Costing the earth?

Striding_Edge_Sept19

This is the third general election that has taken place during the lifetime of this blog and, looking back at the posts I wrote in the run-up to the 2015 and 2017 elections, I see one overwhelming difference.  In both 2015 (see “A plague on both their houses”) and 2017 (see “How green is my party?“) I lamented the lack of focus that the major parties gave to the environment.   The same is not true this time, though the way that the major parties approach the issue varies considerably.

I’m not going to go through the manifestos in detail: there is a good comparison of environmental policies on the BBC website that already does this.  Instead, I want to take a step back and look at the broader context and, bearing in mind the present lead that the Conservatives have in the polls, I think it is appropriate that their environmental pledges get the closest scrutiny.   As is always the case, manifestos are glossy brochures that can be frustrating documents for anyone interested in the nuts-and-bolts of implementation. However, in the case of the Conservatives, we have a better idea of what their promises will look like in practice as their Environment Bill was working its way through Parliament when the election was called. The implication from their manifesto is that this will be picked up again (perhaps with some modifications) if they are re-elected.

Greener UK, a coalition of environmental organisations in the UK sees “many welcome measures” in the Environment Bill, but has a number of concerns (summarised in this briefing paper.   There was a hope that some of these could be addressed during the second reading and committee stages but my fear would be that a Conservative government with a Parliamentary majority would be in a position to drive through the bill without taking these concerns into account.  One of my key concerns (shared with Greener UK) is that the proposed Office for Environment Protection is not sufficiently independent from DEFRA to be truly independent (see “(In)competent authority”).

My major concern, however, is not what the Conservatives promise to do, but whether they will provide the administrative infrastructure that allows this to happen.   Earlier this week, I spent a day in a meeting with representatives from the Environment Agency, Natural England, a utility company and a rivers trust, considering the actions needed to manage one catchment in northern England.   There were ten of us around the table, all bringing different perspectives and expertise to the discussion.   This is a timely reminder that, whilst the politicians might want to present a series of neat prescriptions to drive environmental improvement, the reality will always be more complicated.   Conservative (and Labour, Lib-Dem and Green) promises need to be backed up by the resources (and adequate staff) to allow the ambition encapsulated in the manifestos to be fine-tuned to local circumstances.

And this is where the Conservative manifesto falls down.  The Institute for Fiscal Studies has looked at the spending pledges of all the major parties and found that neither Labour nor the Conservatives have spending plans that are in line with their manifesto commitments (you can see an analysis of the IFS report by the BBC’s Reality Check team, based on the IFS report here.  However, whilst Labour and the Lib-Dems are, at least, honest about the need to raise extra revenue through taxation and borrowing (even if Labour’s sums don’t add up), the Conservatives actually make a promise not to raise rates of income tax, VAT or national insurance.   That might play well, at a purely superficial level with voters with a libertarian bent, but will lead either to a gulf between ambition and delivery or to broken promises.

As 2020 dawns, the manifestos will be largely forgotten and newspaper headlines will focus on the crisis in the NHS.  We’ll still see articles about the climate emergency but, unless provoked by extreme events such as flooding, concrete pledges on spending will be few and far between.   The Environment Agency will still be limping along with senior management trumpeting a “more with less” ethos whilst their Poor Bloody Infantry will, if the polling is accurate, have barely enough resources to make the shrewd interpretations of the limited data now available that are necessary for effective decisions at a local scale.

At the root of my concerns is a question about whether the preoccupation of neoliberals with “small government” can ever be compatible with environmental management. Turning this around, the reason why academics and environmental professionals tend to be pro-EU is that they understand that the scale of the problems facing us is such that the broadly Keynesian, interventionist approach that the EU espoused was more likely to bring changes than laissez-faire economics.  That is not to say that the EU gets everything right (the Common Agricultural Programme, for example, the Common Agricultural Programme, for example, desperately needs an overhaul), just that it is going to be difficult for the UK to achieve the goals set out in all the major party manifestos unless we are prepared to pay for a public sector with a budget that is proportional to the task in hand.

