Ecological yin and yang …

One of the sites we visited had a lot of fine, silty material at the margins, washed into the river following floods a few weeks before our visit.   There were a few light green patches on this silt which were dotted with oxygen bubbles as the algae made the most of the bright sunlight.  Under the microscope the green patches resolved into filaments of the blue-green alga Phormidium (probably P. autumnale or a relative).  You can see from the picture that this is a much simpler organism than the Stigonema that we met in the previous post, consisting just of straight, unbranched filaments.  However, it is effective at growing around the silt particles, creating a “mat” of algal filaments. The Phormidium filaments are capable of limited gliding motion which means that they can adjust their position to get the maximum benefit from the light.

Phormidium_in_Atma

A patch of Phormidium autumnale (or a close relative) growing on silt at the side of the Atma River, July 2013.   The air bubbles are about two millimetres across.

The next time there is a flood in the Atma, these banks of silt will probably be washed away, along with the Phormidium colonies.   However, we have seen very similar colonies form more substantial growths in the River Ehen (see post of 24 April 2013), perhaps reflecting a more stable habitat though these, too, could be washed away by the larger floods.

It is often hard to convince people of the importance of algae in lakes and rivers.   My work can seem abstract and esoteric but these oxygen bubbles help us put it all into perspective.   Put simply, the algae are the engines of rivers, particularly fast-flowing rivers such as the Atma where higher plants cannot get established.  They use the sunlight to create simple sugars out of carbon dioxide and water and this, in turn, is the food for the midge larvae and other bugs which are, ultimately, eaten by fish.  The oxygen is a by-product of this process but also plays a role in keeping the river healthy. All of the other organisms in the river need oxygen if they are to survive, so there needs to be a source that can constantly replenish the supply.   Algae contribute to the yin and the yang of freshwater ecology: capturing the sun’s energy and then balancing this by producing the oxygen that other organisms need to release this energy again for their own needs. Those of us who study algae tend to get bogged down with putting names on all the microscopic shapes we find and too easily forget to explain the role that they play.

Phormidium_from_Atma

A network of filaments of Phormidium autumnale (or a close relative) growing amongst silt particles in the Atma River.  The inset shows a single filament (scale bar: 10 micrometres = 1/100th of a millimetre).

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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_in_Atma

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_in_Atna

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_in_Atma

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.

A brief excursion to Norway

There is a heat wave in the UK as I write, but I am wearing a fleece, cagoule and waterproof over-trousers and wishing I had brought a wooly hat and gloves too. I am standing beside a stream 350 m north of Oslo, at an altitude of about 1000 metres, in the Rondane National Park in Norway.  We are at the tree line here, and there are still patches of  snow on the hillsides around us.  At my feet I can see low shrubby growths of the dwarf birch (Betula nana), a classic feature of “tundra” vegetation, interspersed amidst expanses of light-coloured lichens.

Rotina_mountains

The mountains of the Rondane National Park, Norway, seen from the Nedre Dørålseter Turisthytte, July 2013.

I am here to help a colleague, Susi Schneider with some fieldwork and, in the process, to learn the Norwegian approach to ecological assessment.  However, my travels around Europe have already taught me that differences in scientific approach have to be set into broader contexts of environment and culture, and the patches of moose droppings scattered amongst the lichen are enough to remind me of the many differences that exist.

The streams in the upper part of the Atna River, which drains this part of the national park have extensive covers of a slippery, brown growth.  If you remove a stone and run your fingers through it, it has a slimy, viscous feel.   The overall visual effect is, frankly, off-putting but this is an entirely natural phenomenon: an alga called Hydrurus foetidus.   Under the microscope, the yellow-brown cells can be seen to be arranged in rows within this mucilage, branching at intervals to give a feathery appearance.   Hydrurus belongs to a group of algae called Chrysophytes, which are related to the diatoms, yet also distinct in many ways.  It can be found in the UK but only in the depths of winter in remote places.  It is much more common in Norway, even in July, partly because it is further north and partly because there are so many near-pristine streams here.

Hydrurus_in_Atna

Hydrurus foetidus in the Atna River, Rondane National Park, Norway.  Left hand image shows Hydrurus smothering a submerged cobble; right hand image shows the mucilaginous growths on a stone removed from the water.

Susi’s conductivity meter gives us an extremely low reading, just 4 microSiemens /cm, meaning that this stream water is about as pure as distilled water and we both wonder out loud how any organism can find the sustenance to grow here.  There must, we presume, be occasional flushes of nitrogen, phosphorus and the other building blocks of life, perhaps following rain showers, one of which had soaked us a couple of hours earlier.  In any case, most of the biomass that we can see is the slimy mass around the cells, composed of carbohydrate, the most basic product of photosynthesis.  The recipe is simple: shake stream water with the carbon dioxide that is found naturally in the air (a turbulent stream is ideal for this purpose), then pour the mixture through the Hydrurus cells.  The result, judging by the number of midge larvae feeding on it, is delicious.

