Phylogenetic connection among close genera of Aphanizomenonaceae (Cyanobacteria): Amphiheterocytum gen. nov., Cylindrospermopsis and Sphaerospermopsis

Okeanomitos corallinicola gen. and sp. nov. (Nostocales, Cyanobacteria), a new toxic marine heterocyte-forming Cyanobacterium from a coral reef

Cyanobacterial mats supplanting coral and spreading coral diseases in tropical reefs, intensified by environmental shifts caused by human-induced pressures, nutrient enrichment, and global climate change, pose grave risks to the survival of coral ecosystems. In this study, we characterized Okeanomitos corallinicola gen. and sp. nov., a newly discovered toxic marine heterocyte-forming cyanobacterium isolated from a coral reef ecosystem of the South China Sea. Phylogenetic analysis, based on the 16S rRNA gene and the secondary structure of the 16S-23S rRNA intergenic region, placed this species in a clade distinct from closely related genera, that is, Sphaerospermopsis stricto sensu, Raphidiopsis, and Amphiheterocytum. The O. corallinicola is a marine benthic species lacking gas vesicles, distinguishing it from other members of the Aphanizomenonaceae family. The genome of O. corallinicola is large and exhibits diverse functional capabilities, potentially contributing to the resilience and adaptability of coral reef ecosystems. In vitro assays revealed that O. corallinicola demonstrates notable cytotoxic activity against various cancer cell lines, suggesting its potential as a source of novel anticancer compounds. Furthermore, the identification of residual saxitoxin biosynthesis function in the genome of O. corallinicola, a marine cyanobacteria, supports the theory that saxitoxin genes in cyanobacteria and dinoflagellates may have been horizontally transferred between them or may have originated from a shared ancestor. Overall, the identification and characterization of O. corallinicola provides valuable contributions to cyanobacterial taxonomy, offering novel perspectives on complex interactions within coral reef ecosystems.

An updated classification of cyanobacterial orders and families based on phylogenomic and polyphasic analysis

Cyanobacterial taxonomy is facing a period of rapid changes thanks to the ease of 16S rRNA gene sequencing and established workflows for description of new taxa. Since the last comprehensive review of the cyanobacterial system in 2014 until 2021, at least 273 species in 140 genera were newly described. These taxa were mainly placed into previously defined orders and families although several new families were proposed. However, the classification of most taxa still relied on hierarchical relationships inherited from the classical morphological taxonomy. Similarly, the obviously polyphyletic orders such as Synechococcales and Oscillatoriales were left unchanged. In this study, the rising number of genomic sequences of cyanobacteria and well-described reference strains allowed us to reconstruct a robust phylogenomic tree for taxonomic purposes. A less robust but better sampled 16S rRNA gene phylogeny was mapped to the phylogenomic backbone. Based on both these phylogenies, a polyphasic classification throughout the whole phylum of Cyanobacteria was created, with ten new orders and fifteen new families. The proposed system of cyanobacterial orders and families relied on a phylogenomic tree but still employed phenotypic apomorphies where possible to make it useful for professionals in the field. It was, however, confirmed that morphological convergence of phylogenetically distant taxa was a frequent phenomenon in cyanobacteria. Moreover, the limited phylogenetic informativeness of the 16S rRNA gene, resulting in ambiguous phylogenies above the genus level, emphasized the integration of genomic data as a prerequisite for the conclusive taxonomic placement of a vast number of cyanobacterial genera in the future.

Monilinema gen. nov., a homocytous genus (Cyanobacteria, Leptolyngbyaceae) from saline-alkaline lakes of Pantanal wetlands, Brazil

Cyanobacteria morphotypes with simple morphology, mainly thin filamentous homocytous strains, comprise a taxonomically complex group and represent a challenge in systematic studies. However, the polyphasic approach applied nowadays to investigate the cyanobacterial diversity has emerged as a powerful tool to undercover cryptic taxa and to set up a more natural classification system. Yet, studies exploring the cultured diversity of cyanobacteria from extreme tropical environments have paved the way to the discovery of new cyanobacteria taxa, enabling the description of Pantanalinema, Alkalinema, and Cephalothrix as novel genera from saline-alkaline lakes in Pantanal wetlands (Brazil). The present study investigated a set of cyanobacterial strains resembling Leptolyngbya morphotypes by means of its morphology, 16S ribosomal RNA (16S rRNA) phylogeny, and ITS secondary structures. Based on the shape and structure of their trichomes, the low levels of 16S rRNA identity coupled with the distinct phylogenetic position with regard to well establish genera, and their divergent ecological feature, these strains must be accommodated into a novel genus, named as Monilinema gen. nov., described under the provisions of the International Code of Nomenclature for algae, fungi, and plants. Accordingly, based on the singularity of the D1-D1' and V3 helixes, these strains should be considered as a single species, Monilinema alkalinum.