Castle Eden Dene in November

Castle_Eden_Burn_Nov19

For the first time this year, I heard Castle Eden Burn before I saw it.  Walking down from the car park, the distant roar of water was apparent almost as soon as the canopy of largely leafless branches closed over me.  A few trees still held their leaves – spectacularly golden on beech and birch, in particular, and the Dene’s famous yews were still green, of course – but the forest was dressed for winter now, much as it was on my first visit this year, back in January (see “Castle Eden Dene in January”).  Then, I was surprised that there was no water in the Burn.  On this trip, however, I wore my chest waders.  Back in August, I had compared Castle Eden Burn to a wadi (see “The presence of absence in Castle Eden Dene”) so the heavy rain of the previous few weeks had led me to suspect that today would be different.

The water surging through the Dene was very turbid, so collecting stones to examine involved feeling around on the river bed with my hand until I located one that was not sufficiently bedded into the substratum to remove.   That’s not ideal, but needs must and I got the five cobbles I needed, each with a distinct biofilm, slimy to the touch.  This is the first time, after eleven months, that Castle Eden Burn’s substratum has looked and felt remotely like the substratum from most of the other rivers I know in this part of the world.

Under the microscope, I see lots of particulate matter but also plenty of algae.   Apart from a few filaments of the cyanobacterium Phormidium, these were mostly diatoms.   The green algae I described in “When the going gets tough …” back in May were not obvious.  The diatoms were mostly largely motile cells of Navicula, with a few sigmoid cells of Nitzschia clausii and some smaller cells whose identity I will need to confirm once I have cleaned the sample and prepared a permanent slide.  The Navicula species, in particular, are typical inhabitants of local rivers during winter and early spring, all tolerant to a wide range of conditions.   I suspect that the rainfall has washed a lot of fine particulate debris from the industrial estates in the upper catchment into the river, and these diatoms will have the resilience to cope with such types of pollution.  A large storm sewer overflow also empties into the burn about a kilometre upstream of where I was standing and this, I suspect, has been flowing over the past month or two.

I also saw a few cells of Achnanthidium minutissimum, which I generally associate with cleaner conditions.  I suspect, however, that numbers will be relatively low compared to its more pollution-tolerant brethren.   Again, I can give a more authoritative answer once I have cleaned the sample and performed a full analysis.

CEB_diatoms_Nov19

Diatoms from Castle Eden Burn, November 2019.  a., b.: Navicula trpunctata; c. – e.: Navicula lanceolata; f., g.: Rhoicosphenia abbreviata; h., i.: Nitzschia clausii; j., k.: Navicula gregaria; l. Achnanthidium minutissimum.   Scale bar: 10 micrometres (= 1/100thof a millimetre).   The photograph at the top of the post shows Castle Eden Burn just downstream from the point I sampled.

I originally set out to visit Castle Eden Burn six times during 2019 and this was the last of those. I’ve written about most of these visits already but not about my September visit.  There was, on that occasion, little new information to justify a separate post but I will include the sample I collected in my final overview of the algae of Castle Eden Burn, just as soon as I get this final sample cleaned and analysed.   Before then, I have one more post to write about the diatoms, based on some more detailed observations of a few of the species, and then it will be time to think about where to focus my observations during 2020.

Diatoms from the Troodos mountains

Troodos_snowscape_Apr19

Back in April, I wrote two posts about the algae from a stream draining a chromite mine in the Troodos mountains in Cyprus (see “Survival of the fittest (1)” and “Survival of the fittest (2)”).  I also planned to write a post about the diatoms growing in the stream but the slide I prepared has been sitting on my desk over the summer whilst I was distracted by other things.  However, I have just started looking at some samples from metal-enriched streams in the northern Pennines and, curious to see whether a Cypriot chromite mine had similar effects, I blew the dust off the slide and slipped it under my microscope.

