Whatever doesn’t kill you …

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.

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 (= 100^th 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, 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 (= 100^th 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 ..

A 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.

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.

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.

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.

The littoral ecology of Lough Down

My recent overviews of the major groups of algae have been useful as a way of highlighting which families and orders I’ve neglected.  That’s mostly because my posts are largely reactions to the circumstances I find myself in, rather than as a comprehensive overview of the world of freshwater algae.  My travels, this week, have brought me to Lough Down, a little-known Irish lake to which many of us have journeyed over the past few weeks.   Today, I thought I would peer at the littoral zone in the hope that I might find an alga from an order I have not previously written about.

I seem to be in luck: there has not been much rain recently and there is recently-exposed mud.  When I look closely, I see tiny green spheres, each the size of a pin head, dotted across the mud surface.   These are vesicles of the alga Botrydium and, despite their bright green colour, they actually belong to the Xanthophyceae (see “When a green alga is not necessarily a Green Alga …”).   Below this pear-shaped vesicle there is a system of branched rhizoids which anchor the plant to the surface (see lowermost photograph).  Surprisingly, the whole plant is a single cell, a similar situation to the one we encountered in Vaucheria, another representative of the Xanthophyceae (see “The pros and cons of cell walls”).

The vesicle itself is green, and a close look reveals the presence of many chloroplasts and also (less easy to see without special stains) nuclei.   You can also see, on the image below, crystals of calcium carbonate which are deposited on the vesicle.   However, if the marginal mud where Botrydium thrives is flooded again, the cell contents divide into a large number of spores, each with a single nucleus and two flagellae, which are liberated.  On the other hand, if the pond continues to dry, then a different type of spore is produced, as the cell contents retracts into the rhizoids where it forms thick-walled spores, which can survive long periods of desiccation.   Once the mud is dampened again, these spores germinate into motile spores.

A vesicle of Botrydium granulatum spotted with crystals of calcium carbonate.  The photograph at the top of the post shows vesicles on the bed of a pond (both photos: Chris Carter).

Botrydium is a small genus, with just eleven species listed on AlgaeBase, of which just one, B. granulatum, is recorded from the UK and Ireland.  It fills in one glaring gap in my coverage of the Xanthophyceae, leaving just two Orders still to feature.  These are the ones containing the awkward little single cells and colonies that are difficult to identify and easy to confuse with similar-shaped forms in the Chlorophyta.   I’ll get around to writing about these one day.   Maybe I’ll find representatives from them on a future visit to Lough Down.  Who knows how much time I will have to become acquainted with this fascinating lake?

A complete Botrydium granulatum plant, showing the vesicle on the left with a series of rhizoids extending out below.  The lowermost rhizoids are obscured by soil particles (photo: Chris Carter).  All images in this post are from material collected from Pitsford Water, Northamptonshire.

 Some other highlights from this week:

Wrote this whilst listening to: J.S. Bach’s St Matthew’s Passion and Laura Marling’s new album, Song for Our Daughter.  My systematic review of Dylan’s back catalogue has reached the incomparable Blonde on Blonde.

Cultural highlights:  Portrait of a Lady on Fire.   French arthouse film.  I know, I know …

Currently reading:  Drawing to the close of The Mirror and The Light .

Culinary highlight:   Buying a bag of strong white flour after many abortive attempts.   And a Simnel Cake, in celebration of Easter.

 

Blessed are you that hunger …

At the time of writing, four of my five working days are given over to ecology whilst the fifth is spent volunteering for the local Foodbank, which is gearing itself for a huge run on the stocks built up from generous donations over the Christmas period.   It occurred to me last week that I spend four days extolling hunger in an ecological context whilst spending the fifth trying to alleviate it in a human one.

“Hunger” in an ecological context is a them to which I have returned several times over the years.   We set the threshold for “good ecological status” for attached algae at a point that we thought coincided with the algal community switching from species that were adapted to “stressed” (i.e. nutrient-poor) conditions to those adapted to compete when nutrients were not in short supply (see “What does it all mean?” and references therein).   I’ve also talked, in some of my posts, about the adaptations some algae have to scavenge scarce nutrients (“A day out in Weardale”).   We’ve then gone on to try to work out what that means in terms of nutrient concentrations in UK and European rivers (references at the end of the post).

So I was pleased to see a paper appear last week that confirms some of these hunches.   Broadly speaking, Eleanor Mackay and colleagues have shown, using in situ bioassays, that as the concentration of inorganic nutrients decreases so the algae make more use of phosphorus and nitrogen bound into organic complexes.  As the algae get more “hungry”, in other words, they become more adept at scavenging for the resources that they need.

