The intricate life of a colonial alga …

The annual Algal Training Course in Durham always has a field trip out to Cassop Pond, a small pond at the foot of the Permian Limestone escarpment in County Durham that has featured in a few of my posts over the years (see “A return to Cassop”).  This year, the group came back with some samples from the pond’s margins bearing a suspension of green dots just visible to the naked eye which, when examined under the microscope, turned out to be the colonial green alga Volvox aureus.  These are spherical, with the cells at the periphery, joined together by thin strands of protoplasm. The smaller colonies were scooting about, propelled by the pairs of flagellae borne by each of the cells that constitute the colony, whilst the larger ones (mostly “pregnant” with one or more daughter colonies) were sessile.

Volvox aureus colonies just visible to the naked eye in a drop of water from Cassop Pond, July 2019.   The drop is 13 millimetres across.

Colonies of Volvox aureus (each bearing daughter colonies) from Cassop Pond, July 2019.  Scale bar: 50 micrometres (= 1/20^thof a millimetre).

A close-up of part of a colony of Volvox aureus from Cassop Pond, July 2019.  Scale bar: 20 micrometres (= 1/50^thof a millimetre). 

Watching a Volvox colony swimming around under the microscope is a beguiling experience, but its movement is not random.  Consider: there may be a 1000 or more cells in the larger colonies, each with two flagellae.  If all beat their flagellae at random, the colony would not get anywhere, as the force in one direction would be cancelled out by forces in all other directions.   But Volvox colonies do actually move with intent.   Look closely at the individual cells in the photos below and you will see that each has a red-coloured eye spot (the light-detecting organelle actually lies beneath the red layer, which acts as a filter).   People with more patience than me have noticed that the eye spots in different parts of the colony differ in size, suggesting a level of organisation that may not be immediately apparent.  We also know that the daughter colonies tend to form at the posterior end of the colony (assuming “posterior” and “anterior” in a spherical colony are defined by the direction of travel) and also that only a small number of cells (larger than the others) are responsible for the division that produces these.

In theory, a spherical object is going to offer less resistance and so sink faster than an object of the same size that had a greater surface area : volume ratio. This should mean that they are not able to stay in the light-rich surface layers where they can photosynthesise and grow.   In practice, Volvox colonies are able to adjust their position by using their flagella but this requires them to pump some of the energy they have obtained from photosynthesis into the flagella’s motors.  Another advantage in Volvox’s favour is a relatively low density of the colony as a whole.  The individual cells are separated by strands of protoplasm which creates a lattice through which water can penetrate, so the overall density of the colony is closer to that of the surrounding water than would be the case if the cells were tightly packed.

Volvox is most often found in the summer in relatively nutrient rich lakes, where nutrients are sufficiently plentiful to support a rich crop of algae.  A motile colony that is not too dense is well-placed to adjust its position to stay in the surface layers and harvest the sunlight.  Moreover, the size of the colony probably means that it is too big for the filter-feeding zooplankton that grazes on the algae.   At the same time, however, Volvox begins to experience some of the problems associated with multicellular life (see references in “The pros and cons of cell walls …”).   As large multicellular organisms ourselves, a nuanced discussion about the pros and cons of multicellularity may seem to only have one possible outcome.   However, Volvox inhabits a world where plenty of single-celled organisms thrive and where a colonial lifestyle offers a small competitive advantage.  It means that it is quite happy drifting around at the time of year when many of us would like nothing better than to don swimming trunks and soak up some sun in a local pool.   Study algae for too long and you end up realising that only losers need to evolve.

Cells from a Volvox aureus colony from Cassop Pond, July 2019. You can see the red eye-spots in some of the cells in the left-hand image (bright-field) whilst the protoplasmic strands joining cells together can be seen in the right-hand image (phase contrast).   Scale bar: 10 micrometres (= 1/100^thof a millimetre). 

References

Canter-Lund, H. & Lund, J.W.G. (1995).  Freshwater Algae: Their Microscopic World Explored.  Biopress, Bristol.

Reynolds, C.S. (1984). The Ecology of Freshwater Phytoplankton. Cambridge University Press, Cambridge.

