My post on Rivularia from softwater habitats (‘“Looking’ is not the same as seeing”’) prompted an email from Bryan Kennedy in Ireland with some pictures of Rivularia from a moorland stream in Co. Mayo in the west of Ireland, once again from a catchment completely lacking limestone. Bryan estimates the calcium concentration in the water to be between 5 and 10 milligrams per litre, which means that the water here is very soft. He also comments that it was recorded in the 1970s from the Caragh catchment in south-west Ireland (average calcium concentration: 2.15 milligrams per litre: see Heuff & Horkan, 1984).
A tributary of the Yellow River, Co. Mayo, Ireland (left) with dark brown / black colonies of Rivularia beccariana on a submerged stone (right). Photos: Bryan Kennedy.
The photomicrographs show the colony structure very well with filaments radiating out from the centre. The major difference between these and the Rivularia biasolettiana I photographed in Upper Teesdale (“Blue skies and blue flowers in Upper Teesdale”) is that colonies of the latter contain calcite crystals, though these were not visible in my images. The right hand image shows the structure of Rivularia filaments very clearly; the tapering blue-green filament gradually narrowing to a colourless hair. Note the colourless cell at the base of the filament. This is the “heterocyst”, and is the location where nitrogen fixation takes place. This is a very useful adaptation in the nutrient-poor habitats where Rivularia is found, as it means that, like peas and beans, it can capture nitrogen directly from the atmosphere, rather than relying upon dissolved minerals.
Nitrogen-fixation, however, needs a lot of energy and organisms do not fix nitrogen if there is a ready supply available from other sources. Once nitrogen is abundant, species such as Riviularia are at a competitive disadvantage and it is no surprise that Rivularia and it’s close relatives are found only in remote parts of the country, given the extent to which nitrate fertiliser washes off the land and into streams and rivers. Even in upland areas, there are often nitrogen compounds in rain water, much of it originating in the exhaust emissions from our cars. One wonders if Rivularia might have been much more widespread a hundred years ago than is the case now.
A close-up of a Rivularia beccarina colony from the tributary of the Yellow River, Co. Mayo, Ireland. Photos: Bryan Kennedy.
Heuff, H. & Horkan, K. (1984). Caragh. Pp. 363-384. In: Ecology of European Rivers (edited by B.A. Whitton). Blackwell Scientific Publications, Oxford.