The principal effect of toxic pollution is to reduce the number of species found and, in this respect, my sample from the outflow of the Hadjipavlou mine outflow was true to form, containing just eight species.  The most abundant of these was Meridion circulare, accounting for one in four of all the cells.  What is more, many of the cells were visibly distorted (see images a., c. and d., in particular, in the plate below).  This is quite a common phenomenon in metal-polluted streams (see “A twist in the tale”) though I have not seen it quite so obviously in Meridion circulare before. My own pet theory is that one of the enzymes involved in laying down the silica cell wall has a metal co-factor that is displaced by heavy metals.

Meridion_circulare_Hadjipavlou_Apr19

Meridion circulare from thepebbles from the stream draining Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.  Scale bar: 10 micrometres ( = 1/100th of a millimetre).   The photograph at the top of the post shows snow on the Troodos mountains near the mine.

The only other diatom that was at all common in the sample was Hantzschia amphioxys, which also occurred alongside a smaller population of Hantzschia abundans.  I’ve not come across Hantzschia in metal-enriched streams before: it is a species that is most often associated with habitats that are not permanently submerged.  That may be the case at Hadjipavlou but the water that flows from mines comes from groundwater rather than rainfall so would not be subject to the strong seasonal variations that we associate with Mediterranean streams.  It is hard to draw a firm conclusion from a single visit.   Unlike Meridion circulare, however, neither population of Hantzschia showed any obvious distortion, perhaps due to the Hantzschia cells being more heavily silicified than those of Meridion circulare.

The extent to which cellular distortions are obvious does vary between species, as can be seen in “A twist in the tale …”  which compared three different representatives of the same genus in a metal-polluted stream.  I chose the word “obvious” with care as I do think that these phenomena are more easily seen in long thin cells than in shorter ones.  In the same Pennine streams where distorted Fragilaria are common, for example, I can also see distorted cells of smaller diatoms such as Achnanthidium minutissimum.  But you need a keen eye to spot these reliably.   Some other people have used fluorescent stains to look at other cellular irregularities, such as the position of the nucleus and damage to the nuclear membrane, but these require specialist approaches whereas distortions to cell outlines can be spotted from a standard analysis.

Hantzschia_spp_Hajipavlou_Apr19

Hantzschia abundans (k., l.) and Hantzschia amphioxys (m. – p.) in the from the stream draining Hadjipavlou chromite mine in the Troodos mountains, Cyprus, March 2019.  Scale bar: 10 micrometres ( = 1/100th of a millimetre). 

A few years ago I was involved in a study of diatoms from streams in Cyprus and I dug out some of these data in order to put the Hadjipavlou sample into context.  One immediate surprise was that many of the “reference” (i.e. pristine or near-pristine) samples in that survey also had relatively low diversity.   The 45 samples in this subset had, on average, nine species, and a mean Shannon diversity index of 1.7, compared to eight species and a Shannon diversity index of 1.42 for the Hadjipavlou sample.   I’ve never been a fan of diversity indices as measures of ecological quality (see “Baffled by the benthos (2) and links therein”) although I suspect that average diversity at Hadjipavlou measured over a period of time will always be low whereas average diversity at unimpacted sites is more likely to fluctuate. Equally, low diversity coupled with a second strand of evidence, such as distorted valves, is a useful sign to an ecologist that something untoward is happening.

diversity_indices

Number of taxa (left) and Shannon diversity (right) recorded in 45 samples from “reference” sites (i.e. minimal evidence of anthropogenic alteration) in Cyprus.  The arrows indicate the location of the Hadjipavlou stream within this dataset. 

The irony of writing about a heavily-polluted stream in the Troodos mountains is that the geological conditions which created the metal-rich veins hereabouts also create conditions for many plants endemic to Cyprus.   The serpentine and other ultramafic rocks create metal-rich soils within which few plants can survive (more about these here. I suspect that few of the plant enthusiasts drawn to Cyprus will ever cast more than a cursory glance at the green flocs adorning the abandoned mines of the Troodos mountains.