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Fade to grey …

Prudhoe is a small town in Northumberland whose most famous inhabitant doesn’t exist*.   I came here to have a look at a pond in Priestclose Wood, a nature reserve operated by Northumberland Wildlife Trust which hit the local headlines recently for a suspected pollution incident.  You can see the scum on the surface in the photograph above and it does have an oily appearance, so anyone might be forgiven for calling the Environment Agency and asking them what was going on.

The query worked its way through the Environment Agency and ended up in my in-box in the middle of last week, with a specimen falling onto my doormat a few days later.   Having had a good look at it through my microscope, I drove out to the pond on a damp afternoon to take a look myself.   It is just a small pond, perhaps 30 metres across, set amidst the oak, birch and rowan-dominated woodland, which means that much of the lake is in almost permanent shade and, perhaps more important for the development of surface films, sheltered from the wind.  The surface film was just as I had been led to expect, despite the efforts of folk from the wildlife trust tried to disperse it last week in case the newts which lived here were threatened.   It was greyish-brown in colour, and covered the entire surface.  When I stirred it with a twig, it broke up, quickly closing up again as the water settled.   I then skimmed a sample bottle across the surface layer and harvested a yellow-brown suspension which I brought home for a closer look.

The Chromulina scum on the surface of the pond in Priestclose Wood: the left hand image was taken after stirring the surface with a stick to break up the oily layer; the right hand image shows the golden-brown algae that I scooped from the surface.  The picture at the top of the post shows the pond with its covering of algae.

The fresh sample was dominated by large numbers of tiny cells darting around.  Closer observation showed this to be oval, with a single yellow-brown cup-shaped chloroplast and what looked like one flagellum.   Over time, however, these cells slowed down, became rounder and started to aggregate in groups.   These, rather than the motile cells, proved rather easier to photograph.   I suspected that we were looking at a Chrysophyte, and Dave John later confirmed it to be Chromulina ferrea (the chloroplasts lack a pyrenoid, otherwise it would be C. aerophila).   If that is the case then there will be a second, much smaller flagellum too, but which is much harder to see with the light microscope.

Both of these species were described by John Lund in 1942 from ponds in Richmond Park whilst he was a PhD student at Queen Mary College London.   They are “neustonic”, meaning that they are adapted to live at the air-water interface, which also explains why they form the surface film that we saw in the pond at Priestclose Wood.   John Lund gives a detailed description of just how the behaviour of the alga leads to the formation of these films.   However, apart from John Lund’s original observation, the only other record in the British Freshwater Algal Flora is from a pond near Orpington in Kent, close to Dave John’s house.   Such is the nature of phycological records: it is not necessarily the algae that are rare so much as the people who notice them.

Chromulina cf ferrea from the pond in Priestclose Wood, Prudhoe, Northumberland, July 2019.   The left hand image shows a clump of sessile cells, photographed at 400x magnification; the right hand images show sessile cells at x1000 using brightfield (upper) and phase contrast (lower) illumination.  Scale bar: 10 micrometres (= 1/100^thof a millimetre). 

The local paper comments that the pond usually has a covering of duckweed at this year and blames the algae for killing this off.  The reality may be more complicated: duckweed (Lemna minor) can appear and disappear rapidly in a pond without any obvious cause (see “The green mantle of the standing pond …”) so it is equally possible that the duckweed disappeared for an unrelated reason (a virus, perhaps?) and this created an opening into which the Chromulina was able to expand.   We’ll probably never know the truth.   Maybe the duckweed will be back next year; maybe not.

Looking back at earlier posts, I see that the only other time a chrysophyte was the subject, I ended bemoaning circumstances where the these alga were both a “natural” part of the habitat’s biota whilst, at the same time, lacking in aesthetic appeal (see “A brief excursion to Norway”).   The same situation seems to apply here: an otherwise attractive woodland pond now covered with a greyish film which is, as far as I can tell, a “natural” phenomenon.  It is a shame if these are the only times that the lay-public encounter the chrysophytes as some of them are very beautiful under the microscope.   But, at the same time, the is no law that says nature has to be pretty.  Maybe it is our preconceptions that sometimes need adjusting …

* Ruth Archer, from the BBC Radio 4 series The Archers

References

John, D.M., Whitton, B.A. & Brook, A.J. (2011).  The Freshwater Algal Flora of the British Isles.  2^ndedition.  Cambridge University Press, London.

Lund, J.W.G. (1942). Contributions to our knowledge of British Chrysophyceae.  New Phytologist41: 274-292.

