The dark side of the leaf …

Whites_Level_lower_epiphytes

Having mentioned in my previous post that the epiphytes on the top and bottom surfaces of a Potamogeton polygonifolius leaf were different, I have produced a companion piece to the painting I showed in that post.   The new painting is of the lower surface, and shows a greater number of diatoms than are present on the upper surface.  In order to explain why this is the case, it is helpful to look at the structure of the Potamogeton leaves.  The first image, therefore, shows a section through a leaf. It is quite a thick section but we can see the upper epidermis, the palisade mesophyll cells below this, which have plenty of chloroplasts in order to capture the sunlight that the plant needs for photosynthesis.  Below this, we can see parenchymous tissue arranged to create some large internal air spaces which contribute to the leaves buoyancy. Finally, at the bottom, there is a single layer of epidermal cells.   All this is crammed into a thickness of about half a millimetre.

Potamogeton_polygonifolius_leaf_section

Part of a section of a leaf of Potamogeton polygonifolius.  The leaf vein is on the left, thinning to the leaf blade on the right.  The leaf blade is about half a millimetre thick.   The picture at the top of the post shows an artist’s impression of diatoms and Chamaesiphon cf. confervicolus on the lower surface of a Potamogeton polygonifolius leaf. 

 Viewed from the underside, these parenchymous tissues create polyhedronal chambers, ranging from about 100 to 200 micrometres (a tenth to a fifth of a millimetre) along the longest axis.  There are also a few stomata scattered across the leaf surfaces (see the right hand image below).

With this in mind, take a look at my impression of the epiphytes growing on the lower surface of a Potamotgen polygonifolius leaf.   There are a number of cells of Chamaesiphon cf confervicolius, as seen on the upper surface, but there are several cells of the diatom Achnanthidium minutissimum, growing on short stalks, plus a few long, thin cells of Ulnaria ulna, growing in small clusters on the leaf surface (there were a few other species present, but such low numbers that I have not included them here).    It might seem strange to think of two surfaces of a leaf having such different communities of epiphytes but that’s because we’re thinking like large land-dwelling organisms, not like algae.   The longest alga visible in the image of the leaf underside is Ulnaria ulna, at about a 10th of a millimetre in length.  Therefore, to get a realistic impression of the two images, we really need to put a distance of five of these between them, and then pack the gap with chloroplast-rich mesophyll cells inside the Potamogeton leaf.   Allowing for foreshortening, this distance is about five times the height of the image.

Pot_polygonifolius_leaf

The structure of a Potamogeton polygonifolius leaf viewed from the underside.  The left hand image (100x magnification) shows a leaf vein running diagonally across the lower right hand side along with the polyhedron-shaped chambers; the right hand image (400x magnification) shows the outline of one of these chambers superimposed behind the epidermal cells with a stomata with two guard cells visible just above the centre.   Scale bar: 20 micrometres (= 1/50th of a millimetre). 

The epiphytes on the upper surface of the leaf get first dibs at the meagre Pennine sunlight, which then has to pass through the upper layers of the Potamogeton leaf, where the mesophyll cells will continue to feast on the tastiest wavelengths, leaving relatively meagre pickings for the epiphytes that hang around on the underside of the leaf.

Chlorophyll, the molecule that makes plants green, absorbs light over a relatively narrow range of wavelengths – predominately red and blue – and this means that there are plenty of other wavelengths awaiting an organism with different pigments.   Diatoms have chlorophyll, but they also have some carotenoids (principally fucoxanthin) that grabs energy from the green part of the visible light spectrum (which is reflected, rather than absorbed by chlorophyll) and passes it to the cell’s photosynthetic engine.  Having this capability means that they can survive in relatively low light, which is why we see more diatoms on the underside of the Potamogeton leaf than on the top.

And that, best beloved, is the story of how Potamogeton got its epiphytes …

 

Some other highlights from this week:

Wrote this whilst listening to: more Bob Dylan.   I’ve got to the mid-70s, which means the live version of Like a Rolling Stone on Before the Flood plus the great Blood on the Tracks.  Also, as I was reading Ian Rankin, I listened to John Martyn’s Solid Air.

Cultural highlights:  we’re watching the BBC adaptation of Sally Rooney’s Normal People

Currently reading:  Ian Rankin’s Rather be the Devil.

