Abstract :
[en] Cyanobacteria are major components of Polar ecosystems. They comprise a considerable amount of the total biomass, are the main primary producers, and play important roles in the initial development of terrestrial and aquatic habitats. Although the cyanobacterial diversity in the Polar Regions has been well studied, research has been mostly limited to traditional methods such as microscopy, culturing, and Sanger-based molecular ecology approaches. High-troughput sequencing (HTS) has revolutionized our knowledge of microbial diversity and distribution, but a thorough investigation of polar cyanobacterial communities using HTS has not yet been performed. As a result, knowledge of the distributional patterns of polar cyanobacterial communities and the mechanisms underlying their structure is fragmentary.
In this thesis, we applied the HTS approach to investigate the diversity and distributional patterns of cyanobacteria in a range of polar habitats. The main goals were:
1. To evaluate the use of HTS as a tool for assessing the cyanobacterial diversity in polar environments, including an assessment of different bioinformatics pipelines and the improvement represented by HTS in comparison to traditional methods;
2. To assess the spatial patterns of cyanobacterial communities across polar environments and the environmental factors shaping cyanobacterial community composition;
3. Tho study the successional dynamics of cyanobacterial communities following the colonization of recently deglaciated habitats.
Due to its higher analytical depth, HTS was proven a valuable tool for the study of the cyanobacterial biota of Polar Regions. Nevertheless, the analysis of artificial communities highlighted the importance of strict bioinformatics control of HTS reads, as PCR and sequencing errors might bias the observed structure of the communities. Given the strict bioinformatics workflow applied, our results suggest a previously undetected polar cyanobacterial rare biosphere. On one side, the HTS approach revealed that microbial mat and biological soil crust (BSCs) across different Antarctic and Svalbard (High Arctic) regions are somewhat homogeneous, being dominated by filamentous cyanobacteria from the orders Pseudanabaenales and Oscillatoriales. At the phylotype level (97.5% similarity threshold), on the other hand, cyanobacterial communities were highly variable along environmental gradients within and between habitats. A study of benthic mat communities in Antarctic lakes revealed cyanobacterial communities highly structured by lake conductivity, and comparable compositions in lakes from distant Antarctic regions. In a study of BSC communities in four nunataks and mountain ridges in the Sør Rondane Mountains (Dronning Maud Land), highly dissimilar communities were observed between locations. Finally, two studies of BSC communities in Petunia Bay (Svalbard) revealed a marked, linear cyanobacterial community turnover along gradients of soil development.
Altogether, the results obtained in the present thesis show that HTS is a powerful yet underused tool for the study of cyanobacterial diversity, not only in Polar Regions, but also on a global scale. This can be attributed to the very high sequencing depth achieved by HTS, which provides more robust assessments of community turnover and with a stronger statistical support than Sanger- based molecular ecology approaches.
