Algae behaving selfishly …

My most recent trip to Ennerdale Water was on a wonderful windless winter day, offering perfect reflections of the snow-dusted peaks beyond the lake. It was a cold day but I was well wrapped-up and could enjoy both the long-distance views and the close-ups of nature around the lake’s margins.   One of the small streams that I crossed as I skirted the perimeter of the lake had patches of green algae growing on its submerged stones and even a quick examination showed it to be coarser than the green algae that covered most of the larger stones on the lake bed itself, as well on those in the River Ehen, just below the outfall.   When I managed to get specimens under my microscope I saw that the algae on the lake bed was Spirogyra (which I have seen here before; see “A lake of two halves”) whilst that in the inflow stream was Oedogonium.

I’ve written about Oedogonium before, and lamented the problems we face when we try to identify the species within this large genus (see “The perplexing case of the celibate alga”).   Ironically, a couple of weeks after I wrote this, I encountered a population of Oedogonium in another Cumbrian stream that did have sexual organs (see “Love and sex in a tufa-forming stream”).  However, this was the exception that proves the rule, as I have not seen a sexually-mature population of Oedogonium since.  The population I found beside Ennerdale was not sexually mature either but it did show a different, but equally effective, means of going forth and multiplying.

In the left hand diagram below we see a vegetative cell from an Oedogonium filament that has split open, allowing a vesicle to be extruded within which a single zoospore has formed.   This has a ring of flagella at one end, resembling a monk’s tonsure (you can just see these flagella in the photograph).   The other two photographs show the monk’s bald pate, though the fringe of flagella is not very clear.    The transparent vesicle swells and eventually ruptures, releasing the zoospore, which swim around for an hour or so, before settling on a new substratum and growing into new filaments.

Zoospores of Oedogonium from a stream flowing into Ennerdale Water, January 2018.   Scale bar: 25 micrometres (= 1/40th of a millimetre). 

In my material, the new filaments were mostly attached to mature Oedogonium filaments; however, this is probably partly an artefact and, in the field, they would almost certainly also settle on rocks and other surfaces too.   You can see, in the diagram below, how the “bald” end of the zoospore has started to differentiate into a holdfast that will secure the cell to the substrate whilst, over time, the other end will start to divide to produce the first cells of the new filament.  The whole process is described in a series of papers by Jeremy Pickett-Heaps (see reference list below).

Why did I see zoospore formation in this particular sample?   I don’t know for sure but it may be because I let a longer than usual time elapse between collecting and examining the sample.   This one had sat around in a cool box and fridge for four days, whereas I usually manage to check them within 24 hours.   Neglect can be a useful tool in the phycologist’s arsenal, as many freshwater algae see no need to indulge in anything more taxing than routine cell division for as long as the habitat keeps them replenished with whatever light, nutrients and other resources that they need.   Only when this is no longer the case do the algae start to channel resources into survival strategies.

Oedogonium zoospores germinating into new filaments, both epiphytic on mature filaments.   From a stream flowing into Ennerdale Water, January 2018. .   Scale bar: 25 micrometres (= 1/40th of a millimetre). 

Although I used the phrase “go forth and multiply” in an earlier paragraph, these Oedogonium cells are actually “going forth” rather than “multiplying” as the process we are watching only produces a single new cell.  However, were this zoospore to be released in a stream rather than a sample bottle, then there is a good chance that it would have been washed downstream and that a few of the many zoospores might have settled on a suitable habitat away from the constraints of their former home.   Asexual reproduction is a dispersal mechanism that results in the spread of genetically-identical copies of the parent cell.  For a sessile organism, this strategy allows a single genotype to move on from less-favourable locations and to exploit the potential of nearby locations.

The word “reproduction” is misleading as the mixing of genetic material that we associate with sex doesn’t take place.  The end product is a clone of a successful Oedogonium filament growing somewhere else.   However, taking the “sex” out of “asexual” removes a huge potential for innuendo, and readers who have battled this far through a post on nondescript green filaments deserve a reward.  So let’s finish with Woody Allen’s definition of masturbation as “sex with someone you love” and suggesting that the cytological huffing and puffing involved in zoospore production may not have the romance of sex but nor does it lead to any of the complications which result from sex either.   The alga gets offspring that are 100% identical to itself, just slightly further downstream and there is no risk of mixing with inferior genotypes.   That’s about as “selfish” as the “selfish gene” can get.