Culinary highlight:   A rather fine vegetarian chilli, from Felicity Cloake’s column in The Guardian last week.   Served with corn bread, using a recipe we got from a hand-me-down American housekeeping magazine during our time in Nigeria.

 

Whatever doesn’t kill you …

Potamogeton_epiphytes_April20

The previous post focussed mostly on the higher plants that I found in the short stream that connects White’s Level with Middlehope Burn.  I mentioned the mass growths of algae that I found growing immediately below the entrance to the adit, but I did not talk about them in any detail, instead spinning off on a tangent while I mused on why the water cress had a purplish tinge.

When I did find time to examine the algal floc, I found it to consist of a mix of three different algae, the most abundant of which was Tribonema viride, but there were also populations of a thin Microspora (not illustrated) and Klebsormidium subtile.   I talked about Tribonema in the drainage from the Hadjipavlou chromite mine in Cyprus last year (see “Survival of the fittest (1)”) and both Microspora and Klebsormidium are also genera that are known to frequent these habitats.  Indeed, there is evidence that the populations that grow in these extreme habitats have physiological adaptations that help them to cope with the conditions.  Brian Whitton, my PhD mentor, led several studies on these adaptations in the streams of the northern Pennines in the 1970s, and Patricia Foster did similar studies in Cornwall at about the same time.   There is probably a mixture of physiological strategies involved, including the production of low-molecular weight proteins, which bind the toxic metals, and the production of extracellular mucilage.  Most of the populations I find in such habitats have a distinctly slimy feel due to the production of extracellular polysaccharides, and it is possible that these play a role in trapping the metal ions before they can get into the cell and cause damage.

Tribonema_viride_WhitesLevel_Apr20

Filamentous algae from the drainage channel below White’s Level, upper Weardale, April 2020.  a., b. & c.: Tribonema cf. viride, showing the characteristic H-shaped cell ends.   d.  Klebsormidium cf. subtile.  Scale bar: 10 micrometres (= 100th of a millimetre).   The picture at the top of the post shows an artist’s impression of Chamaesiphon cf. confervicolus on the upper surface of a Potamogeton polygonifolius leaf. 

I also had a look at the algae growing on the submerged leaves of Potamogeton pergonifolius in the channel between the adit and Middlehope Burn.   One easy way of examining them is to add a small amount of stream water then shake the leaves vigorously in a plastic bag.  The result is a brownish suspension of algae that can be sucked up with a Pasteur pipette and placed on a microscope slide.  When I did this, I found a community that was dominated by a short cyanobacterium, closest in form to Chamaesiphon cf. confervicolus.  The other abundant alga in the sample was Achnanthidium minutissimum, which is often common in minewaters, along with smaller numbers of a few other species.  The total number of species in the sample was just 12, which is low by the standards of streams without metal pollution, but such suppression of all but the hardiest species is another characteristic effect of heavy metal pollution.

I’ve added a “cf” (from the Latin conferre, meaning “compare to”) to my identification of Chamaesiphon confervicolus because this is the closest name, based on a comparison with images in the Freshwater Algal Flora of Britain and Ireland.  However, it is not an exact match.  Whether this is because the metals have strange effects on Chamaesiphon (as we saw for diatoms in “A twist in the tale …”) or whether our knowledge of the species within this genus is imperfect is not clear.  But discretion is the better part of valour in this instance.  Chamaesiphon species fall into two groups: those that live on stone surfaces (see “Survival of the fittest (2)”) and those that live on algae and plants, such as the one we see today (another is illustrated in “More from the River Ehen”).   They consist of a single, elongate but gently tapering cell, attached at one end to the plant and enclosed in a sheath.   The upper end of the filament forms small spherical buds (technically “exospores”).  One reason that I am wary of calling this population C. confervicolus is that most illustrations of this species show a stack of exospores in the sheath, whereas the White’s Level population all had just a single exospore.

Chamaesiphon_confervicolus

Chamaesiphon confervicolus, growing on Potamogeton polygonifolius in White’s Level outflow, April 2020.   Note the exospores at the end of the cell.  f. and g. show the sheath very clearly.  Scale bar: 10 micrometres (= 100th of a millimetre). 

