Abstract
Unicellular cyanobacteria inhabit a wide range of ecosytems and can be found throughout the phylum offering space for taxonomic confusion. One example is strain PCC 6712 that was described as Chlorogloea sp. (Nostocales) and later assigned to the genus Chroococcidiopsis (Chroococcidiopsidales). We now show that this strain belongs to the order Pleurocapsales and term it Hyella disjuncta based on morphology, genome analyses and 16S-23S ITS rRNA phylogeny. Genomic analysis indicated that H. disjuncta PCC 6712 shared about 44.7% orthologue genes with its closest relative H. patelloides. Furthermore, 12 cryptic biosynthetic gene clusters (BGCs) with potential bioactivity, such as a mycosporine-like amino acid BGC, were detected. Interestingly, the full set of nitrogen fixation genes was found in H. disjuncta PCC 6712 despite its inability to grow on nitrogen-free medium. A comparison of genes responsible for multicellularity was performed, indicating that most of these genes were present and related to those found in other cyanobacterial orders. This is in contrast to the formation of pseudofilaments—a main feature of the genus Hyella—which is weakly expressed in H. disjuncta PCC 6712 but prominent in Hyella patelloides LEGE 07179. Thus, our study pinpoints crucial but hidden aspects of polyphasic cyanobacterial taxonomy.
Highlights
Pleurocapsalean cyanobacteria represent a phylogenetically well-framed group but are still poorly investigated on several levels
The overall results comprising morphology studied under different culture conditions, media and substrates, 16S rRNA based phylogeny and main informative domains of the secondary structures of the 16S–23S ITS sequence led to the conclusion that strain PCC
6712 must be assigned to the genus Hyella in the order Pleurocapsales. This is corroborated by in-depth analyses of biosynthetic gene clusters responsible for, e.g., multicellularity and nitrogen fixation obtained from comparisons on genome level; it is formally described as H. disjuncta sp. nov
Summary
Pleurocapsalean cyanobacteria represent a phylogenetically well-framed group (see, e.g., [1,2]) but are still poorly investigated on several levels. The problem is that a large number of genera in this order lack sequence data and are very difficult to transfer in pure cultures [1]. The order is highly diverse and species rich, only a few genera have been studied to an extent where a highly accurate taxonomic placement based on modern standards has been achieved. This is the case for the genus Pleurocapsa, for which in 2019 the neotype and epitype for P. minor and P. fuliginosa were introduced [3]. In order to further strengthen taxonomy within the Pleurocapsales, additional strains need to be revised whose classification were based primarily on morphology, and they may be incorrect
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