Hydrurus_foetidus

Hydrurus foetidus at two different magnifications under the microscope.  250 micrometres = a quarter of a millimetre.   Photographs by Chris Carter.

This still leaves us with a conundrum: that the goal of the EU legislation to which both Norway and the UK are signatories is natural or near natural ecosystems yet here we have just such an ecosystem yet one distinctly lacking in aesthetic appeal.  Nature is not only red in tooth and claw: it can also be brown, slimy and somewhat unappealing to the naked eye.  Quite how we convince the lay public of this is something I still haven’t fully solved.

Hilda Canter-Lund photography award 2013 winner

Much of the pioneering work on the fungal parasites of algae such as Asterionella was performed by Hilda Canter-Lund during her time at the Freshwater Biological Association, which makes a nice link with this post, as the winner of the 2013 Hilda Canter-Lund photography award has just been announced on the British Phycological Society website.  Hilda Canter-Lund was an extremely accomplished photographer of the microscopic world, producing pictures that combined high technical and aesthetic merits and was a Fellow of the Royal Photographic Society.  The award was set up in her memory by the British Phycological Society.

I was extremely pleased that Chris Carter won this year.  He made it to the shortlists in 2010 and 2011 and, as readers of this blog will already know produces pictures of an extremely high standard (see posts of 1 March and 14 May).   Chris’ winning entry shows the reproductive organs of a stonewort, Chara virgata from a pond in Northamptonshire, where he lives.   The visual focus s the bright orange antheridium, about 0.4 millimetres across, with interlocking shield cells caught just before they rupture.

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Chara virgata: Chris Carter’s winning image in the 2013 Hilda Canter-Lund photography award.

Chris’ career was spent developing infra-red sensors in the electronics industry, with natural history and photomicroscopy as spare-time enthusiasms.  Now he has retired from the electronics industry he has more time to spend on these enthusiasms, with some spectacular results.   Despite all the advances in optical technology and digital imaging over the last decades, good microscopic images require an extraordinary amount of patience and technical know-how.   His winning image encapsulates perfectly the standards that Hilda Canter-Lund set herself.

Chris-Carter

Chris Carter, out in the field collecting algae.

Little bugs have littler bugs upon their backs to bite ‘em ….

My post about John Snow pointed out that he made the link between contaminated water and disease without actually knowing what we now know about germs.  In effect, Snow had made an inference based on the association between cases of cholera and the closest pump but correlation, as we tell our students, is not the same as causation. Elsewhere in London and beyond, others were desperately searching to identify the culprit itself.

Various theories had been put forward, dividing roughly into those suggesting a chemical origin and those suggesting a biological cause. One of the proponents of the latter was a doctor called Arthur Hill Hassall who looked down his microscope at samples he had collected from the reservoirs which supplied London’s drinking water and thought he had found the answer.   The drawings he published in The Lancet show water teeming with algae and if this sounds preposterous, remember that this was still 20 years before Pasteur and Koch discovered bacteria, a group of organisms far too small to be seen with the microscopes available to Hassall.

Hassall published the first authoritive guide to the freshwater algae of Britain and described  several new species including a diatom called Asterionella formosa which is very common in the plankton of lakes in the spring.   The Latin name translates as “beautiful little star” and finding it in a sample always brings a wry smile to my face, as I recall the walk-on part this and other algae played in the story of the struggle to unravel the causes of cholera.

Asterionella_Dannemarche

Asterionella formosa collected from Dannemarche Reservoir in Jersey in June 2013 by Dave John.  The scale bar indicates 10 micrometres (1/100th of a millimetre).

The individuals photographed here come from a reservoir in Jersey.  I spend a day each year teaching on an algal identification course based in Durham. It relies on water samples brought along by the tutors and participants which means that there is always a rich assortment of material from all over the country to examine.  I was checking this sample before the class when I noticed the beaded appearance of the Asterionella.   Under higher magnification, these “beads” resolved into yet tinier organisms, unicellular fungi called “chytrids” which had infected the alga.

Asterionella_&_chytrid

Just as cholera was able to spread rapidly through the densely-populated regions of London in the nineteenth century, so chytrids thrive when Asterionella is most abundant.  It is a reminder that diseases and infections are a natural feature of animal and plant populations, not just human scourges and, indeed, are an entirely natural way of regulating population numbers:

Little bugs have littler bugs
upon their backs to bite ‘em.
And littler ones have littler ones,
and so on, ad infinitum …
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