The pros and cons of cell walls …

When I wrote about Vaucheria in a recent post I mentioned that it is “siphonous”, meaning that there are no cross walls dividing the filament into individual cells. Instead, the organism consists of branching tubes containing many separate nuclei and chloroplasts (see “When the going gets tough …”). What I did not do was explain why this might be of benefit to a stream-dwelling organism.

Many scientists over the years have considered the benefits that accrued when simple unicellular organisms banded together to form the first multicellular organisms. They’ve come up with a number of theories, all of which may apply in some cases. However, having accepted that multicellular organisms have a number of advantages over single cells, we have to ask why organisms such as Vaucheria seem to have gone one step further: not only have several cells banded together but they also seem to have lost the cell walls that usually separate the individual units. Most biologists, nurtured in the belief that the cell is the basic unit from which organisms are built, will find Vaucheria’s growth form to be a surprise.

The usefulness of cells, in an evolutionary context, is that this opens the way to specialisation and, over time, to the development of tissues within organisms dedicated to particular tasks. We could think of cells as tiny bags, each containing mixtures of enzymes most relevant to their function. Cells in a leaf, for example, will need to focus on producing enzymes required for photosynthesis whilst those in the trunk will be programmed to produce structural tissues. Cells, in other words, facilitate this division of labour within plants. However, a simple filamentous alga has far fewer needs than a mighty oak tree: the water around them provide both support (so complicated structural tissues are not needed) and a supply of nutrients (removing the need for internal plumbing). If there is less need for specialisation, then there is also less need to invest energy in building cell walls which, despite their advantages, also limit the capacity of cells to share resources. Cells of many algae and all higher plants have channels called “plasmadesmata” which link them together; however, these require energy to function.

If you will excuse a topical, somewhat teleological and rather tortuous metaphor, all plants have the choice of either a “trade deal” (investing energy in plasmadesmata in return for resources from neighbouring cells) or “open borders” (ditching cell walls and sharing resources). For most land plants, the former makes more sense; different lineages of algae, on the other hand, have dabbled in both strategies and Vaucheria represents a successful example of the latter. “Successful”, you sneer, “… it thrives in small clumps in polluted rivers. How is that successful?”. That, if I may say so, is a very terrestrio-centric view of the world: Vaucheria lives in a world where it does not need extensive investment in the tissues that comprise the organisms that are paraded before our eyes on the Living Planet. It has all it needs and neither boasts nor worries about tomorrow.

Siphon of Vaucheria with a side-branch, collected from the River Browney, Co. Durham and photographed at 400x magnification. Note the absence of cell walls. Scale bar: 20 micrometres (= 1/50th of a millimetre). The photograph at the top of the post shows more siphons, at 100x magnification. Scale bar: 100 micrometres (= 1/10th of a millimetre).

We can find siphonous algae not just in the Xanthophyta (the division to which Vaucheria belongs) but also in some lineages of the green algae, including the important marine genus Caulerpera. Cladophora, no stranger to these posts (see “Summertime blues …”) is siphonocladous, rather than strictly siphonous, meaning that it is divided into large cells, each of which contains many nuclei rather than the single nucleus that is characteristic of most cells throughout the plant and animal kingdoms. Balanced against this, there is a much greater number of multicellular algae that retain cell walls, some of which show considerable differentiation of tissue. Modern land plants arose from this latter largely because of the opportunities this differentiation offered. Being siphonous (or siphonocladous) has proved to be a good strategy under certain circumstances but, in turn, limits the options for the species to extend into new habitats.

In my earlier post I used the metaphor of a “sausage skin” to describe Vaucheria. Most of the interior of the organism is taken up by a vacuole, with the chloroplasts and other cell machinery pressed into a narrow band just inside the cell wall. If you watch closely, you can see the chloroplasts in living Vaucheria moving very slowly – a process called “cytoplasmic streaming”. In very bright light the chloroplasts gather at the sides, so protecting each other from harm (see “Good vibrations under the Suffolk sun …” for another way around this problem). The nuclei and mitochondria (the cell’s “batteries”) can also move around and studies have shown all three move by different mechanisms. Being a siphonous organism offers more prospects for this means of adaptation to local circumstances but, overall, the pros are outweighed by the cons, and there are far more genera of multicellular algae than there of siphonous or siphonocladous algae. Vaucheria and other siphonous algae are clearly very successful in a few habitats but the big picture suggests that being truly multicellular offered organisms far more options in the long term.

