As we worked our way down the Atma River, the diversity of algae increased, although the river did not yield up its secrets easily. At each site, Susi had to make a careful scrutiny of the stones on the river bed using an Aquascope to find a series of spots, blobs and tufts which, experience had told her, were likely to consist of algae. The Hydrurus, which we met in the previous post, was conspicuous but many of the others were very easily overlooked.
Susi using an Aquascope to search for algae in the Atma River, Norway, July 2013.
The small jelly-like growths on the top surface of several of the submerged stones are a case in point. It takes a practised eye to spot these on the apparently smooth rock surfaces but, under the microscope, they resolve into distinct colonies of small green cells, each with a tiny cup-shaped chloroplast. This is Tetraspora gelatinosa, a green alga which I often find in spring in the UK, often attached to vegetation at the edges of lakes. The colonies grow by simple division of the cells, with the “daughters” often remaining in close proximity, which is why the genus is called “Tetraspora”.
Tetraspora gelatinosa: the left hand image shows the gelatinous growths on the upper surface of a stone from the river bed; the right hand image shows the cells in their mucilaginous matrix (scale bar: 20 micrometres = 1/50th millimetre); inset: a group of four Tetraspora cells from within the matrix.
Elsewhere in the same stretch of river we found dark olive-green patches at and around water level, so that they spent part of the time submerged and part exposed to air, but never so high on the boulders that they dried out entirely. These were formed by a blue-green alga (Cyanobacterium) Stigonema mamillosum. Most blue-green algae live either as isolated cells or simple filaments but Stigonema have a relatively advanced morphology, with filaments that are several cells wide and branched. The individual cells have the characteristic blue-green colouration that gives the group its name, but the sheath within which they live has a brownish hue. This is common in blue-green algae that live in areas subject to bright light and is due to a compound called scytonemin which acts like a natural sunscreen, protecting the cells from the damaging effects of ultra violet radiation.
Stigonema mamillosum: the left hand image shows the Stigonema colonies (arrowed) growing in the “splash zone” just above water level on a boulder in the Atma River in Norway. The scale bar is one centimetre long. The central image is a low magnification view of the colonies, showing the side branches arising from the central filament whilst the right-hand image shows a higher magnification view of the filament (scale bar: 50 micrometres = 1/20th millimetre).
The dense network of Stigonema filaments acts like a sponge, trapping water so that the colony did not dry out and, at the same time, creating a habitat within which other algae could survive. I saw some thinner blue-green algal filaments growing on the Stigonema as well as several diatoms here.
The public’s perception of blue-green algae is usually negative because they often proliferate in lowland lakes and reservoirs where they can produce toxins, which limits recreational use of the water. However, my experience is that many types of blue-green algae are extremely sensitive to pollution and, as a consequence, are good indicators of high quality habitats. One of our challenges for the next few years is learning how to build this information into our assessments.