References

Licursi, M., & Gómez, N. (2013). Short-term toxicity of hexavalent-chromium to epipsammic diatoms of a microtidal estuary (Río de la Plata): Responses from the individual cell to the community structure. Aquatic Toxicology 134-135: 82-91.  https://doi.org/10.1016/j.aquatox.2013.03.007

Messy bedrooms …

Sand_Loch_May19

When I was tramping around the Shetland Islands earlier this year (see “Hyperepiphytes in the Shetland Islands“), looking at the algae that live in the freshwater lochs, I noticed some meandering hieroglyphs made from fine sediment on the tops of some of the stones in the littoral zone.   I see these occasionally at other places too, and know that they are the “galleries” of caseless caddis flies.  Caddis flies are close relatives of the butterflies and are best known because many of their larvae use “found materials” (in contemporary art jargon) to construct cases to protect themselves.  Some species use fine gravel, silt and sand, some use fragments of plants, some have cases that are very neat, some have a more haphazard approach to construction.  However, a few families of caddis flies eschew cases and, instead, build these galleries.

Many caddis fly larvae, whether cased or not, are grazers, scraping the algae off the rocks on the bed of the stream or lake.   There is evidence that the cases offer some protection against predators such as trout which, by increasing survival rate, means that it is worthwhile for the caddis larvae to divert some of their hard-earned energy into building these.   Presumably, their caseless cousins gain the same advantage to building their galleries but recent research has suggested that these galleries offer a further benefit.

Think of caddis larvae as adolescent caddis flies.  Now imagine that the caddis gallery is the equivalent of an adolescent’s bedroom.   Horribly messy, in other words.   Let’s leave that image of a teenager behind (as most human teenagers know their way to the bathroom) and consider what happens to all that waste material that emerges from the far end of a caddis larva’s digestive system.   This nutrient-rich “ manure” encourages algae, meaning that our caseless caddis flies are, in fact, gardeners and are able to tap into this extra energy resource within their galleries in order to grow.   That brings us back to the analogy with teenagers, as these also frequently graze in their bedrooms (the diatom Campylodiscus is even the same shape as a Pringle, whose empty containers litter the bedroom floor of my own progeny).   I guess it is a good thing that caddis larvae don’t wear socks as, with six legs and two prolegs, the mess inside the gallery would be indescribable.

Psychomiiddae_Sand_Loch_May19

Galleries of caseless caddis flies (possibly Psychomiidae) on the top surface of a cobble from Sand Loch, Shetland Islands with (right) a close-up of a single gallery. The photograph at the top of the post shows Sand Loch in May 2019.

A recent study in the Lake District has shown that this “gardening” means that the algae which grow in the fine sediment from which the galleries are constructed are different to those found elsewhere on the rock surface, with a greater proportion of diatoms, which are considered to be more palatable to invertebrates than other types of algae.  Some caddis flies are thought to go even further, and can selectively remove and discard the algae that are least palatable (some Cyanobacteira, for example).

It is possible that up to 40% of the larva’s energy needs are met from the gallery itself.   The tube is, in fact, not a static construction: the larva pokes its head out in order to graze the algae immediately in front of the gallery, and extends the gallery as the food supply within easy (and safe) reach is exhausted.   At the same time, it is consuming the alga-rich rear part of the gallery (reminiscent of Hansel and Gretel eating the gingerbread house?).   A gallery only has a life-span of 10 days in the laboratory; whether this is the same under field conditions is not clear but that gives us some idea of the transience of these structures.   This rapid turnover means that the caddis larva is always feeding on succulent early-succession species, rather than the tougher and less digestible algae that might appear in more mature biofilms.

I also see similar galleries on the bed of the River Ehen from time to time but have been told that these are formed by non-biting midge (chironomid) larvae, rather than by caddis.  I presume that the same processes are happening in these although I have not been able to find much written in the literature.