References
Coneva, V., & Chitwood, D. H. (2015). Plant architecture without multicellularity: quandaries over patterning and the soma-germline divide in siphonous algae. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2015.00287

Herron, M. D., Borin, J. M., Boswell, J. C., Walker, J., Chen, I.-C. K., Knox, C. A., … Ratcliff, W. C. (2019). De novo origins of multicellularity in response to predation. Scientific Reports. https://doi.org/10.1038/s41598-019-39558-8

Canter-Lund, H. & Lund, J.W.G. (1995). Freshwater Algae: Their Microscopic World Explored. Biopress, Bristol.

Ott, D.W. & Brown, R.M. (1974). Developmental cytology of the genus Vaucheria 1. organisation of the vegetative filament. British Phycological Journal 9: 111 – 126.

Pennisi, E. (2018). The momentous transition to multicellular life may not have been so hard after all. Science, New York. doi:10.1126/science.aau5806

Raven, J.A. (1997). Minireview: multiple origins of plasmadesmata. European Journal of Phycology 32: 95-101.

Vroom, P.S. & C.A. Smith (2001). The challenge of siphonous green algae. American Scientist 89: 525-531.

Hilda Canter-Lund shortlist 2019

We’ve just announced the shortlist for the 2019 Hilda Canter-Lund but, unfortunately, the British Phycological Society’s webmaster is presently on a research cruise and has limited bandwidth so we haven’t been able to put them onto the BPS website yet.   Meanwhile, here is a sneak preview of what to expect when the shortlist finally does appear.  We had over fifty submissions this year, and it was a hard job to select the six images that make up the shortlist.  We always try to get a balance between different genres of images and, this year, we have two images of microalgae, two of marine macroalgae and two that sit in the middle ground – macroscopic images of microscopic organisms, if that makes sense.  If not, read on and all will become clear.

The first of the microscopic images is Cyanobacterial Entanglement, an almost abstract image of Cyanobacteria filaments taken by Forrest Leffler from the University of Florida. Alongside this we have Majestic Micrasterias, an image of the desmid Micrasterias furcata taken by William Murray from a sample from a lake in Delaware.   Whereas Forrest Leffler exploited abstract qualities in his image, William Murray achieves a sufficiently high level of detail that would not look out of place in an identification guide.   His image is very much in the tradition of Hilda Canter-Lund, which is one of the reasons why the judges recommended its inclusion on the shortlist.

Forrest Leffler’s Cyanobacterial Entanglement and William Murray’s Majestic Micrasterias.

A similar abstract versus representation tension is apparent in the two images of macroalgae on the shortlist.   Serial shortlist contender (and 2014 winner) John Huisman offers us a beautiful image of the red alga Martensia denticulata, photographed at Cape Perron, Western Australia, whilst Zoe Loffler from James Cook University in Queensland takes us to the other side of the continent to enjoy a riot of colour in her image of seaweed taken during a family camping trip.

John Huisman’s The next generation: Martensia denticulata, with cystocarpsand Zoe Loffler’s Symphony of Seaweed.

The final two images sit at the borderline between the macroscopic and microscopic worlds.  Damian Sirjacobs’ from the University of Liège in Belgium submitted an untitled image showing a bluish haze created by the diatom Haslea growing over macroalgae in shallow water in Calvi Bay, Corsica, whilst Wright State University’s Leon Kantona’s Pedestal of Productivity shows filaments of the Cyanobacteria Phormidiumand Oscillatoria amidst a yellow-brown mass of diatoms in an aquarium towards the end of a long-term photophysiology experiment.   You can also see oxygen bubbles surrounding the filaments due to the high rates of photosynthesis.

Damian Sirjacobs untitled view of the diatom Haslea growing amidst other algae, and Leon Katona’s Pedestal of Productivity.

The thumbnails in this post don’t really show the images at their very best; however, we hope to get them mounted on the BPS website within a few days, so that you can enjoy them all at higher resolution.  Meanwhile, the BPS Council are voting to decide the winners and I will be writing more about these just as soon as a decision has been reached.