The picture at the top of this post shows an artist’s impression of the Chamaesiphon cf confervicolus on the upper surface of the Potamogeton leaf.   I wanted to get some idea of the size, shape and arrangement of the epidermal and stomatal cells on the Potamogeton leaves and resorted to the tried and tested technique of painting a layer of clear nail varnish onto the leaf surface, then peeling this off when it had dried.  This had the added (and unexpected) benefit of also pulling of the epiphytes, giving some idea of their arrangement on the leaf surface at the same time.   One extra observation that this yielded was that upper surface was dominated by Chamaesiphon, growing in clusters, whilst the lower surface had greater representation of diatoms.   I’ve also tried to portray the chloroplasts in the stomata guard cells.  Plant epidermal cells generally do not contain chloroplasts, as their purpose is to protect the mesophyll cells that are the main centres of photosynthesis.  Guard cells of stomata, however, need energy to open and close the stomata so these are the exception to this rule.  I had not even been sure that I would see stomata on the upper surface of the cell, as these are mostly found on the underside of leaves; however, Potamogeton appears to have stomata on both surfaces.  As ever, there is a certain amount of evidence along with a dose of extrapolation.   Imagined, but not imaginary …

You can find a description of the terrestrial plant life of Slitt Mine and its environs in this post on Heather’s blog.

References

Foster, P.L. (1982).  Metal resistances of Chlorophyta from rivers polluted by heavy metals. Freshwater Biology 12: 41-61.

Harding, J.P.C. & Whitton, B.A. (1976).  Resistance to zinc of Stigeoclonium tenue in the field and the laboratory. British Phycological Journal 11: 417-426.

Robinson, N.J. (1989).  Algal metallothioneins: secondary metabolites and proteins.  Journal of Applied Phycology 1: 5-18.

Say, P.J., Diaz, B.M. & Whiton, B.A. (1977).  Influence of zinc on lotic plants. I. tolerance of Hormidium species to zinc.  Freshwater Biology 7: 357-376.

Sorentino, C. (1985).  Copper resistance in Hormidium fluitans (Gay) Heering (Ulotrichaceae, Chlorophyceae).  Phycologia 24: 366-368.

(Note that Hormidium is the old name for the genus Klebsormidium.  There is an orchid genus called Hormdium and, as this was described first, it takes priority.)

 

Some other highlights from this week:

Wrote this whilst listening to: Bob Dylan’s New Morning and Pat Garrett and Billy the Kid.   Also, Samuel Barber’s Prayers of Kirkegaard.

Cultural highlights:  The Netflix series Unorthodox, about a young woman fleeing a Hassidic community in New York.

Currently reading:  Agatha Christie’s A.B.C. Murders.

Culinary highlight:   Arroz con leche (Spanish rice pudding) served with peaches poached in madeira.

A reasonable excuse for exercise ..

High_Mill_falls_Apr2020A redefinition of the travel restrictions hereabouts means that “driving to the countryside and walking (where far more time is spent walking than driving)” it is now “likely to be reasonable” within the terms of Regulation 6 of the The Health Protection (Coronavirus, Restrictions) (England) Regulations 2020. That means that, rather than plan another post about the fascinating ecology of Lough Down, I can look a little further afield.   As both Heather and I are writing chapters for a forthcoming book on the Natural History of Weardale, we turned our eyes to the hills, largely for exercise and a change of scenery, but also as part of our background research for these chapters.

We parked the car at Westgate and followed a path alongside Middlehope Burn, a tributary of the Wear with a long history of lead mining and, as such, a case study in how man has shaped the ecology of Weardale, both terrestrial (Heather’s domain) and aquatic.  The first part of the walk is through Slitt Wood, where the stream cascades over a series of low step-like waterfalls, alternately sandstone and limestone, illustrating the bedrock geology of the area.   The air is full of birdsong and there are patches of primroses feasting greedily on the light that is still plentiful on the forest floor at this time of year.   However, this idyll is short-lived as, passing through a gate we emerge into a grassed area surrounded by derelict mine buildings.  Early on a Saturday morning in the midst of the pandemic, we have the place to ourselves and it is a struggle to imagine this place as a busy industrial site.   Similar sites are scattered throughout Weardale and the surrounding dales; all are now closed but once they would have employed large numbers of people.  There would have been the miners, working underground, of course, but also gangs of people (including women and children) breaking down and sorting the ore as it was brought to the surface, plus ancillary workers involved in construction, both above and below ground.