Organisms that can significantly alter the habitat in which they live, and affect the conditions experienced by other species in the habitat are termed “ecosystem engineers”.  Beavers are good examples, as their dams can have significant effects on organisms extending for hectares.  Yet, in their own small way, caseless caddis larvae are also ecosystem engineers.  As are adolescent boys.   Which makes me wonder, having only talked until now about the algae in their galleries, whether caseless caddis larvae also have patches of mould extending up their walls.

Chironomid_galleries_Ehen_March19

Galleries made by chironomid larvae on a boulder in the River Ehen, March 2019.

References

Hart, D. D. (1985). Grazing insects mediate algal interactions in a stream benthic community. Oikos 44: 40-46. https://doi.org/10.2307/3544041

Johansson, A. (1991). Caddis larvae cases (Trichoptera, Limnephilidae) as anti-predatory devices against brown trout and sculpin. Hydrobiologia 211: 185-194. https://doi.org/10.1007/BF00008534

Ings, N. L., Hildrew, A. G., & Grey, J. (2010). Gardening by the psychomyiid caddisfly Tinodes waeneri: Evidence from stable isotopes. Oecologia 163: 127-139. https://doi.org/10.1007/s00442-009-1558-8

Ings, N. L., Grey, J., King, L., McGowan, S., & Hildrew, A. G. (2017). Modification of littoral algal assemblages by gardening caddisfly larvae. Freshwater Biology 62: 507-518. https://doi.org/10.1111/fwb.12881

Otto, C., & Johansson, A. (1995). Why do some caddis larvae in running waters construct heavy, bulky cases? Animal Behaviour 49: 473-478. https://doi.org/10.1006/anbe.1995.0061

The devil lies in the detail …

Our latest ring test* slide took us on a vicarious journey to the beautiful River Don in Aberdeenshire.  Maybe because I have been doing this job for so long, but the quality of the landscape was clear to me as I peered through my microscope 500 kilometres away: the range of diatoms that I could see would not have thrived anywhere with more than the lightest touch from humankind.

One of the clues for me lay in some of the smallest diatoms on the slide.   It took some discussion amongst my fellow experts, but we eventually came up with a list of five different species of Achnanthidium (all illustrated below) which, together, constituted about a third of all the diatoms on the slide (admittedly, because they are small, they constitute rather less than a third of the total volume of diatoms, but that is another story ….).   The mere presence of several Achnanthidium species is, in my experience, usually a sign of high habitat quality (see “Baffled by the benthos (2)”) but unravelling the identities of the different species with a light microscope is challenging.

Achnanthidium-minutissimum-Medwin_WaterAchnanthidium minutissimum from Medwin Water, Scotland. Photographs from the Diatom Flora of Britain and Ireland by Ingrid Jüttner.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

Achnanthidium_pyrenaicum_Towie

Achnanthidium pyrenaicum from the River Don, Towie, Aberdeenshire.  Photographs by Lydia King.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

The genus Achnanthidium is a good example of the delicate co-existence between “identification” and “taxonomy” in the world of diatoms.   Individuals from this genus are usually small so anyone using a light microscope for routine analyses will be working right at the optical limits of their equipment whilst anyone with a serious interest in taxonomy will depend upon a scanning electron microscope for the insights needed for critical differentiation between species.

This divergence between the working methods of “identifiers” and “taxonomists” means that it is rarely possible to name every individual of Achnanthidium with complete confidence.  The ones that present clearly in valve view (i.e. face-up) can mostly be assigned to a species based on features we can see with a light microscope, but it is not always straightforward for those seen in girdle view (i.e. on their side) or which are partly obscured by other diatoms or extraneous matter on the slide.   In this example from the River Don, we also noticed that smaller individuals of A. gracillimum lost their characteristic rostrate/sub-capitate ends and were, as a result, not easy to differentiate from A. pyrenaicum.

