A thousand little mosses …

Castle_Eden_Burn_July19

Nature doth thus kindly heal every wound. By the mediation of a thousand little mosses and fungi, the most unsightly objects become radiant of beauty. There seem to be two sides of this world, presented us at different times, as we see things in growth or dissolution, in life or death. And seen with the eye of the poet, as God sees them, all things are alive and beautiful.
Henry David Thoreau (journal entry, March 13, 1842)

I was back in Castle Eden Dene earlier this week for my regular visit and, once again, encountered a dry stream bed.  This was no great surprise but, having written about the algae of dry river beds in earlier posts from Castle Eden Dene (see “When the going gets tough“ for the most recent instalment), I thought that I would focus on some of the other vegetation that I could see in and around the stream and, in particular, the bryophytes.   I asked Gaynor Mitchell, who wrote her MSc thesis on the bryophytes of the Dene, to come along and help me with these as my skills never really extended beyond those mosses and liverworts that live permanently submerged in streams and, as we have seen, there is rarely enough water in the burn here for such species to thrive.

There is a rich carpet of mosses on the woodland floor in much of Castle Eden Dene but, in the stream bed and its immediate environs, it is thalloid liverworts that are the most conspicuous bryophytes. Two species, in particular, stand out: the first is Conocephalum conicum, which has broad ribbon-shaped branches and an upper surface covered with pores – which just visible as light coloured dots to the naked eye.   The other is Pellia epiphylla, which was particularly noticeable on the top surface of boulders that are, I suspect, rarely covered, even when the burn is very full.   P. epiphylla had smaller thalli than C. conicum and, importantly, lacked the distinct pores on the upper surface.

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Conocephalum conicum from Castle Eden Dene, July 2019. The pores are clearly visible on the thallus in the lower image.

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Pellia epiphylla from the top of a boulder in Castle Eden Burn, July 2019

Alongside Pellia epiphylla on the boulder tops were shoots of the moss Thamnobryum alopercum.  The populations on top of the stones were rather non-descript to the naked eye, being stems growing horizontally across the rock surface. However, amidst these, we found a few of the upright stems which have a distinctly tree-like appearance.   We found more characteristic growths on the woodland floor nearby and my now-dated copy of Watson does, in fact, comment that this species has these two distinct habitats and also that it is a good indicator of calcareous conditions (for anyone who had not noticed the towering limestone cliffs in Castle Eden Dene, I presume?).   Lower down (and, thus, more frequently submerged), we saw Rhynchostegium confertum though this, too, is a species more often associated with terrestrial rather than aquatic habitats.  More significantly, the mosses I associate with streams in north-east England – Rhynchostegium riparioides, Fontinalis antipyretica and Leptodictyon riparium – are all missing from Castle Eden Burn.

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Tree-like shoots of Thamnobryum alopercum from the forest floor in Castle Eden Dene in July 2019.  Growths on rocks in Castle Eden Burn were smaller but there were enough upright stems for it to be recognisable with the naked eye. 

Gaynor’s sharp eye spotted many other mosses and liverworts, though more in the woodland around the stream than in the stream bed itself.  As well as mosses and liverworts, the stream’s vegetation also consisted of a number of grasses and patches of Chrysoplenium alterniflorum, opposite-leaved golden saxifrage.

The story that the vegetation is telling is, I would venture, that Castle Eden Burn is a shaded terrestrial habitat that is occasionally wet, rather than an aquatic habitat that is often dry.  I dug out an old account of the Winterbourne Stream, an intermittent stream in the chalk downlands of southern England for comparison, and found little overlap in the species recorded.   Care is needed for this comparison as the focus of the surveys is different (the Winterbourne account, for example, includes no bryophytes and spans perennial as well as intermittent sections) but there was a mix of genuinely aquatic and amphibious species, including Callitriche sp. and aquatic Ranunculus, which I did not see in Castle Eden Burn.    I suspect that Castle Eden Burn spends longer as a dry stream bed than the upper parts of the Winterbourne.  However, we also must remember that the Winterbourne data are now almost 50 years old, so that stream, too, may have changed much in the interim.

All this adds to my opinion that Castle Eden Burn – and the streams flowing through the other coastal denes in County Durham – are a unique and understudied habitat.  And that’s before I start thinking about the animal life here…

Chrysoplenium_alterniflorum_CED

A patch of Chrysoplenium alterniflorum, opposite-leaved golden saxifrage, on the bed of Castle Eden Burn, July 2019.

Reference

Berrie, A.D. & Wright, J.F. (1984).  The Winterbourne Stream.   pp.179-206.  In: Ecology of European Rivers (edited by B.A. Whitton).  Blackwell, Oxford.

Mitchell, G. (2015).  Bryophytes: changes in diversity and habitat in Castle Eden Dene (1975-2011).   Northumbrian Naturalist: Transactions of the Natural History Society of Northumbria 79: 39-66.