A couple of hundred metres beyond the site of the main shaft at Slitt Mine, I spot an adit (a shaft driven horizontally into a hillside) and make my way towards it.  These are intriguing habitats for ecologists interested in the interactions between man and nature and I was intrigued to see what was growing in this one, White’s Level.  The mine’s levels and shafts act as natural drainage channels, collecting water that has percolated through the rocks but, because the miners have driven the levels along the mineral veins, the water comes into contact with lead, zinc and cadmium during the course of its underground journeys, emerging with concentrations far in excess of those deemed safe by toxicologists.  However, the channel immediately downstream of the entrance of White’s Level was lush with vegetation.   I could see thick wefts of filamentous algae giving way to beds of water-cress (Rorippa nasturtium-aquaticum) and bog pondweed (Potamogeton polygonifolius).  The latter two were surprising as, in my experience, most of the streams draining north Pennine adits are dominated by algae rather than by higher plants.

Whites_Level_April20

The stream flowing from White’s Level to Middlehope Burn, April 2020.  The left-hand image shows the beds of water-cress very clearly whilst the right hand image shows the filamentous algae growths immediately below the entrance.   The picture at the top of the post shows Middlehope Burn at High Mill Falls, just upstream from Westgate.

The water cress had a distinctive purplish tinge which is probably a response to stress.  We’ve encountered this type of colour-change in response to stress elsewhere (see “Escape to Southwold”).   In this post, and in “Good vibrations under the Suffolk sun …” I talked about how plants have to regulate the amount of energy from sunlight in order that their internal photosynthetic machinery is not overwhelmed.  Those posts were both written after a hot weekend in July, but this was a chilly and overcast April morning in the Pennines where the prospect of plant cells being overcome by heat seems faintly ludicrous.   Here, instead, is my alternative hypothesis.

Although White’s Level and the other mines in the northern Pennines were driven by the demand for lead, lead is a relatively insoluble element and zinc, which is found alongside the lead in the metal-rich veins of the northern Pennines, is more soluble and, therefore, has a greater toxic influence on the plants and animals in these streams.  Zinc affects the metabolism of plants in several ways, one of the most important of which is to reduce the effectiveness of the chlorophyll molecules which are responsible for photosynthesis.  It does this by nudging the magnesium atom, which lies at the heart of every chlorophyll molecule, out of place.

Whites_Level_macrophytes_Apr20

Macrophytes in the stream flowing from White’s Level to Middlehope Burn, April 2020.   Left: Potamogeton polygonifolius; right: Rorippa nasturtium-aquaticum.

What this does, then, is alter the balance of the equation that tries to balance energy inputs and photosynthesis.   If your chlorophyll molecules are hobbling along, then the point at which they are overwhelmed by even the meagre Pennine sunlight shifts so that  the need for the plants to manufacture their on-board sunscreen kicks in sooner.   Just a hypothesis, as I said: if you have a better explanation, please let me know.

A few hundred metres further on, there is another lush growth of water cress in the stream flowing out of another adit, Governor and Company Level, this time even extending beyond the metal grille designed to keep the curious from harm.  I most associate watercress farms with the headwaters of chalk stream, which are characteristically spring-fed and, therefore, have very stable conditions.   The adits of the northern Pennines are, this respect, very similar to springs insofar as their flow, temperature and chemical conditions vary little over the course of a year.   In that respect, it is perhaps less of a surprise that we find water cress growing so prolifically here.   The zinc, admittedly, is a complication we don’t find in most springs but, that apart, the adits could be thought of as man-made springs, creating a series of almost unique, but largely overlooked habitats.

In the next post, I’ll talk about the algae that I found in the White’s Level channel.

Governor_Company_Level_Apr20

A prolific growth of water cress in the drainage channel below Governor and Company Level, April 2020. 

Some other highlights from this week:

Wrote this whilst listening to: Still working through Dylan’s back catalogue: John Wesley Harding, , Nashville Skyline and Self-Portrait, the latter a blip in an otherwise superb run of albums.   Next up is New Morning but I want to re-read the chapter in Dylan’s Chronicles Volume One where he describes the genesis of this album before listening.

