Arctic). Cyanobacterial cell size ranges between 0,5 and 40 µm; they were firstly considered to be blue-green algae, because of their ability to photosynthesis. Species of the cyanobacterial phylum are morphological divers. Morphotypes of one species can differ extremely in their appearance living under varying environmental conditions. Therefore, the determination of cyanobacterial species from environmental samples is very challenging.

Reasons for difficulties in determination of cyanobacteria are:

• two different systems of cyanobacterial classification, which stay in conflict to each other

International Code of Botanical Nomenclature and

Bacteriological Code of Nomenclature

• difficulties in culturing cyanobacteria in the lab
• huge variety of morphotypes per species
• wrong described cyanobacterial strains in sequence databases

Despite their very small size, they play a major role in various global nutrient cycles and are one of the most efficient organisms when it comes to carbon dioxide fixation, which has a huge impact on the world climate. Moreover, cyanobacteria occur in symbiosis with dozens of other organisms (e.g. green algae, diatoms, liverworts, mosses, ferns, cycads, fungi, protocista, sponges, bryozoa, didemnid ascidians, shrimps and mammals). Furthermore, they stabilize sediments e.g. in the Wadden Sea against hydrodynamic erosional forces and produce bio active compounds, which bear a great potential for the pharmaceutical industry.

During my master thesis, I performed cyanobacterial diversity analyses of 22 endolithic (living inside or on the surface of stones) cyanobacterial samples in relation to a salinity gradient in the intertidal zone of the Croatian Adria. Samples of different salinity levels (low, medium and high) were taken from rock pools of 5 separated locations. In order to find out, which taxa occur under certain salinities, the molecular technique of 454 next-generation sequencing (on 16S rRNA gene region) and morphological determinations (microsopy) have been used.

Resulting diversity indices and richness estimates, revealed a high level of diversity, particularly in high salinity samples. The most common cyanobacteria included the genera Hormathonema, Solentia, Hyella and Enthophysalis. Pyrosequencing yielded significantly higher diversity coverage than compared with phenotypic identification. Most of the endolithic cyanobacterial sequences detected in tidal rock pools matched most closely with isolates and clones from similar environments, suggesting that the most microbial members are adapted and suited to inhabit marine endolithic habitats. The relative diversity of the community is greater than those described from harsh endolithic habitats of cold and hot deserts.

The results of this study give an insight into the complex challenges, which lay in modern methods of biodiversity analyses of environmental cyanobacterial communities and may show a possible way forward in suitable describing methods of the diversity of these fascinating microorganisms.

The cyanobacterial appearance may be tiny, but their role in ecosystem function is huge!