## 3 Discussion
However, an interesting paper has recently been published by [PERSON] and colleagues [26], who performed a metagenomic study of the _A. filiculoides_ genome. They found Burkholderiales, Caulobacteriales, and Rhizobiales as the most abundant microbial groups accompanying the fern. Deeper analysis revealed microorganisms belonging to the genera _Microbacterium_, _Hypomicrobium_, _Shinella_, _Ralstonia_, _Rhizobium_, and _Hydrocarboniphaga_[26]. We found these data different from ours. In our study, Burkholderiales constituted 23% and Rhizobiales 11% of the epiphyhic microbiome, which is one-third of the whole microbiome identified. In addition, we obtained two isolates belonging to _Microbacterium_. Moreover, other genera, including _Delftia_ were not detected. The differences in microbiome composition may be connected with _Azolla_ sp. environment. Our laboratory culture was sustained for 9 years on IRRI medium. [PERSON] and colleagues [26] tested bacterial communities with both natural (ditch) and controlled (sterilized, IRRI collections). They stated that _Azolla_ sp. has control over the bacterial community assembly within its closed leaf pockets and it differs between sources of _Azolla_ sp. Since there are no more similar studies, we consider our study as a next very important step providing deep knowledge on the _A. filiculoides_ microbiome. For microbiome phenotyping, we found only one paper describing the capability of _A. filiculoides_ and _A. pinnata_ endosymbiotic _Arthbroacter_ sp. for IAA production, where the auxin concentration remained at the level of 1.5-10 \(\mu\)g mL\({}^{-1}\) at an L-tryptophan dose of 100-600 \(\mu\)g\(\cdot\)mL\({}^{-1}\)[30]. Other studies demonstrated different efficiencies of IAA production by various microorganisms. In the study by [PERSON] et al. [31], the highest production of IAA (\(>\)20 \(\mu\)g mL\({}^{-1}\)) was recorded for _Pseudomonas_ sp., whereas these values in _Bacillus_ sp. and _Agrobacterium_ sp. amounted to 3-7 and 16 \(\mu\)g mL\({}^{-1}\), respectively. [PERSON] et al. [32] showed IAA production of 87.9 \(\mu\)g mL\({}^{-1}\) for _Bacillus_ sp. In turn, [PERSON] et al. [33] proved that _Delftia_ sp. JD2 was able to synthesize IAA up to 80 \(\mu\)g\(\cdot\)mg\({}^{-1}\) dw when exposed to Cr(VI) ions. Strains from the genus _Bacillus_ were reported to display both cellulase and protease activity [32; 34]. [PERSON] et al. [31] reported high protease activity in _Bacillus_ sp., which produced 2-3 cm diameter halos, i.e., approximately twice as big as in our study. _Agrobacterium_ sp. formed 2.3 cm halos, whilst 2.95 cm zones were noted in our study. In contrast to our study, [PERSON] et al. [34] indicated no ability to decompose the pathogen cell wall by _Microbacterium_ sp. The ability to solubilize P in _Bacillus_ sp. was reported by [PERSON] et al. [32]. PSB, PSM, and PMPSB microorganisms were reported in the study of [PERSON] and colleagues [35]; however, the authors mentioned only strains from the genera _Pseudomonas_, _Enterobacter_, and _Pantoea_, whereas [PERSON] et al. [36] presented Delftia sp. as PSB for the first time. The production of siderophores by _Bacillus_ sp. was reported by [PERSON] et al. [31] and [PERSON] et al. [32]. [PERSON] et al. [33] reported that _Delftia_ sp. JD2 produced siderophores in Cr(VI) stress conditions. It is worth emphasizing that other authors presented the ability to promote plant growth by microorganisms originating from soil or isolated from terrestrial vegetation. However, the data regarding aquatic microorganisms are scarce. Most of the microorganisms described were not found in the _A. filiculoides_ microbiome and only _Bacillus_ sp. was often identified. By displaying the highest potential in growth promotion among all isolates, _Delftia_ sp. AzoEpi7 particularly attracted our attention. Members of the genus _Delftia_ are aerobic non-endospore forming Gram-negative rods that inhabit diverse ecological niches.