New taxonomy of cyanobacteria, or how not to get lost in the systematic jungle

taksonomia sinic

The last quarter century has seen major changes in algal taxonomy, both at the level of major systematic categories and newly described species. Even if one were limited to just one group, such as cyanobacteria, keeping up with changes can be difficult. You don’t have to be a phycologist to notice this. One cyanobacteria researcher, Jan Kaštovský, recently published his analysis and thoughts on the subject in the journal Hydrobiologia under the significant title “Welcome to the jungle!: An overview of modern taxonomy of cyanobacteria” [1]. This title calls directly what many biologists express behind the scenes – the modern changes experienced by the taxonomy of cyanobacteria, but not only, resemble a jungle in which it is easy to get lost.

Phylogenetic tree rearrangements – on which branch to place this taxon?

The development of modern molecular and phylogenetic methods has meant that the taxonomy of cyanobacteria has undergone dynamic changes over the past quarter century. Well-known species are now operating under new names. The dietary supplement spirulina is no longer crafted specimens of the genus Spirulina, but Arthrospira. One of the most common bloom-forming species, Anabaena flos-aquae, is now Dolichospermum flos-aquae. The species that Lampert and Gliwicz used for an experiment fundamental to aquatic ecology showing that the presence of filamentous cyanobacteria causes large planktonic animals to recede, entailing further changes in the trophic cascade [2] is no longer Cylindrospermopsis raciborskii, but Raphidiopsis raciborskii. The species is known for its progressive invasion on all continents, most notably for producing a toxin named cylindrospermopsin after it.

At the level of rows, too, there are shifts and highlighting new ones. Not so long ago, within the class Cyanophyceae, it was enough to know two: the filamentous Hormogoniales and the coccal Chroococcales. Admittedly, a few more were also distinguished, but their knowledge was niche. Nowadays, more than twenty orders are distinguished, and one does not have to be a specialist to come across the names Nostocales, Oscillatoriales or Synechococcales, but the name Hormogoniales is no longer encountered. And some changes are really surprising – a few years ago a certain species was moved not only to another genus or even order, but it turned out that it was not a cyanobacteria (i.e., a bacterium), but a crayfish (i.e., a nuclear organism). Today we know it as Hildenbrandia cuprea.

The condition may be of interest to taxonomy specialists or science popularizers like me, but for many practitioners it is a bit of a nuisance. If one is interested in lake restoration, it matters little whether the problematic taxon belongs to the genus Anabaena, Cylindrospermopsis or Oscillatoria, or Dolichospermum, Raphidiopsis and Planktothrix, respectively. Ecological requirements, sensitivity to antibiotics (although they are not likely to be used to control cyanobacteria) or interactions with other organisms do not change from this. In theory, recognizing that a population is Raphiodiopsis raciborskii rather than Raphidiopsis mediterranea could have legal significance in Europe, changing the status of a species from native to alien, but in practice, no one uses such legal categories for cyanobacteria.

Sometimes it is not clear what to consider as a separate genre, and what is only a variety or even a form. Underlying the description of several new genera was the separation of planktonic and benthic species (e.g. Dolichospermum/Anabaena), as the differences between them are due to anatomy, such as the presence of aerotopes. The combination of Cylindrospermopsis with Raphidiopsis, on the other hand, is a counter example – stemming from the recognition that such differences are insignificant in the face of proximity at the molecular level. The taxonomy of cyanobacteria appears to be not only complex, but also evolving.

New taxonomy of cyanobacteria – is this jungle really so confusing?

A traditional phycologist might liken modern cyanobacteria taxonomy to a jungle, and this is the comparison Kaštovský uses in his review of the issue in “Hydrobiology.” The inclusion of a phrase in the title that I think is an obvious reference to the title of a Guns N’Roses song(Welcome to the jungle!) is meant to indicate the confusion of the issue. In his introduction, the author quotes passages from Kipling about the ease of getting lost in such a world, but in his conclusion he concludes that although the situation seems confusing and hostile at first glance, there is no need to panic.

He came to this conclusion based on a review of several hundred publications from two decades (2000-2022) in which new descriptions of cyanobacterial species – both newly discovered and previously known but renamed – were published. Despite appearances, simply counting new species is not trivial. Even excluding fossil species, it is not always clear how to treat a particular description. In theory, to be valid, it must meet the criteria of the code of botanical nomenclature (traditionally still including algae and fungi), but in reality some authors do not quite follow them. Kaštovský assumed that the names of 3733 species established before 2000 and 1073 after that date are currently in effect.