Close to the edge in Wastwater …

I’m back in the Lake District for this post, standing beside Wastwater, the most remote and least disturbed of England’s lakes and, especially obvious on a sunny day in June, the most spectacularly-situated.  I stood on the western shore looking across to the screes and, beyond to the mass of Scafell Pike, England’s highest peak, looming up in the distance.

When I was done admiring the scenery I adjusted my focus to the biology of the lake’s littoral zone and some dark brown – almost black – marks on the boulders in the littoral zone.  In contrast to the grand vista stretching away to the north, these were beyond unprepossessing and my attempts to photograph them yielded nothing worth including in this post. However, I had seen similar looking marks in Ennerdale Water and there is a photograph in “Tales from the splash zone …” that should give you some idea of what I was seeing.

Under the microscope, my expectations were confirmed.  As in Ennerdale Water, these patches were composed of Cyanobacteria – gradually tapering trichomes of Calothrix fusca and more robust trichomes of Scytonema calcareum, both encased in thick, brown sheaths which, when viewed against the granite boulders on which they lived, resulted in the dark appearance of the growths.  To the untrained eye, these barely look like lifeforms, let alone plants yet they offer an important lesson about the health of Wastwater.

Calothrix cf fusca from the littoral zone of Wastwater, June 2019. Scale bar: 20 micrometres (= 1/50^thof a millimetre)

Though hard to see amidst the tangle of filaments in these population, both Calothrix and Scytonema have specialised cells called “heterocysts” that are capable of capturing atmospheric nitrogen (you can see these in the photographs of Nostoc commune in “How to make an ecosystem (2)”.   Nitrogen fixation is a troublesome business for cells as they need a lot of energy to break down the strong bonds that bind the atoms in atmospheric nitrogen together.   That means that plants only invest this energy in nitrogen fixation when absolutely necessary – when the lack of nitrogen is inhibiting an opportunity to grow, for example.   The presence of these Cyanobacteria in Wastwater is, therefore, telling us that nitrogen is scarce in this lake.

The dogma until recently was that phosphorus was the nutrient that was in shortest supply in lakes, so attention has largely focussed on reducing phosphorus concentrations in order to improve lake health.   Over the last ten years, however, evidence has gradually accumulated to show that nitrogen can also be limiting under some conditions.   That, in turn, means that those responsible for the health of our freshwaters should be looking at the nitrogen, as well as the phosphorus, concentration and, I’m pleased to say, UK’s environmental regulators have now proposed nitrogen standards for lakes.   That marks an important shift in attitude as, a few years ago, DEFRA were quite hostile to any suggestion that nitrogen concentrations in freshwaters should be managed.   In this respect, the UK is definitely out step with the rest of Europe, most of whom have nitrogen as well as phosphorus standards for freshwaters.

Scytonema cf calcareum from the littoral zone of Wastwater, June 2019. Note the single and double false branches.   Scale bar: 20 micrometres (= 1/50^thof a millimetre)

Wastwater flows into the River Irt and, a few kilometres down from the outflow, I found another nitrogen-fixing Cyanobacterium, Tolypothrix tenuis.  Once again, I could not get a good photograph, but you can see images of this in an earlier post from the River Ehen in “River Ehen … again”.   Nitrogen fixing organisms, in other words, are not confined to the lakes in this region, which raises the question why the UK does not have nitrogen standards for these as well (see “This is not a nitrate standard …”).   In rivers such as the Irt and Ehen that are already in good condition, it might only take a small increase in nitrogen concentration for the ecology to change.   Whether the loss of these nitrogen-fixing organisms will be noticed is another question.

For now, I am just happy to see that nitrogen in lakes has finally made it to the regulatory agenda.  It has taken about 15 years for the science to percolate through the many layers of bureaucracy that are an inevitable part of environmental management.  Give it another decade and maybe we’ll get nitrogen standards for rivers too.

References

Maberly, S. C., King, L., Dent, M. M., Jones, R. I., & Gibson, C. E. (2002). Nutrient limitation of phytoplankton and periphyton growth in upland lakes. Freshwater Biology. https://doi.org/10.1046/j.1365-2427.2002.00962.x

Moss, B., Jeppesen, E., Søndergaard, M., Lauridsen, T. L., & Liu, Z. (2013). Nitrogen, macrophytes, shallow lakes and nutrient limitation: Resolution of a current controversy? Hydrobiologia. https://doi.org/10.1007/s10750-012-1033-0

P.S. any guesses as to which 1970s prog rock group I was listening to over the weekend?  The clue is in the title.

