More from the Atma River …

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.

More about Very Hungry Chironomids

I wrote about a Very Hungry Chironomid in a post back in March after watching a midge larva munch its way through a patch of diatoms in the River Ehen.  I’ve spent the time between then and now trying to capture the image in paint.  This is partly to remind myself of the artificiality of all that we look at under the microscope, because so much disruption is involved in getting microscopic organisms from their natural habitats on river beds to a microscope slide.  But it is also serves partly as a meditation on the organisms themselves, what they look like in their natural habitat, and how they interact with the organisms around them.

My problems are intensified because it is a long time since I looked at insect larvae in any detail.   The last time was, I am fairly sure, practical classes as a second year undergraduate, and I had to do a lot of background reading to remind myself just how different the insect mouthparts are to the vertebrates.  I had my own videos and others I found on YouTube as source material, as I tried to work out how a midge larva would work its way through a patch of diatoms.  My sketchbook now has page after page of sketches, annotated with diagrams cut from papers and textbooks on insect mouthparts, and I exchanged emails with Les Ruse, a colleague who is an expert on this group.


A chironomid (non-biting midge) larva feeding on diatoms in the River Ehen.

My picture is an approximation of the view that I saw back in early March.  Though only a couple of millimetres long, a chironomid is still two orders of magnitude larger than the algae on which it was feeding.  This is approximately the same size ratio as a cow and the grass on which it feeds and, consequently, it is difficult to capture both the intricacy of the diatoms and the immensity of the larva.   The picture shows the two prolegs at the front of its body.  These are unjointed stumpy projections which are capable of limited movement, and enable the larva to drag itself along.  The front prolegs have a ring of hooks with which it held onto the stalks of the diatom as it pulled itself into a position where its mandibles could shear through the stalks.  The rear-view here is deliberate as it is hard to see the other mouthparts in sufficient detail to draw them, forcing me to search for books and papers with illustrations that I could use as source material.


A sketch of the mouthparts of a chironomid larva similar to that found in the River Ehen.   The head is approximately 1/10th of a millimetre across

Even this was complicated because there are almost 600 species of chironomid recorded from Britain and Ireland alone, and mouthparts vary from species to species, depending on their habitat and food preferences. The diagrams in books are often very generalised so I had to go back and forth between these and my photographs and videos to try to work out how the various parts fitted together.   The second image in this post is the last of the 13 pages of sketches that I made.   The mandibles (a) are impressive shear-like organs on either side of the head which move obliquely and which, in this species, seemed to be hacking through the diatom stalks.  The movement of the mandibles carried the food towards the mouth where other mouthparts, the maxillae (b), mentum (c), which roughly equates to the lower lip, and the labium (d), the upper “lip”, direct these into the mouth.  Note the fine hair like structures on the maxillae which assist in this process.  Finally, there are maxillae palps (e), which are jointed, mobile structures which sense the characteristics of potential foods.   Compare this arrangement with our own mouths, where the jaws work vertically and where the taste receptors are inside rather than outside.   It is no wonder that I needed 13 pages of my sketchbook to figure it all out.