Cultural highlights:  My book group looked at Pride and Prejudice but, being deep into The Mirror and The Light, I did not had time to read this.   We watched the 2005 film version starring Kiera Knightly instead.   Turned out that three of the six participants in the book group had also watched the film the night before our Zoom meeting, rather than (re-)reading the book itself.

Currently reading:  Finally finished The Mirror and The Light which was, definitely, worth the effort.  Started Kate Atkinson’s Big Sky.

Culinary highlight:   Home-made tortellini filled with mushroom paté, served with a consommé made from turkey stock from the freezer.  Culinary ambition hereabouts always goes sky high in the week of the MasterChef finals.

A twist in the tale …

hamsterley_forest_jan19\

After my sojourn in East Durham, described in the previous post, I have travelled back to the Pennines for this one, crossing the River Wear at Wolsingham before driving up onto the fells and finally dropping down to the woodlands that are Hamsterley Forest.  This is a large man-made plantation, dating from the 1930s and popular for recreation. In January, however, the forest is quiet, and I only have a few mountain bikers and a lone dog walker for company as I peer into the peaty waters of Euden Beck.   This stream rises on the open fells of Hamsterley Common, between Weardale and Teesdale, before flowing through the forest and joining Spurl’s Wood Beck just downstream from where I am standing, to become Hamsterley Beck.  This then joins the Wear a few kilometres downstream from Wolsingham.

eudon_beck_jan19

Euden Beck, just above the forest drive in Hamsterley Forest, January 2019.  The photograph at the top of the post shows a view towards Hamsterley Forest. 

There is a mixture of diatoms growing on the stones here but I am most interested in the genus Fragilaria today.   One of the curiosities of this genus is that we often find several representatives growing at the same site at the same time, reminiscent of the old adage about London buses (“you wait ages, and then three come along at once”).   I’ve written about this before (see “Baffled by the benthos (2)” and “When is a diatom like a London bus?”) and Euden Beck is another good example of this conundrum in practice.

Today, I could see quite a few cells of Fragilaria teneraand smaller numbers ofF. gracilisplus a newly-described species that I will talk more about later in the post.  Fragilaria teneraforms long, needle-like cells, often clustering together to form sea urchin-like masses growing out from either a filamentous alga or particle to which they are attached (see “Food for thought in the River Ehen” for an illustration).  Most of the ones that I saw in my samples from Euden Beck were either single cells or pairs of cells, presumably following a recent division. Note how the second cell from the left in the figure below is not as straight as the others.   This is something that I often see with Fragilaria populations in streams in the northern Pennines, and indicates that there may be heavy metal pollution in the water.  There are a lot of abandoned lead mines in the northern Pennines and, sure enough, when I looked at a large scale map, I found one that I had not previously noticed in the upper part of Euden Beck’s catchment.

fragilaria_tenera.euden_jan19

Live cells of Fragilaria tenera(a. – d.) and F. heatherae from Euden Beck, January 2019.   a., b. and e. are valve views; c. and d. are girdle views.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

The next image shows these valve abnormalities even more clearly, with almost all of the cells showing aberrations in their outline.   These images are from an older sample; the curiosity here is that whilst most of the Fragilaria tenera valves were twisted, fewer of the valves of Fragilaria gracilisare twisted, whilst few of the valves of the third Fragilaria species show any abnomality in their outline at all.   This species is very common in northern Pennine streams, and I have often seen distorted valves of this species in streams polluted by mine discharges.  This makes the discrepancy between the outlines of this and Fragilaria tenera in Euden Beck particularly intriguing.

fragilaria_tenera_rt#55

Fragilaria tenera from a sample collected from Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King.

I say “Fragilaria gracilis” with a modicum of trepidation as a recent study in which I have been involved, suggests that there may well be at least two species.  These are, as far as we can tell, indistinguishable using characteristics that can be seen with the light microscope though we know that they are genetically quite distinct, and both are widespread, turning up not just in the UK but in other parts of Europe too.

The third species, to the best of our knowledge, does not match the description of any other Fragilaria species, and we are in the process of publishing it as a new species, Fragilaria heatherae.   We have found it a number of samples, not just from the UK but also from sites elsewhere in Europe.   These, by comparison with the other two species, show very little distortion at all.   Whilst several authors have noted this phenomenon in the past, the physiological cause is still not understood. My guess is that the metal ions are displacing a metal co-factor in an enzyme that is involved in the process of laying down the silica cell wall.   Fragilaria seems to be particularly susceptible, but this may be because their long needle-like cells show the distortions more clearly than in some genera but, based on the evidence from Euden Beck, there are clearly differences in susceptibility between species.