Pickett-Heaps, J. (1971).   Reproduction by zoospores in Oedogonium. I. Zoosporogenesis.   Protoplasma 72: 275-314.

Pickett-Heaps, J. (1971).   Reproduction by zoospores in Oedogonium. II. Emergence of the zoospore and the motile phase. Protoplasma 74: 149-167.

Pickett-Heaps, J. (1972).   Reproduction by zoospores in Oedogonium. III. Differentiation of the germling.  Protoplasma 74: 169-173.

Pickett-Heaps, J. (1972).   Reproduction by zoospores in Oedogonium. IV. Cell division in the germling and the possible evolution of the wall rings.   Protoplasma 74: 195-212.

See also “The River Ehen in March” for some further perspectives on asexual reproduction in algae.

View from near our sampling site on Croasdale Beck, looking towards Ennerdale Bridge, January 2018.



Love and sex in a tufa-forming stream …

The reason behind my trip to the Lake District a couple of weeks ago was to teach a short course on identification of freshwater macroalgae with Allan Pentecost (see “Heatwave? What heatwave” and subsequent posts for more about last year’s course).   One of the sites we visit with the students is a small stream flowing off Whitbarrow, a Carboniferous limestone outcrop in southern Cumbria.   The bed of the stream is covered with tufa, formed from calcium carbonate precipitated from the water. We bring the students here because there is usually a good variety of cyanobacteria for them to learn to recognise in the field and to sample for later investigation in the laboratory.   Amongst these cyanobacterial growths, however, we also saw a few patches of green filaments on the stream bed, which we also took back with us.


Sampling Whitbarrow tufa stream in May 2015.

These filaments turned out to be growths of the green alga Oedogonium. You may remember that I wrote a post last year with the title “The perplexing case of the celibate alga …” in which I commented that Oedogonium, though a common genus in freshwaters, is difficult to identify to species because this requires the reproductive organs which are rarely seen in the wild.

Our population of Oedogonium, however, was fertile, and this enabled us (Allan, to be strictly honest, as he knows the algae of tufa-forming streams extremely well) to name it.   The images below show the distinctive swollen oogonia within filaments of narrow cells (compare these with the much broader cells observed in “A case of mistaken identity?”). These oogonia look as if they have already fused with the male antheridia to form zygotes, which will eventually be released. These zygotes can lie dormant for a long time, which makes sexual reproduction a useful technique for overcoming adverse conditions (see also: “The River Ehen in March”). Not very romantic, I know, but that’s the reality of life at the unprepossessing end of biodiversity.


Oedogonium calcareum from Whitbarrow tufa stream, May 2015, showing oogonium. Arrows indicate position of “caps” (scar tissue from intercalary cell division) a. scale bar: 20 micrometres (= 1/50th of a millimetre); b. & c.: scale bar: 10 micrometres (= 1/100th of a millimetre).

Pearl mussels with some unexpected bedfellows …

I was thinking about my recent encounters with epiphytic algae (see “Cladophora and friends“) during my most recent visit to the River Ehen, when I noticed algae attached to some of the pearl mussels on the stream bed.   We’ve seen this a few times before but today I had my Olympus TG2 camera at the ready and was able to take a few photographs.   There have been occasions when the quantity of algae on the shells has been greater than this, and there has been concern about how this algae may affect the mussel’s ability to feed.   It is one of several possible stresses on the pearl mussel population that we are investigating at the moment.

When I put a small piece of algae that I removed from the mussel shell onto a slide and had a look at it through my microscope, I saw that it was Oedogonium, which we have already met in some posts from earlier this year (see “More about Oedogonium” and links therein).   My guess is that this is not a sophisticated relationship between host and alga, but simply that the mussel shells represent a convenient and relatively stable substratum for opportunistic filamentous algae, which seem to thrive in the River Ehen at this time of year.


Pearl mussels (Margaritifera margaritifera) on the bed of the River Ehen, several with bright-green growths of Oedogonium attached to the shells.   Note, too, the moss Fontinalis antiypretica in the upper part of the picture.

