The larger stones at one of the sites on the River Ehen I visited last week had curious leathery mats on their upper surface that I could peel away intact with my fingers. They are dark-brown, almost black in appearance but, if you look closely, there is a distinct greenish tinge to the upper surface. It is not easy to get a clear view of these under the microscope but, after gently teasing out the mat with a dissecting needle, the composition becomes clear. The mats are composed of a dense network of intertwined blue-green filaments, belonging to a species called Phormidium autumnale. Mats of this species are often found on the upper surfaces of rocks, where they are likely to dry out from time to time. When this happens, the mat dries down to paper-like fragments that can blow in the wind.
Leathery mats of Phormidium autumnale on a boulder removed from the River Ehen in April 2013.
A microscopic view of filaments of Phormidium autumnale from the mats in the photograph above. The filaments are approximately 5 micrometres (1/200th of a millimetre) in diameter.
These filaments are almost identical in appearance to some of the oldest fossils ever recorded – from the Gunflint chert of Australia, estimated to be about 2000 million years old. This nondescript mat in a northern English stream offers us a glimpse into a world some ten times older than that of the dinosaurs yet, somehow, these organisms have survived all the vicissitudes that led to the extinctions of so many other forms.
Naming these organisms is a challenge; more so, in some ways, than for the diatoms (see Berlin and barcodes). There is little more for the eye to grasp than a filament divided into cells by simple cross-walls. Consequently, those who study the blue-green algae have the same struggles as I described in my earlier post and, again, molecular approaches are coming to our aid. If anything, the outcomes are more unsettling for the blue-green algae than was the case for diatoms, with some of the evidence (see below) suggesting that the taxonomy based on the optical microscope may need a complete rethink. As ever, we are too easily deflected by our anthropocentric view that evolution results in ever more complicated and sophisticated organisms. Look at how all three-door hatchback cars are broadly similar, regardless of the manufacturer. They all inhabit a similar environment and, consequently, the models have evolved to the most efficient form to thrive there. The same is true for these filamentous blue-green algae, only it has taken place over a much, much longer period. And now, unless you are to blue-green algae what Jeremy Clarkson is to cars, frankly, they all look pretty much the same.
Marquardt, J. & Palinska, K.A. (2007). Genotypic and phenotypic diversity of cyanobacteria assigned to the genus Phormidium (Oscillatoriales) from different habitats and geographical sites. Archives of Microbiology 187: 397-413.
Teneva, I., Dzhambazov, B., Mladenov, R. & Schirmer, K. (2005). Molecular and phylogenetic characterization of Phormidium species (cyanoprokaryota) using the cpcB-IGS-cpcA locus. Journal of Phycology 41: 188-194.