The next group of algae I’m going to consider in my review of higher taxonomy and systematics, the Cyanobacteria, or “blue-green algae”, present some significant challenges. Not least of these is a shift over the past forty years from being classified according to the rules of botanical nomenclature to being classified according to the International Code of Nomenclature of Bacteria. The former assumed species could be defined from field material on the basis of morphology, with “type specimens” preserved in herbaria; the latter uses axenic (i.e. pure) cultures as the basic taxonomic unit, and allows a wider range of attributes than just morphology. In recent years, as those who follow this blog will know, properties such as gene sequences have also been used to define species although the Code of Botanical (now Biological) Nomenclature still requires a description of the any new species that are described, with an expectation that the morphology will take a prominent role in that description. As this post will show, morphology is no longer such a reliable indication of how Cyanobacteria are organised as it was in the past.
For practical purposes, many Cyanobacteria fall into the same size range as other algae, live in communities that include many protist groups and can be identified using similar techniques as would be employed to identify other algae. They also have a form of photosynthesis that produces oxygen as an exhaust gas, in contrast to other bacteria which are capable of photosynthesis. This means that a default view of the Cyanobacteria as “algae” is a reasonable starting point for a field ecologist. However, at an intercellular scale, the Cyanobacteria are very different to other algae, and we should never lose sight of the fact that they actually belong to a different Domain to other algae.
The problems are clear when I compare the morphology-based classification that I used when I first taught classes on algae in 1990 with the classifications that are accepted now. Then, Cyanobacteria were divided into three or four orders, typically:
- Chroococcales – single cells or cells loosely-bound into irregular gelatinous colonies
- Oscillatoriales – filamentous forms lacking heterocysts
- Nostocales – filamentous forms with heterocysts
The high-level classification, in other words, was based solely on whether or not the organism formed filaments and, if so, whether or not it possessed heterocysts (specialised cells responsible for nitrogen fixation). This made logical sense when your primary source of insight is morphology. Unfortunately, more recent studies have shown that it bears little relationship to the genetic relationships amongst the organisms that have been revealed over the past thirty years or so. A more recent organisation is given in the diagram below.
First, note that this shows subclasses, rather than orders, within the class “Cyanophyceae” (the only class in the division Cyanophyta). There is rarely unanimity amongst experts on the appropriate organisation of high-level classifications so just bear with me on this one. Of the four sub-classes, one, Nostocophycidae, contains a single order (Nostocales) which includes all the heterocyst-bearing forms. No change there. However, the other two classes diverge very much from the older classifications in that they both contain a mixture of filamentous and non-filamentous forms.
The organisation of the Cyanobacteria (blue-green algae) division into four sub-classes. Filled boxes indicates the classes that are represented in UK and Irish freshwaters. Organisation follows Algaebase. The image at the top of this post shows a Microcystis bloom at Ladybower Reservoir (photo: Chris Carter)
The Oscillatoriophycidae is a good example, with five sub-classes, four of which are represented in the UK and Ireland. Two of these have featured in several posts (see Appendix) so you can see for yourself just how different they are in appearance. The Oscillatoriales includes filamentous forms without heterocysts whilst the Chrococcales has taxa that either exist as single cells or in masses loosely-bound within gelatinous colonies. A similar situation exists within the Synechococcophycidae; indeed, some genera that would formerly have been considered to be relatives of taxa within Oscillatoriales (e.g. Schizothrix and Heteroleibiana) are now included in families in this group. There is, however, still more work to be done to unravel all the relationships within this sub-class. The current understanding is that there is a single order (“Synechococcales”) but a great number of families. Similarly, all the heterocystous forms are grouped into a single order, the Nostocales, within the Nostocophycidae, also divided into a large number of families.
Organisation of the Oscillatoriophycidae showing the orders that include genera found in UK and Irish freshwaters.
I always stress that taxonomy and identification are two distinct crafts: the taxonomist calls on a wide range of tools to find natural groupings of species at different levels whilst an ecologist only needs a parsimonious route to an unambiguous identification. For the purposes of identification, recognising whether an organism is filamentous or not is a logical early step, even though both options will contain representatives of both Oscillatoriophycidae and Synecchococcophycidae. We need to recognise that some of the characteristics that contribute to our taxonomic understanding (gene sequences, arrangement of thylakoids) are useless from the point of view of someone trying to name an organism encountered in a field sample but, at the same time, the taxonomist’s standpoint will not necessarily capture all of the features that explain how an organism contributes to energy and nutrient flow within ecosystems.
Calothrix stagnalis: a member of the Nostocales. Note the heterocysts at the base of the filaments (photo: Chris Carter)
Mai, T., Johansen, J.R., Pietrasiak, N., Bohunciká, M. & Martin, M.P. (2018). Revision of the Synechococcales (Cyanobacteria) through recognition of four families including Oculatellaceae fam. nov. and Trichocoleraceae fam. nov. and six new genera containing 14 species. Phytotaxa 365: 1-59.
Palinska, K.A. & Surosz, W. (2014). Taxonomy of cyanobacteria: a contribution to consensus approach. Hydrobiologia 740: 1-11.
Links to posts describing representatives of the major groups of Cyanobacteria found in freshwaters. Only the most recent posts are included, but these should contain links to older posts (you can also use the WordPress search engine to find older posts).
|Synechococcales||Chamaesiphon: A bigger splash…
Heteroleibleinia: River Ehen … again
|Chrococcales||Aphanothece: No excuse for not swimming …
Gloeocapsa: The mysteries of Clapham Junction …
|Oscillatoriales||Microcoleus: How to make an ecosystem
Oscillatoria: Transitory phenomena …
|Pleurocapsales||Watch this space …|
|Spirulinales||Watch this space …|
|Nostocales||Nostoc: How to make an ecosystem (2)
Rivularia: Both sides now
Scytonema, Stigonema, Tolypothrix: Tales from the splash zone
Some other highlights from this week:
Wrote this whilst listening to: Leonard Cohen’s posthumous album Thanks for the Dance. And, as I used it to name a post, Joni Mitchell’s Both Sides Now.
Cultural highlights: David Hockney: Drawing from Life at the National Portrait Gallery. Great examination of the importance of drawing and observation to artistic practice. By coincidence, another post I’ve cited is named after one of Hockney’s paintings
Currently reading: Robin Wall Kimmerer: Gathering Moss (Oregon State University Press). A collection of essays on the natural and cultural history of mosses.
Culinary highlight: dinner at The Sichuan on City Road in London.