The graph at the top of this post is the graphical abstract from the paper which summarises this, whilst the one below shows the response to organic sources of phosphorus as a function of the concentration of “soluble reactive phosphorus” (the standard measure of “inorganic” phosphorus).  I’ve added an arrow to the right-hand side of this which shows roughly the current UK threshold, based on the work mentioned above.   Ellie’s graph seems to be confirming that, once that limit is exceeded, the algae are no longer “hungry”, meaning that they no longer need the nutrients bound into organic complexes.  Because organic phosphorus utilisation depends upon production of phosphatase enzymes to break down the organic complexes to releasee the phosphorus, there must be a greater energetic cost to the organism than if there was a ready supply of inorganic phosphorus that they can access.  I have, I must admit, never seen any figures that quantify this cost.

Fig. 5c from Mackay et al. (2020):  The relationship between “soluble reactive phosphorus” and dissolved organic phosphorus use by algae in in situ bioassays.  The “response ratio” is the natural logarithm of the ratio between the chlorophyll concentration of the treatment and the chlorophyll concentration of the corresponding control.  The arrow on the right-hand side indicates the approximate position of the regulatory threshold for phosphorus (see note at end of post).  The figure at the top of the post is the graphical abstract from Mackay et al. (2020). 

Part of me, then, is reassured that the regulatory threshold for phosphorus is roughly in the right place.  The Environment Agency’s reliance on a single measure of inorganic phosphorus, measured infrequently, is often criticised by hydrochemists but we can take some comfort from knowing that other forms of phosphorus (more difficult to analyse and quantify) only become important at concentrations lower than the current UK targets.   There is still part of me, however, that sees room for improvement.  That there are relationships between algae and other plants and phosphorus is not in doubt, and I am sure that a shift in strategies for nutrient acquisition help to define this relationship, particularly at low concentrations.  However, the relationships are not very strong and predictions about the ecological benefits of lowering phosphorus concentrations are imprecise.

Adding another strand of evidence to the current decision-making process makes scientific sense, and looking at how organisms respond to nutrients, rather than just measuring chemistry and describing community structure, seems like a sensible way of doing this.   In situ bioassays clearly have potential, as this paper shows; however, they are time consuming.   An alternative would be to measure phosphatase activity directly.  The Environment Agency did, in fact, fund research on this in the late 1990s and David Harper used these assays in a DEFRA-funded project in the early 2000s, but they have never become routine.  That’s a shame because, particularly for catchment-level investigations, they could add a useful additional insight.

The downfall of all these methods is not science, but the “more-with-less” ethos that has prevailed in the UK public sector for the past decade.  Everyone recognises that diffuse nutrient pollution offers a challenge that current monitoring and decision-making processes struggle to address.  However, most of the serious research effectively concludes with “if you spend a lot more money, you’ll discover that the problem is more complicated than you initially thought”.   That’s a difficult message to pass up through managerial hierarchies trying to keep a cash-starved regulatory agency moving forward.

Reference

Mackay, E. B., Feuchtmayr, H., De Ville, M. M., Thackeray, S. J., Callaghan, N., Marshall, M., Rhodes, G., Yates, C.A., Johnes, P.J. & Maberly, S. C. (2020). Dissolved organic nutrient uptake by riverine phytoplankton varies along a gradient of nutrient enrichment. Science of the Total Environment 722: 137837.   https://doi.org/10.1016/j.scitotenv.2020.137837

Poikane, S., Kelly, M. G., Salas Herrero, F., Pitt, J. A., Jarvie, H. P., Claussen, U., Leujak, W., Solheim, A.S., Teixeira, H. & Phillips, G. (2019). Nutrient criteria for surface waters under the European Water Framework Directive: Current state-of-the-art, challenges and future outlook. Science of the Total Environment 695: 133888. https://doi.org/10.1016/j.scitotenv.2019.133888

 

Note: regulatory threshold for inorganic phosphorus

The arrow indicating the approximate position of the regulatory threshold for phosphorus uses the current UK TAG phosphorus standard.   This is site specific, using altitude and alkalinity as predictor variables.  This means that a range of thresholds is possible and the position of the arrow reflects the average alkalinity (50 mg L^-1 CaCO₃) and altitude (75 m) in a database of river samples collected as part of DARES project. Note, too, that P standards are based on the Environment Agency’s standard measure, which is unfiltered molybdate reactive P.  This approximates to “soluble reactive P” or “orthophosphate-P” in most circumstances but the reagents will react with phosphorus attached to particles that would have been removed by membrane filtration.

 

Some other highlights from this week:

Wrote this whilst listening to: My lockdown project of listening to all Bob Dylan’s albums in sequence has brought me up to Bringing It All Back Home and Highway 61 Revisited.

Cultural highlights:  Bait, a low-key black and white British film from 2019.  Definitely sits in the “sub hero” genre that I much prefer to the crash, bang, wallop of most Hollywood blockbusters.

Currently reading:  About three-quarters of the way through Hilary Mantel’s The Mirror and The Light now.  Jane Seymour is gone; Anne of Cleeves coming up next.

Culinary highlight:   Grilled mackerel with sautéed potatoes, probably.  A close second was home-made tortellini filled with mushroom paté and served with garlic mustard (Alliaria petiola) butter.   Mrs K is forager-in-chief hereabouts.