Draft Genome Sequence of the Planktic Cyanobacterium Tychonema bourrellyi, Isolated from Alpine Lentic Freshwater

ABSTRACT

We describe here the draft genome sequence of the cyanobacterium Tychonema bourrellyi, assembled from a metagenome of a nonaxenic culture. The strain (FEM_GT703) was isolated from a freshwater sample taken from Lake Garda, Italy. The draft genome sequence represents the first assembled T. bourrellyi strain.

GENOME ANNOUNCEMENT

Tychonema bourrellyi (J. W. G. Lund) (1) is a filamentous cyanobacterium historically found in northern regions, such as northern Europe and Canada (2). Tychonema is a cold-stenotherm genus with thin filaments (4 to 6 µm wide) and mainly colonizes pelagic freshwater environments. T. bourrellyi is an important cyanobacterial species, since some strains can produce the neurotoxins anatoxin-a (ATX) and homoanatoxin-a (HTX), which can be harmful for human and animal health. Recently, Salmaso et al. (3) found that T. bourrellyi is spreading and diversifying in Alpine freshwater lakes, with 49 distinct strains being identified in lakes previously dominated by Planktothrix species. However, despite the relevance of this species for human health, no sequenced T. bourrellyi strains are currently available.

We reconstructed the draft genome of T. bourrellyi from a nonaxenic culture. The culture originated from a water sample collected in summer 2014 by vertical tows from 20 to 30 m to the surface with a plankton net at Lake Garda. A single filament of Tychonema was isolated under the microscope and, after being washed repeatedly with Z8 Cellstar medium (Greiner Bio-One GmbH, Germany), was incubated at 20°C at constant-light illumination (3). The Tychonema culture was filtered through 0.22-µm filters, and the PowerWater DNA isolation kit (Qiagen) was then used to extract the DNA. The paired-end libraries (Illumina) were prepared and run on an Illumina HiSeq 2500 platform (100-nucleotide [nt]-long reads). Raw reads were assembled using metaSPAdes version 3.10.1 (4), with default parameters, which generated 7,029 contigs larger than 1,000 bp, with a total size of 74 Mbp. From this assembly, the draft genome of T. bourrellyi was binned using the manually supervised anvi’o protocol based on the abundance and tetranucleotide frequency distributions (5), quality controlled by CheckM (6) (95% predicted completeness with no indication of contamination or strain heterogeneity), and scaffolded through SSPACE (7).

The assembled draft genome was confirmed to belong to the T. bourrellyi species by PhyloPhlAn analysis (8) and BLASTn searches of the 16S and rbcLX genes, which were 98.7% and 100% identical, respectively, to previously reported T. bourrellyi genes (3). The genome is 5.08 Mb assembled into 271 scaffolds, with an N₅₀ value of 29,997 bp and a GC content of 44.63%. The genome annotation was performed with Prokka version 1.11 (9), which identified 4,493 coding sequences and 49 tRNAs. A candidate of a polyketide synthase family protein gene cluster potentially linked with the neurotoxic activity was identified in the genome at 40% amino acid identity (over an alignment length of 50%).

To our knowledge, this work represents the first sequencing of a Tychonema bourrellyi genome. Further analysis of the anatoxin genes involving more strains of this species are needed to unravel the genetic basis of T. bourrellyi toxicity.

Accession number(s).

This genome project has been deposited at DDBJ/ENA/GenBank under the accession number NXIB00000000. The version described in this paper is version NXIB02000000.