Once again, I seem to be ending a post having asked more questions than I have answered. That is always frustrating but another way of looking at this is to realise that the frontiers of ecology are only ever a short drive away from where you are now.  It is very nice to cross oceans to visit rain forests and coral reefs, but there are adventures to be had closer to your doorstep.

fragilaria_gracilis_rt#55

Fragilaria gracilis from a sample collected from Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King.

fragilaria_heatherae_rt#55

Fragilaria heatherae” from a sample collected in Euden Beck in June 2012.  Scale bar: 10 micrometres (= 1/100thof a millimetre).   Photographs: Lydia King

References

Duong, T.T., Morin, S., Herlory, O. & Feurtet-Mazel, A. (2008). Seasonal effects of cadmium accumulation in periphytic diatom communities of freshwater biofilms.  Aquatic Toxicology90: 19-28.

Falasco, E., Bona, F., Ginepro, M., Hlúbiková, D., Hoffmann, L. & Ector, L. (2009). Morphological abnormalities of diatom silica walls in relation to heavy metal contamination and artificial growth conditions.  Water SA35: 595-606.

McFarland, B.H., Hill, B.H. & Willingham, W.T. (1996). Abnormal Fragilaria spp. (Bacillariophyceae) in Streams Impacted by Mine Drainage. Journal of Freshwater Ecology 12: 141-149.

A year in the life of the River Wear …

After six bimonthly visits to the River Wear at Wolsingham during 2018, I can now step back and have a look at the complete dataset to see what patterns emerge.   Over the course of the year, I have visited the site six times and recorded a total of 107 species: 5 Cyanobacteria, 32 green algae, 69 diatoms and one red alga.  The true figure is probably higher than this, as the green algae include a number of “LRGT” (see “Little round green things …”) and certainly did not receive the same level of attention as the diatoms.

This crude enumeration of species, however, disguises some interesting seasonal patterns with, as I described in “Summertime Blues” and “Talking about the weather …”, abundant growths of green algae during the heatwave and associated low flow periods.  This can be seen clearly in the bar chart showing the seasonal changes in the river: diatoms predominate in the early part of the year whilst green algae are very scarce.  The bloom of the green filamentous alga Ulothrix zonata that I expected to see in March was missing due, I suspected, to the hard weather we experienced in late Feburary (see “The mystery of the alga that wasn’t there …”) but, by the summer, the river had taken on a very different complexion and was dominated by small green algae.   The last sample of the year, collected in November, showed a return to diatom dominance with a late autumn showing of Ulothrix zonata(see “The River Wear in November …”).

wear_summary_2018

Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2018.  

Looking back at records of a similar exercise in 2009, I see that the beginning and end of the year were quite similar, with thick biofilms dominated by diatoms; however, the algae in the summer of 2009 were very different to those I found in 2018.  My 2009 exercise involved visits every month rather than every other month and I see that I recorded more Cyanobacteria in June and July 2009 than I found in Summer 2018.  These were mostly filaments of Phormidium retziiand tufts of Homoeothrix varians, which I assumed to be a consequence of intense grazing (there is evidence that invertebrates find Cyanobacteria to be less palatable than other algae).  By July, Cyanobacteria comprised over half the total biovolume of algae; however, there was a major spate soon after my visit.  I was surprised to find, when I visited in August, a noticeably thicker biofilm smothering the rocks and, when I looked closely, this was dominated by the small motile diatom Nitzschia archibaldii.   The Cyanobacteria had disappeared almost completely.   I attributed this change to the invertebrate grazers being washed away by the spate, allowing the algae to grow unhindered.  As the biofilm grew in thickness, so the algal cells start to shade each other, and a diatom that can glide through the biofilm has an advantage over any that are stuck to one place.  Diatoms remained dominant for the remainder of the year, although my November sample came just after another storm and the stones I sampled were completely bare.

wear_summary_2009

Relative proportions (by approximate biovolume) of the main groups of algae found in the River Wear at Wolsingham during 2009.   A sample was collected in November but no living algae were recorded from it.