Watson, E.V. (1981).  British Mosses and Liverworts. Third Edition.  Cambridge University Press, Cambridge.

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Hilda Canter-Lund competition winners 2019

Zoe_Loffler_Synphony_of_Seaweed

This year’s Hilda Canter-Lund competition for the best algal-themed photograph has been won by Zoe Loffler for her image “Symphony of Seaweeds” taken on a at low tide near Apollo Bay, Victoria, Australia, while on a family camping trip.  She took the photo using a Google Nexus 5x Smartphone.  Zoe grew up diving in temperate waters near Melbourne, Australia. She completed her undergraduate degree and PhD at James Cook University in Townsville, Australia, studying the ecology of canopy-forming seaweeds (particularly Sargassum spp.) on coral reefs. She is now based in Sydney, and enjoys snorkelling and diving in temperate waters where there is such a wonderful diversity and abundance of seaweeds. The image meets Henri Cartier-Bresson’s maxim of the “decisive moment” (see “How to win the Hilda Canter-Lund prize”) and Zoe comments in her caption that the photo shows all who are unfamiliar with seaweeds that “they are not just brown and smelly!”.

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Zoe Loffler: winner of the 2018 Hilda Canter-Lund prize for algal photography, for her image “Symphony of Seaweed”, shown at the top of the post.

Since 2016 we have also offered a second prize which is awarded to a photograph in a contrasting style to the overall winner.  This year, that prize goes to Damien Sirjacobs of the University of Liege in Belgium  for his image “Blue Haze”. This shows a bloom of benthic blue diatoms of the genus Haslea (H. ostrearia, H. provincialis) covering a community of macroalgae (Padina pavonica, Acetabularia acetabulum, Halopteris scoparia, Dictyota sp.) in the shallow water of Calvi Bay (Corsica, France). For scale, the circular caps on the end of the Acetabularia stalks are 5 – 10 mm in diameter.   The image was taken at a depth of four metres in May 2018 with a LUMIX TZ10 in an underwater housing, under natural light conditions, while scuba-diving along rocky shores of the Revellatta peninsula (Calvi Bay).

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Damien Sirjacobs’ image: Blue haze”.  

There is a lot to interest readers of this blog in Damiens’s image; first of all, Acetabularia is another challenge to the generally-accepted view that multicellularity is the only option for large organisms.  Although the plant is quite large and, unlike Vaucheria is more elaborate than a simple tube of cytoplasm (see “The pros and cons of cell walls …”) .  The whole organism is, in fact, just one giant cell with a single nucleus.  

The diatom Haslea that grows over Acetabularia and the other macroalgae in Damien’s picture creates a blue haze due to a pigment called “marennine” which is found in vacuoles inside the cells (as you can see in the photograph below).   When marennine-containing species of Haslea are abundant around oyster beds (as is the case in parts of Brittany), then the pigment turns the gills of the oyster green and such oysters are highly sought after by gastronomes.   Whether or not these oysters really taste better is debatable but marennine certainly does have some antimicrobial properties.

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Damien Sirjacobs: co-winner of the 2019 Hilda Canter-Lund prize for algal photography.

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Whilst Zoe’s image has direct visual appeal, and most people will recognise it almost straightaway as depicting seaweeds, Damien’s image has a more other-worldly quality.   Unless you are familiar with the habitats and organisms, then it is difficult to interpret what is portrayed (see “Abstracting from reality …”).   One of the challenges of photographing algae is that we are dealing with the real yet little understood aspects of biodiversity, creating a multi-layered problem: first, of capturing an impression of the organism(s) but, also,  of interpreting the image to a lay-audience.   In the case of Blue Haze we have that intriguing combination of beauty, mystery and economic relevance.   That is what makes phycology such a fascinating subject.

Reference

Gastineau, R., Prasetiya, F.S., Falaise, C., Cognie, B., Decottignies, P.,  Morançais, M., Méléder, V., Davidovich, N., Turcotte, F., Tremblay, R., Pasetto, P., Dittmer, J., Bardeau, J.-F., Pouvreau, J.-B. & Mouget, J.-B. (2018). Marennine-like pigments: blue diatom or green oyster cult?   pp. 529-551.  In: Blue Biotechnology: Production and Use of Marine Molecules (edited by Stéphane La Barre and Stephen S. Bates).  Wiley VCH Verlag GmbH & Co. KGaA

The intricate life of a colonial alga …

Cassop_Pond_July19

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.

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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.

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

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

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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 (= 1/100thof 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.

Fade to grey …

Priestclose_pond_July19

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.

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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.

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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 (= 1/100thof 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.  2ndedition.  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 …

Vaucheria_x100_190630_#1

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.

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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.