In the latter group, slightly more than half are descriptions of species hitherto unknown to science, and the rest have gained new names. A particularly large number, nearly 200, were broadcast in 2001. In other years it was only a few dozen per year, and only in 2021. The number has exceeded a hundred. In comparison, an average of 21 species were described per year between 1828 and 1999.

Kaštovský explains this acceleration with two reasons – an increase in the availability of molecular methods to prove the species distinctiveness of a population and an increase in the expectation from scientists for more publications; although this argument is poorly defended. The first argument is weakened by one of the results of the article’s author’s analysis, which indicates that still as much as 43 percent of the species were described traditionally, based on morphological features. As for the second reason, although it is hard to deny the exponential growth of scientific publications in general, just presenting papers describing a new species to the world has always been prestigious, and researchers are unlikely to delay.

Lots of new discoveries, but are they all equally significant?

The fact is that with new species being discovered by machine, the prestige of such a discovery may have diminished. Kaštovský notes that some of them are questionable, or at least of little value. This is especially true for those based on only one molecular criterion, assuming a difference of 1 to 2.5 percent. in the DNA or RNA sequence as sufficient to distinguish the species, without being tempted to point out the phenotypic differences that follow.

He bluntly calls such discoveries cheap, and notes that demonstrating cryptic species, i.e., indistinguishable by appearance, does not attract wider interest. It is significant that the notion of a taxonomic operating unit, present in many contemporary molecular analyses that examine not so much individuals but genomes of environmental DNA, which are hard to attribute to specific organisms, does not appear throughout the article. Such a study itself is cited only once, in the context of the Tara Oceans research project.

In general, it appears that the species richness of oceanic planktonic cyanobacteria is surprisingly low compared to that found in inland waters and terrestrial habitats. This is one reason why the taxonomy of oceanic cyanobacteria is not changing much. In the case of inland planktonic cyanobacteria, where species are more abundant, they do not affect as large a fraction – 18 percent. This may be due to the fact that this is the most intensively studied group and some of the reshuffling occurred before the period under study – in the 1990s. last century.

The opposite is true for terrestrial cyanobacteria – formerly not studied intensively enough and reduced almost exclusively to the genus Nostoc – are now the group with the largest number of new species. The taxonomy of terrestrial cyanobacteria is changing rapidly. Kaštovský does not cite this example, but this group includes taxa recently described by Iwona Jasser’s team based on studies of the Pamir deserts in Tajikistan, such as Hillbrichtia. [3] Among the newly described, species from temperate zones predominate, which, given the general trend, allows us to assume that many taxa from the intertropical zone are still waiting to be described.

At the other end are taxa that, except for explorers from decades or even a hundred years ago, no one has managed to find in the wild. There are no changes here, and it is unclear if there ever will be. In contrast, there are taxa that were described as new in the two decades under review, but have since been eliminated, such as Sphaerocavum now considered synonymous with Microcystis.

Among the obstacles to good species differentiation, Kaštovský mentions the fact that different traits are detected by very different methods – using molecular techniques, light or electron microscopy. This makes it hard to make a clear identification. It also punctuates the lack of willingness of many cyanobacterial taxonomists to accept that some common traits are the result of convergence rather than evidence of relatedness.

Returning to the jungle metaphor, Kaštovský points out that it is, after all, a magnificent ecosystem, and the taxonomy of cyanobacteria is easier to understand with information from publicly available databases such as AlgaeBase or CyanoDB. It must be said that Kaštovsky’s language is lively and full of humor. Such a way of writing in scientific publications has hardly been seen for some hundred years. The slackness of the author and the editors of “Hydrobiology”, who allowed themselves a bit of nonchalance, should be congratulated.


In the article, I used, among other things. From the works:

[1] Kaštovský J., (2023). Welcome to the jungle: An overview of modern taxonomy of cyanobacteria. Hydrobiologia doi:10.1007/s10750-023-05356-7

[2] Gliwicz M.Z., Lampert W., (1990). Food Thresholds in Daphnia Species in the Absence and Presence of Blue-Green Filaments. Ecology, 71, 2: 691-702, http://www.jstor.org/stable/1940323

[3] Jasser I., Panou M., Khomutovska N., Sandzewicz M., Panteris E., Niyatbekov T., Łach Ł., Kwiatowski J., Kokociński M., Gkelis S., (2022). Cyanobacteria in hot pursuit: Characterization of cyanobacteria strains, including novel taxa, isolated from geothermal habitats from different ecoregions of the world. Molecular Phylogenetics and Evolution, 170, 107454, https://doi.org/10.1016/j.ympev.2022.107454

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