Overall, however, the similarities between the years outweighed the differences in the summer assemblages, whilst the composition of communities between late autumn and late spring was remarkably similar across the two years.   The changes in summer 2018 extended beyond just a shift in the balance of algae in favour of greens: there were also changes in the composition of diatoms too.  In fact, the changes in diatoms proved to be quite powerful mirrors of the changes in the community as a whole.  I have demonstrated this in datasets spanning a number of sites in the past but it is reassuring to see that they are also reflecting patterns within one site.   On the other hand, if I only had examined the diatoms, I would have missed some of the most interesting changes in the river over the course of the year.

Another observation is that no single sample from 2018 contained more than a quarter of the total algal diversity that I recorded over the course of the year.  Every month saw some new arrivals and some departures (or, more likely in some cases, a few taxa that were present had dropped below my analytical detection limit).  Some of these were expected (the seasonal dynamics of Ulothirx zonata, for example); others not (e.g. dominance by Keratococcus bicaudatusin the summer).  I discussed this in “A brief history of time-wasting …” and, in honour of that post, am not going to repeat myself here. In an age when our environmental regulators are cutting back on the amount of data that they gather, I shall go into 2019 reflecting on Yuval Noah Harari’s comment that “the greatest scientific discovery was the discovery of ignorance”.

The River Wear in November

Wear_Wolsingham_181119

I was back at the River Wear last week for my final visit of the year.   The heatwave that dominated the summer seems like an aeon ago as I plunged my arm into the cold water to find some stones and take some photographs.  I’m curious to see what is here, though.   The river has surprised me several times already this year.  Has it reverted to type as the British climate has regained a semblance of normality, or will the changes that we saw in the summer (see “Summertime blues …” and “Talking about the weather …”) still have consequences for the algae growing on the river bed?

The river bed itself had many patches of green filamentous algae which, on closer examination, turned out to be my old friend Ulothrix zonata, an alga that is common in these parts and which has a distinct preference for early spring conditions (see “Bollihope Bavakakra” and references therein).   A closer look showed two types of filament present: the normal vegetative ones with a single chloroplast encircling the cell but also some where the cell contents have divided to produce zoospores which are released and which, if they land on a suitable surface, will produce new vegetative filaments.   The “parent” filaments, themselves, are produced as zygotes, produced back in the spring, germinate.  The zygotes are the product of sexual reproduction, triggered by lengthening days (see reference in earlier post) and are dormant through the summer, only germinating once day length shortens and temperatures start falling.

Wear_Wolsingham_bed_Nov18

The river bed of the River Wear at Wolsingham, November 2018, showing conspicuous growths of Ulothrix zonata.

Ulothrix_zonata_Nov18

Magnified views of Ulothrix zonatafilaments from the River Wear at Wolsingham.  The upper image shows a vegetative filament and the lower image shows filaments where the cell contents have divided up prior to the release of zoospores.  Scale bar: 20 micrometres (= 1/50thof a millimetre).

The areas between the patches of Ulothrix zonatawere covered with a thick film, composed primarily of diatoms, in contrast to the situation on my last two visits when non-filamentous green algae predominated.  This time, it was Achnanthidium minutissimumdominated my count (about 70% of cells) although, because they are relatively small, they comprised just under half of the total volume of algae present.   Other diatoms bumped this up to about 70 per cent of the total volume, with motile cells of Navicula and Nitzschia, which were so abundant at the start of the year, beginning to appear in numbers again.   The green cells that dominated my counts in July and September now only constitute about five per cent of the total.   The River Wear, in other words, has shaken off the effects of the summer, just as a healthy human gets over a winter cold, and is now back to its old self.

Wolsingham_181119_#1

A view down my microscope whilst examining samples from the River Wear at Wolsingham showing the predominance of Achnanthidium minutissimum with (on the right-hand side) a filament of a narrow Ulothrix (not U. zonata).  

More green algae from the River Wear

Having discussed some of the recent name changes in green algae in the previous post, I thought that I would continue this theme using some of the other taxa that I found in the samples I collected from the River Wear a couple of weeks ago.   The plate below shows some specimens that, 20 years ago, I would not have hesitated to call Scenedesmus, characterised by coenobia of either four cells or a multiple of four cells arranged in a row.   Over 200 species, and 1200 varieties and forms have been recognised although there were also concerns that many of these so-called “species” were, in fact, variants induced by environmental conditions.  A further problem is that Scenedesmus and relatives do not have any means of sexual reproduction.  This means that any mutation that occurs and which does not have strong negative effects on the organism will be propagated rather than lost through genetic processes.  Working out what differences are really meaningful is always a challenge, especially when dealing with such tiny organisms.