Achnanthidium_gracillium_Towie_Water

Achnanthidium gracillimum from the River Don, Towie, Aberdeenshire.  Photographs by Lydia King.   Scale bar: 10 micrometres (= 1/100thof a millimetre). 

What continues to mystify me is why so many closely-related species can live in such close proximity. It is Achnanthidium that prompt this question here, but other genera display similar tendencies (see “When is a diatom like a London bus?”).  And this immediately generates another question: why are more people not asking this question of diatoms and, indeed, microscopic algae in general?

The answer to that question falls into two parts. The first is that understanding the precise ecological requirements of microscopic algae is not a trivial task, and assumes that you are able to get several closely-related species to live in culture (which, itself, assumes you know the precise ecological requirements of each … you see the problem?).   There is, as a result, a tendency to avoid experimental approaches and, instead, look for how species associate with likely environmental variables in datasets collected from sites exhibiting strong gradients of conditions.   However, this assumes that the forces that drive the differentiation between species work at the same scale at which we sample (see “Our patchwork heritage …” for more on this).

Underlying this, however, is a deeply-held belief, dating back at least forty years, that the niches of freshwater diatoms are determined primarily by the chemistry of the overlying water.   This is a dogma that has served us well when using diatoms for understanding the effects of environmental pollution but which is, ultimately, a limitation when trying to explain why we found five separate Achnanthidium species in a single sample, all exposed to the same stream water.

Achnanthidium_lineare_&_affine_Towie

Achnanthidium lineare (first three images from the left) and A. affine (two images on the right) from River Don, Towie, Aberdeenshire.  Photographs by Lydia King.  Scale bar: 10 micrometres (= 1/100thof a millimetre). 

I will go one step further: this dogma is so deeply held that referees rarely challenge the weak evidence that is produced to demonstrate different responses to environmental conditions between closely-related species.  There are certainly variations in environmental preferences between Achnanthidium species, but these are best expressed as trends rather than unambiguous differences and I have never seen such trends subject to rigorous statistical testing.

I blame better microscopes: greater magnification and resolution has revealed such a baffling amount of diversity that all the energy of bright diatomists is absorbed unravelling this rather than trying to explain what it all means (see “The meaning of … nothing”).  If we were bumbling along with the quality of equipment that Hustedt depended upon, then maybe we would be cheerfully lumping all these forms together and focussing on functional ecology instead.   Maybe.

* see “Reaching a half century” for more about the ring test scheme

Spheres of influence

Back to Moss Dub for this post because Chris Carter has sent me some stunning images of the filamentous desmid Desmidium grevillei that I talked about in my earlier post.   I mentioned that it is surrounded by a mucilaginous sheath, which was just apparent in my brightfield image.   Chris has added Indian ink to the wet mount.  The ink forms a dense suspension in the water but is repelled by the mucilage around the desmid cells, resulting in a much better impression of the extent of the sheath around the cell than is otherwise possible.

Desmidium-grevillei_CCarter_#1_Sept19

Desmidium grevillei from Moss Dub, photographed by Chris Carter using Indian ink to highlight the mucilage sheath around the cells. 

Indian ink is a negative stain, which means that it is the background, rather than the specimen itself, which takes up the colour.   This, in turn, alters the passage of light through the sample and appears to improve the contrast of the final image.   Chris’ images of the apical view show this well, and also illustrates the complicated three-dimensional arrangement of the chloroplasts within each semi-cell.   His photographs also show the pores through which the mucilage is secreted.

The curious thing about this negative stain is that, whilst it appears to emphasis a halo of nothingness around the Desmidium filament, it is actually drawing our attention to something important.   In his presidential address to the British Phycological Society in 1981 A.D. Boney referred to mucilage as “the ubiquitous algal attribute” and goes on to list the many functions that the slimes produced by a wide range of algal groups may perform.  Not all will apply to our Desmidium but Boney does use desmids as examples of some of the roles slime may play: it can be, for example, a buoyancy aid, keeping the desmids in the well-lit regions of a lake or pond and it can protect cells against desiccation if a pond or lake dries out.  It may also play a role in helping desmids adhere to their substrates and there is also evidence that mucilage layers may help to protect algae from toxins.

