River Ehen … again

Some of the submerged stones in the Ehen had dark, almost black tufts, a few centimetres long, attached to the downstream edge and trailing in the water.  The growths resembled the wracks we see on rocky shores but were smaller and slippery to the touch.

Growths of Tolypothrix tenuis on a cobble from the River Ehen, February 2013.

Under the microscope, these resolve into filaments of a blue-green alga (or cyanobacterium) called Tolypothrix tenuis.  The filaments have a characteristic blue-green colour and live in a thick colourless sheath (also visible in the photograph).   You can also see characteristic “false branches” (note how the filaments in the side branch are not actually attached to the main filament) and you may also see some cells in the filaments which are more rounded and slightly lighter in colour than the rest.  These are “heterocysts” and are responsible for nitrogen fixation, the same trick that legumes use to capture nitrogen from the atmosphere so that they can grow in environments where nitrogen is naturally scarce.

Left hand image:  Tolypothrix tenuis photographed at low magnification (left) and at higher magnification (right).  Photos by Chris Carter.

The other remarkable feature of this population was the dense growth of epiphytes on the filaments.   These were the same species as those on Audouinella in the previous post –  Heteroleibleinia rigidula – but the density here was much higher, completely smothering the Tolypothrix filaments in many cases.  The microscopic photographs in this post are by Chris Carter, who has managed to capture these narrow filaments, no more than about a 500^th of a millimetre in diameter, extremely clearly.   All three of the micrographs in this post show the Tolypothrix filaments almost overwhelmed by the narrower Heteroleibleinia.

Heteroleibleinia rigidula growing on a filament of Tolypothrix tenuis from the River Ehen, February 2013.