Scenedesmus and Desmodemus species from the River Wear, Wolsingham, September 2018.  a. and b. Scenedesmus cf ellipticus; c. Desmodesmus communis.   Scale bar: 20 micrometres (= 1/50th of a millimetre).

The onset of the molecular era shed some new light onto these problems but, in the process, recognised differences within the genus itself that necessitated it being split into three, two of which are on the plate below.  Scenedesmus, in this modern sense, has cells with obtuse (rounded) apices and mucilage surrounding the cells whilst Desmodesmus has distinct spines at the apices of marginal cells and, sometimes, shorter ones elsewhere too.   In addition to these there is Acutodesmus, which is similar to Scenedesmus (i.e. without spines) but whose cells have more pointed (“acute”) ends and which does not have any surrounding mucilage.   A further genus, Pectinodesmus, has been split away from Acutodesmus on the basis of molecular studies, although there do not seem to be any features obvious under the light microscope which can differentiate these.

The genera Ankistrodesmus and Monoraphidium present a similar situation.  In the past, these long needle- or spindle-shaped cells would all have been considered to be Ankistrodesmus.   Some formed small bundles whilst others grew singly and this, along with a difference in their reproductive behaviour, was regarded as reason enough for splitting them into two separate genera.   Both were present in the Wear this summer, but only Monoraphidium presented itself to me in a manner that could be photographed.  Two species are shown in the plate below.   Recent molecular studies seem to not just support this division but also suggest that each of these could, potentially, be divided into two new genera, so we’ll have to watch out for yet more changes to come.

Monoraphidium species from the River Wear, Wolsingham, September 2018.  a. and b.: M. griffthii; c. M. arcuatum.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

The final illustration that I managed to obtain is of another common coenobium-forming alga, Coelastrum microporum.   Though the three-dimensional form makes it a little harder to see, cell numbers, as for Pediastrum, Scenedesmus and Desmodesmus, are multiples of four.  I apologise if the picture is slightly out of focus, but it is a struggle to use high magnification optics on samples such as these.  The oil that sits between the lens and the coverslip conveys the slight pressure from fine focus adjustments directly to the sample, meaning that the cells move every time I try to get a crisper view.  That means it is impossible to use my usual “stacking” software.   The answer is to use an inverted microscope so that the lens was beneath the sample.  However, I do this type of work so rarely that the investment would not be worthwhile.

That’s enough for now.   I’m off on holiday for a couple of weeks, so the next post may be from Portugal and perhaps I will also find time to sample the River Duoro as well as the products of the vineyards in it’s catchment…

Coelastrum microporum from the River Wear,Wolsingam, Septmber 2018.  Scale bar: 20 micrometres (= 1/50th of a millimetre).

References

An, S.S., Friedl, T. & Hegewald, E. (2008).  Phylogenetic relationships of Scenedesmus and Scenedesmus-like coccoid green algae as inferred from ITS-2 rDNA sequence comparisons.   Plant Biology 1: 418-428.

Hegewald, E., Wolf, M., Keller, A., Friedl, T. & Krienitz, T. (2010).  ITS2 sequence-structure phylogeny in the Scenedesmaceae with special reference to Coelastrum (Chlorophyta, Chlorophyceae), including the new genera Comasiella and Pectinodesmus.   Phycologia 49: 325-355.

Krienitz, L. & Bock, C. (2012).  Present state of the systematics of planktonic coccoid green algae of inland waters.   Hydrobiologia 698: 295-326.

Krienitz, L., Bock, C., Nozaki, H. & Wolf, M. (2011).   SSU rRNA gene phylogeny of morphospecies affiliated to the bioassay alga “Selanastrum capricornutum” recovered the polyphyletic origin of crescent-shaped Chlorophyta.  Journal of Phycology 47: 880-893.

Trainor, F.R. & Egan, P.F. (1991).  Discovering the various ecomorphs of Scenedesmus: the end of a taxonomic era.   Archiv für Protistenkunde 139: 125-132.