The description of an alga growing on an animal shell makes a nice counterpoint to the recent posts on algae growing on other algae.   There are other accounts of epizoic algae, though nowhere near as many as for epiphytic algae. This might simply mean that we have not looked in the right places.   The most spectacular of the few records that I do know about comes from my Belgian friend, Luc Denys, who scraped the backs of beached whales and found two previously unknown genera of diatoms that seem to grow exclusively on this rather unusual habitat.


Epizoic Oedogonium from the River Ehen, September 2014 at low (x100, left) and medium (x400, right) magnification. Scale bar: 10 micrometres (1/100th of a millimetre).


Denys, L. (1997). Morphology and taxonomy of epizoic diatoms (Epiphalaina and Tursiocola) on a sperm whale (Physeter macrocephalus) stranded on the coast of Belgium.   Diatom Research 12: 1-18.

More about Oedogonium

Having written about a population of Oedogonium that I found in an enriched lowland stream a couple of weeks ago (see “The perplexing case of the celibate alga”), I arrived at the River Ehen last week to find much of the stream bed covered with a dense growth of filamentous green algae that, once again, turned out to be composed largely of Oedogonium.   This soft water site differs in many ways from Stockerley Burn, but a species of Oedogonium seems to be thriving here too. Looking back at my notes, I see that I also found it at the same location this time last year.   The quantities are such that colleagues are concerned for the health of the pearl mussels which also grow here (see “Pearl mussels in the River Ehen”).


Wefts of filamentous algae (mostly Oedogonium) on the bed of the River Ehen, July 2014. Note, too, the flocculant material on the surfaces of the boulders.

The photomicrographs of Oedogonium also show very nicely the reticulate (net-like) nature of the chloroplast.   You can also see two cells of the diatom Tabellaria growing epiphytically on the lower filament.


Photomicrographs of Oedogonium from the River Ehen, July 2014.   Note the cap cells (arrowed) and the reticulate (net-like) chloroplasts.   Scale bar: 25 micrometres (1/40th of a millimetre).

We do not yet have a definitive explanation for the proliferation of Oedogonium in the River Ehen.   The most likely explanation is that the river levels have been low for some time and this means that the gradual accumulation of algae proceeds without the sloughing that occurs when the river is in spate.   The longer the algae can grow without disturbance, the higher the biomass.   When there is a spate and stones are rolled over, the algae that were on top of the stone are deprived of light and gradually die and decompose.   This means that the space between the stones of the substrate, through which oxygen-rich water flows to sustain the young pearl mussels becomes a fetid aquatic “compost heap” which will, in turn, extract this oxygen and make life for the young pearl mussels more difficult. As this is the last healthy pearl mussel population in England, my colleagues in conservation bodies are probably the only people in a region largely dependent on tourists to sustain its economy who are hoping for rain.

The perplexing case of the celibate alga …

Oedogonium presents a real challenge to an ecologist.   As I mentioned in my previous post, there are many species and these are found in a wide variety of conditions. In order to identify the species we need the reproductive organs but, as is the case for several filamentous freshwater algae, these are rarely seen in the wild. I did consult two colleagues on whether it was possible to induce Oedogonium filaments to grow these in the laboratory, but both told me that this was difficult. The theory is that you are more likely to find reproductive organs in situations where the alga has been allowed to dry slowly.   This is a useful survival strategy as the spores are usually very resistant to desiccation and can survive long periods out of water. However, converting theory to practice is not straightforward.

But how, I wondered, was the section on Oedogonium in the Freshwater Algal Flora produced?   ‘From secondary sources’, came back the reply. In other words, the author of this part of the Flora had relied descriptions and illustrations in earlier publications. As the most thorough work on Oedogonium in the UK was performed by the Wests, father and son, in the late nineteenth and early 20th centuries, this means that there has been no thorough overview of Oedogonium here for over 100 years.   I searched the database Web of Science and found just 14 papers that reported studies on the taxonomy of Oedogonium in the intervening years.   Just two of these were from European laboratories: one in 1991 in Czechoslovakia and a Polish study from 1979.   That’s not very much, considering the large number of species and their very broad distribution.