China’s lessons for the Western diet

Just before I set off on my journey to China back in April I heard George Monbiot respond to the question  “what can we do to save the planet”.  His answer was “two things: eat a plant-based diet and avoid air travel”.  One ten-hour flight later I arrived in a country where it is notoriously difficult for a non-Mandarin speaker to avoid meat altogether so, it seems, I failed spectacularly on both counts.   The evidence behind Monbiot’s statements is strong yet I am not alone amongst academic environmental scientists in having a carbon footprint that is way above average.   For this to be justified I need to learn lessons as I travel that offset the environmental costs.  On this trip, those lessons came through the Chinese diet.

Whether we should eschew meat altogether is a moot point.  There are large parts of the UK where arable farming is not practical and livestock rearing makes practical sense, even if current economics leads to overstocking and what Monbiot has termed “sheepwrecking” of the uplands.   I’m more in favour of a substantial reduction in meat consumption, based on some realistic scenarios in a report produced by the French think-tank IDDRI (Institut du Développement Durable et des Relations internationals) and some other recent publications pointing out the environmental benefits of a less meat-rich diet.

Before I went to China I thought about this in terms of eating a higher proportion of vegetarian meals. After my trip I started to think more in terms of a lower proportion of meat in any given meal.  More importantly, meat does not have to dominate a plate but, rather, can act as a flavouring, enhancing the taste of dishes that were, essentially, vegetable-based.   Whilst it was not easy to get a meal that was dish that was through-and-through ‘vegetarian’ in China, few dishes were as meat-heavy as a typical meal in the West.  There are exceptions – Peking Duck being the obvious example – and two fortnight-long trips to this vast country does not make me an expert, but that is the impression that I have formed.

A Sichuan-style hot pot: note the liberal application of whole chillis. If you look very closely you will see Sichuan peppercorns between the chillis, just in case you were thinking that the seasoning was too tame.   The photograph at the top of the post shows Mobikes (and rival brands) for hire in the centre of Chengdu.   A monthly subscription costs less than a US dollar.

The other lesson I brought home from China is that they are not so focussed on the prime cuts and more use made of body parts that a Western cook might well throw away.   The cookery writer Fuchsia Dunlop explains this as a greater interest in the texture, rather than just taste, of food in China, compared to the west.   I’m not sure that duck intestines will appeal to everyone, but I also suspect that many will dismiss the idea without even trying.  But if we are to move to more sustainable diets that includes meat, then we will need to think about how to make use of the whole beast.  We may, actually, be exposed to more of this so-called ‘nose-to-tail’ eating than we think in the west as much of the meat that goes into highly-processed food comes from animal carcasses that have been mechanically-rendered.   The difference is that the Chinese actively embrace and take control of this concept (though they do seem to have an inexplicable fondness for luncheon meat).

Street food in Chengdu close to our Airbnb apartment: total price, including beer, was about £2 each. 

Back in Europe, I find myself less interested in a binary divide between ‘vegetarian’ versus ‘non-vegetarian’ as a result of this trip.   I did not have a Damascene conversion as such, as I have been trying to eat less meat for some time.  I’ve also tried to focus on the provenance of any meat that I buy but, when I did cook meat, it was usually a centrepiece of the meal.   Now, I find myself noticing how Italians toss pasta in a ragùsauce and serve what is, in effect, flavoured pasta rather than the British corruption of ‘spag bol’ where a pile of mince sits on top of the pasta.   That must be a better way to go.

How does this fit into a blog about natural biodiversity?   I often write about how the diversity of organisms is greatest in those lakes and streams that are in the most remote places.  The fertilisers that farmers use to boost production are a major source of nutrients in freshwaters.  These have significant effects on the communities that I see, and on the way that streams function.   One way that ecologists differ from other scientists is that they realise that they can never be wholly independent of the systems that they study. To comment on how agriculture influences freshwater is also to realise that, as a consumer of agricultural produce, I am part of the problem.  And, potentially, part of the solution too.

Smog over Chengdu, photographed from our Airbnb apartment near Zongfu Road.

