Cyanobacteria are among the most important primary producers on the Earth. However, the evolutionary forces driving cyanobacterial species diversity remain largely enigmatic due to both their distinction from macro-organisms and an undersampling of sequenced genomes. Thus, we present a new genome of a Synechococcus-like cyanobacterium from a novel evolutionary lineage. Further, we analyse all existing 16S rRNA sequences and genomes of Synechococcus-like cyanobacteria. Chronograms showed extremely polyphyletic relationships in Synechococcus, which has not been observed in any other cyanobacteria. Moreover, most Synechococcus lineages bifurcated after the Great Oxidation Event, including the most abundant marine picoplankton lineage. Quantification of horizontal gene transfer among 70 cyanobacterial genomes revealed significant differences among studied genomes. Horizontal gene transfer levels were not correlated with ecology, genome size or phenotype, but were correlated with the age of divergence. All findings were synthetized into a novel model of cyanobacterial evolution, characterized by serial convergence of the features, that is multicellularity and ecology.

The cyanobacteria are the most important prokaryotic primary producers on Earth, inhabiting a great diversity of aquatic and terrestrial environments exposed to light. However, the evolutionary forces leading to their divergence and speciation remain largely enigmatic compared to macroorganisms due to their prokaryotic nature, including vast population sizes, and largely asexual reproduction. The advent of modern molecular techniques has facilitated an understanding of the important factors shaping cyanobacterial evolution, including horizontal gene transfer and homologous recombination. We review the forces shaping the evolution of cyanobacteria and discuss the role of cohesive forces on speciation. Further, while myriad species concepts and definitions are currently used, only a limited subset might be applied to cyanobacteria due to their asexual reproduction. Additionally, concepts based solely on phenotypes provide insufficient resolution. A monophyletic species concept which is universal may be ideal for cyanobacteria. Actual identification of the cyanobacteria is difficult due to cryptic diversity, lack of morphological variability, and frequent convergent evolutionary events. Thus, applied molecular techniques such as DNA barcoding will be useful for identifications of environmental samples. Lastly, we show that the real biodiversity of the cyanobacteria is widely underestimated, due in part to low sampling efforts, sensitivity to the molecular markers 123Biodivers Conserv (2015) 24:739-757 DOI 10.1007 employed, and the species definitions employed by researchers. In conclusion, we anticipate a rapid increase in cyanobacterial taxa described and large revisions of the system in the future as scientists adopt a common approach to cyanobacterial systematics.

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Suitable morphological characteristics for identification of zygnematalean algae were determined using a combination of classical light microscopy (LM) techniques, fluorescence microscopy (with blue and green excitation), scanning electron microscopy (SEM) and specialized culture methods. Characteristics of spore germination, growth and reproduction under culture conditions identified Zygnema chalybeo-spermum in a mixture of zygnematalean spores collected from a small fishpond in Czechia. Reproduction in general, particularly aplanospore formation and lateral conjugation was more frequent and occurred earlier in a nutrient poor medium than in a nutrient rich medium, where vegetative growth was more vigorous. Variability in spore size was wide and influenced by the origin (sexual and/or asexual) of the spores. Asexual spores, particularly partenospores were rounded and significantly smaller than sexual ones. Thus spore morphology alone (size and shape) is not a particularly helpful characteristic for species identification. The mesospore of mature spores contained lipids and a sporopolenin-like material (algaenan), which showed green autofluorescence with blue excitation. The mesospore ornamentation, the only characteristic found that is suitable for identification purposes, can be observed easily in LM and SEM after exospore removal by KOH treatment. Detailed LM and SEM observations of the zygospores of all Zygnema species thus could provide basic data necessary for the preparation of an atlas and key for species identification which, after completion with molecular methods, brings clarification into the genetic relationships between morphospecies.

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