Just as we can identify some flowering plants from their vegetative characteristics alone, so some people have tried to identify Oedogonium using just the properties of the filaments. However, there is not very much to go on, apart from the length and width of the cells. The best attempt is that by my colleague Susi Schneider in Norway (see “A brief excursion to Norway”).   She differentiated eight types of Oedogonium in Norwegian rivers based on cell dimensions and noted a significant relationship between these types and phosphorus concentrations in the rivers where they grew.   Interestingly, the narrow forms were associated with low nutrients whilst the broader ones were found in more nutrient rich conditions. The population I found in Stockerley Burn was relatively broad which suggests, using Susi’s criteria from Norway, that this is a nutrient-rich stream. I am, however, reluctant to import Susi’s categories directly to the UK because our rivers are very different from those in Norway. However, I think it would be interesting to see whether the broad principles could be used here, even if we needed a slightly different calibration.

These struggles with Oedogonium also suggest that this is a genus that would benefit from a molecular genetic study, which would be a much more powerful means of differentiating between forms of Oedogonium although, unless we cracked the secret of either finding or culturing fertile Oedogonium it will be difficult to reconcile the DNA results with classical taxonomy. Until then, I fear, Oedogonium, represents yet another case of the “trailing edge” of science, where we may be in danger of forgetting faster than we learn.


Schneider, S.C. & Lindstrøm, E.-A. (2011). The periphyton index of trophic status PIT: a new eutrophication metric based on non-diatomaceous benthic algae in Nordic rivers. Hydrobiologia 665: 143-155.

A case of mistaken identity?

Imagine, just for a moment, that someone makes a list of the plants growing in a river or any other aquatic habitat and includes a category called “unidentified dicotyledon”.   Most botanists would throw up their hands in horror. Yet they probably have a category on their field record sheets for “filamentous green algae” which they use on a regular basis. It takes time, after all, to take a specimen back to the laboratory to check under the microscope and, let’s face it, the identification guides that are available are not very user-friendly and are full of unfamiliar terminology.

A slight variant on this particular sin is to record all the filamentous green algae that you encounter as Cladophora glomerata.  You are on a fairly safe bet here because a) it is a very common alga; and, b) no-one is likely to check.   However, there are a few algae that can be easily mistaken for Cladophora, especially if you are not paying close attention.

Last week, I did a survey of some streams draining into the River Browney, a tributary of the River Wear in County Durham.   Most showed evidence of enrichment which was not surprising as there were small sewage works, arable cultivation and a fish farm within these catchments.   And, as a result, it was no surprise to find that Cladophora dominated the stream beds at several sites.   One site, however, had thick wefts of filaments which looked and felt like Cladophora but, when viewed under the microscope, were quite different.


Stockerley Burn at Bogle Hole,   About half the river bed is covered by thick wefts of filamentous green algae up to about 30 cm in length.

I found three different species of green alga entangled in these wefts. There was some Cladophora glomerata but the most abundant of the three was a species of Oedogonium (see “The River Wear in summer”), characterised by unbranched filaments and cap cells.   66 species of Oedogonium have been recorded from Britain and Ireland but we know little of their ecology. Whilst some forms are common in lowland, nutrient rich rivers and streams such as this one, I have also found Oedogonium in remote, low-nutrient environments such as the River Ehen in Cumbria.   It pays to be careful, in other words, and to make sure that your “Cladophora” really is Cladophora. The easiest way to do this is to check for branching using a hand-lens. If you can’t see branching in the field, take a specimen back to the laboratory and check it under a microscope. Some populations of Cladophora are much more sparsely branched than others, so you may simply confirm your original suspicions. Oedogonium is, in fact, a very distant relation of Cladophora, despite their similarity when viewed with the naked eye. Mistaking Oedogonium and Cladophora is equivalent to  confusing your best friend with a sea squirt (see “Who do you think you are?”).

More about Oedogonium in the next post.


Filamentous algae in Stockerley Burn: main picture shows the wefts of (mostly) Oedogonium; inset shows cells from a single filament with the cap cells arrowed (scale bar: 10 micrometres, 1/100th of a millimetre).