| Summary: | This study aimed to describe the biodiversity of the nitrogen-fixing symbiotic bacteria Frankia, to investigate the occurrence of Frankia in soil and seeds, and finally test the effect of a Frankia symbiosis on growth rate on Sea buckthorn (Hippophae rhamnoides) in an experiment. Data from the high-quality ribosomal gene database SILVA was used to reconstruct phylogenetic trees that revealed a genetic diversity in the genus that includes more strains than 16 described species so far. Strains of Frankia isolated from Sea buckthorn were found in cluster three, which also was the cluster of Frankia strains with the highest diversity in genera of plants possible for the symbiosis (actinorhizal plants).Maps of biogeographical diversity were created from the SILVA gene database and whole genome data from NCBI. Gene sequences of Frankia existed on all continents except Antarctica, which reflects the distribution of actinorhizal plants. From Sweden, only sequences of strains in cluster 1 in symbiosis with Alnus and Myrica were found, although Sea buckthorn is naturally distributed along the Swedish coast. DNA was extracted from seeds of Sea buckthorn and soil, in order to amplify gene sequences affiliated to Frankia, however the preliminary PCR results indicated that there was an issue of inhibition caused e.g. by humic substances – and which therefore would need more optimization.The experiment that was set up to study the relative growth rate (RGR) of Sea buckthorn plants inoculated with Frankia in comparison with controls did not show any difference in RGR during the first month of growth, which is a too short time for any further reliable interpretations.
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