*3.1. Isolation, Purification and Culturing of Nostoc CCNP1411*

*Nostoc* strain CCNP1411 was isolated in 2010 from the Gulf of Gda ´nsk, southern Baltic Sea, by Dr. Justyna Kobos. Based on the 16S rRNA sequence (GenBank accession number KJ161445) and morphological features, such as the shape of trichomes, cell size (4.56 ± 0.30 μm wide and 4.12 ± 0.72 μm long) and lack of akinetes [43,44], the strain was classified to *N*. *edaphicum* species. Purification of the strain was carried out by multiple transfers to a liquid and solid (1% bacterial agar) Z8 medium supplemented with NaCl to obtain the salinity of 7.3 [45]. To establish the strain as a monoculture, free from accompanying heterotrophic bacteria, a third-generation cephalosporin, ceftriaxone (100 μg/mL) (Pol-Aura, Olsztyn, Poland) was used. In addition, the purity of the culture was regularly tested by inoculation on LA agar (solid LB medium with 1.5% agar) [46] and on agar Columbia +5% sheep blood (BTL Ltd. Łód ´z, Poland), a highly nutritious medium, recommended for fastidious bacteria. Cyanobacteria cultures were grown in liquid Z8 medium (100 mL) at 22 ± 1 ◦C, continuous light of 5–10 μmol photons m−<sup>2</sup> s−1. After three weeks of growth, the cyanobacterial biomass was harvested by passing the culture through a nylon net (mesh size 25 μm) and then freeze-dried before further processing.

#### *3.2. Isolation and Sequencing of Genomic DNA*

Genomic DNA of *N*. *edaphicum* CCNP1411 was isolated using SDS/Phenol method as described previously [47,48]. DNA quality control was performed by measuring the absorbance at 260/230 nm, template concentration was determined using Qubit fluorimeter (Thermo Fisher Scientific, Waltham, MA, USA), and DNA integrity was analyzed by 0.8% agarose gel electrophoresis and by PFGE using Biorad CHEF-III instrument (BioRad, Hercules, CA, USA).

Paired-end sequencing library was constructed using the NEB Ultra II FS Preparation Kit (New England Biolabs, Beverly, CA, USA) according to the manufacturer's instructions. The library was sequenced using an Illumina MiSeq platform (Illumina, San Diego, CA, USA) with 2 × 300 paired-end reads. Sequence quality metrics were assessed using FASTQC (http://www.bioinformatics. babraham.ac.uk/projects/fastqc/) [49].

The long reads were obtained using the GridION sequencer (Oxford Nanopore Technologies, Oxford, UK). Prior to long-read library preparation, genomic DNA was sheared into 30 kb fragments using 26 G needle followed by size selection on Bluepippin instrument (Sage Science, Beverly, MA, USA). DNA fragments above 20 kb were recovered using PAC30 kb cassette. 5 μg of recovered DNA was taken for 1D library construction using SQK-LSK109 kit and 0.5 μg of the final library was loaded into R9.4.1 flowcell and sequenced on MinION sequencer.