 

When the going gets tough …

Two months after the visit I described in the previous post I was back at Castle Eden Dene.    The trees were now in leaf and the floor of the forest was carpeted with wild garlic.   The stream, however, had disappeared below the surface and, once again, I could walk along the channel without getting my feet damp.

Having found a rich crop of diatoms on my last visit when the stream was dry I was intrigued to see what was growing on the stones this time, so I used a toothbrush and some water that I had brought along to scrub a few and collected the dislodged material in my white tray.   I was intrigued to see that the suspension that collected in my tray had a distinct green tinge and, when I got a drop of it under my microscope, found it to be dominated by small green cells.  These were superficially similar to the cells of Desmococcus and Apatococcus that I found on the fence in my garden (see “Little Round Green Things …”) but this is a difficult group with not many clear morphological features with which to distinguish genera so I sent a sample off to Dave John for his opinion.

His view is also that groups such as this are almost impossible to identify unless you grow them in the laboratory or have access to DNA sequencing facilities.   He commented that Desmococcus and Apatococcus both have distinctive 2- or 4-celled packets of cells, which were not common in the Castle Eden Dene sample.  Likely candidates are the generaPleurastrumand Pseudopleurococcus, both of which are subaerial or terrestrial.   Perhaps “Little Round Green Things” is as close as we need to go in this particular instance?

A distinctly-green suspension of the biofilm on stones at Castle Eden Dene in May 2019 (left) along with a magnified view showing some of the green cells which dominated the sample (right).  Scale bar: 20 micrometres (1/5^thof a millimetre).   

A short distance further on I found some mats of entwined filaments on the tops of stones which also piqued my curiosity.   Under the microscope, and with the addition of a drop of water to rehydrate them, these filaments revealed themselves to belong to Vaucheria (see “Who do you think you are?”).   Technically speaking, Vaucheria is not filamentous but “siphonous”, meaning that there are no cross walls but, instead, the organism consists of branching tubes containing many separate nuclei and chloroplasts.  The cell walls of Vaucheria, however, rupture easily releasing the chloroplasts and giving the appearance of an empty sausage skin. In this case, there are still quite a few chloroplasts but a healthy Vaucheria filament has a uniformly dense green appearance that none of those that I saw in Castle Eden Burn possessed.

There was more than just vegetative filaments of Vaucheria here: scattered amongst them were some larger, spheroid or jar-shaped cells, which are part of Vaucheria’s sexual reproduction apparatus.   I’ve talked before in this blog about how sexual reproduction is relatively rare in the filamentous algae that we find in lakes and streams (see “The perplexing case of the celibate alga …”) and Vaucheria is another case in point.   Put simply, many algae do not bother with sexual reproduction when conditions are favourable and they can grow through simple cell division.   If you subjected a Vaucheria filament to Freudian analysis, it would probably tell you that one outcome of sexual reproduction was a 50% dilution of its unique genotype. So why bother if you don’t have to?  On the other hand, sexual reproduction in these organisms usually results in a zygote with a thick wall that is capable of resisting tough conditions.   The complete absence of water in Castle Eden Burn would be one such circumstance.   To put it another way, when the going gets tough, the algae get frisky.

Mats of Vaucheria growing on a small boulder in Castle Eden Dene in May 2019.  The picture frame in the left hand image is approximately 30 centimetres. 

Cell walls of Vaucheria, with a few chloroplasts still present, from Castle Eden Burn, May 2019.  Scale bar: 20 micrometres (= 1/50^thof a millimetre). 

However, the structures did not really match any pictures that I could find of oogonia or antheridia in Vaucheria.  I passed my images around some friends, and Gordon Beakes suggested that we might be looking at sporangia of chytrids, a group of fungi that have a string of previous convictions for infecting algae (see “Little bugs have littler bugs upon their backs to bite ‘em ….”).  As I was taking the photographs below, a cloud of tiny spores was released, prompting me to call out “come quickly if you want to see an alga ejaculating” before remembering that we had visitors in the house who might think this a little weird (and not just because I had not yet realised that they were, in fact, fungi).   I even took a video.  I’ll upload it to the Dark Web at some point.  There must be a site for fungal-themed pornography out there, if only I took the time to look…

Sporangia of chytrids on Vaucheria filaments from Castle Eden Burn, May 2019.  The one on the right was releasing spores (arrowed) at the time the photograph was taken.   Scale bar: 20 micrometres (= 1/50^thof a millimetre).