Desmidium-grevillei_apical_view_CCarter_Sept19

Apical view (at four different focal planes) of Desmidium grevillei from Moss Dub, photographed by Chris Carter, September 2019.

But that’s only part of the story.   There is two-way traffic across the membranes of algal cells, with essential nutrients moving into the cell but, in some cases, enzymes moving in the opposite direction.  If nutrients are in short supply then these enzymes can help the cell by breaking down organic molecules in order to release nutrients that can then be absorbed. Those enzymes take energy to manufacture, and the sheath of gunk around the filament means that there is a lower chance of them diffusing away before doing their job (see “Life in the colonies …”).   The same principle applies to sexual reproduction too, with mucilage serving, in some cases, as “sperm traps” or simply as the phycological equivalent of KY Jelly.

It is not just the algae that benefit from this mucilage: the outer layers, especially, can be colonised by bacteria which will also be hoovering up any spare organic molecules for their own benefit with, no doubt, some collateral benefits for the organisms around them.  The connection is probably too tenuous to count as a symbiosis with the desmids but we could think in terms of mutual benefits.

So that “nothing” really is a “something”, and that is before we consider the role of these extracellular compounds in the wider ecosystem.  I mentioned the role of similar compounds in consolidating the fine sediments on coastal mudflats in “In the shadow of the Venerable Bede” to give a flavour of this.   The least prepossessing aspect of the least prepossessing plants can, given time, change landscapes.  That should give us all pause for thought.

Desmidium-grev_apical_pore_CCarater

Close-up of Desmidium grevillei filament with focus on the left-hand cell adjusted to show the apical pores.   Photographed by Chris Carter from material from Moss Dub collected in September 2019.

Reference

Boney, A.D. (1981). Mucilage: the ubiquitous algal attribute.  British Phycological Journal 16: 115-132.

Domozych, D. S., & Domozych, C. R. (2008). Desmids and biofilms of freshwater wetlands: Development and microarchitecture. Microbial Ecology https://doi.org/10.1007/s00248-007-9253-y

Sorentino, C. (1985). Copper resistance in Hormidium fluitans (Gay) Heering (Ulotrichaceae, Chlorophyceae). Phycologia 24: 366-388. https://doi.org/10.2216/i0031-8884-24-3-366.1

 

The little tarn of horrors …

In addition to desmids, we found several other algae in the samples collected from Cogra Moss.  One of these consisted of colonies of cells in mucilaginous masses attached to floating mats of vegetation (which looked like terrestrial grasses).  We decided that these were probably Chrysocapsa epiphytica, the second representative of the Chrysophyta I’ve described in this blog this year (see also “Fade to grey …”).  As is the case for Chromulina, much of what we know about Chrysocapsa epiphytica is down to the patient work of John Lund who first described this species back in 1949.

Chrysocapsa_epiphytica

Colonies of Chrysocapsa epiphytica growing on submerged vegetation at Cogra Moss, Cumbria, September 2019.  Cells are 7.5 – 15 micrometres long and 7.5 – 12 micrometres wide. 

He described the various mucilaginous lobes as “reminiscent of the …. human brain”.  The spherical, oval or ovoid cells form a layer, two to four cells deep, at the surface of the colony.   The cells have the typical yellow-brown colour of chrysophytes and, though it is hard to see the chloroplasts in this photograph, John Lund says that there are usually two, sometimes four, in mature cells.

Its presence in a soft-water lake probably means that it is a species that relies on dissolved carbon dioxide rather than bicarbonate as its raw material for phytosynthesis (see “Concentrating on carbon …” for some background on this).   We know, from laboratory studies, that most chrysophytes rely exclusively on carbon dioxide, and lack the capacity to use bicarbonate.  This confines them to water where the pH is low enough to ensure a supply of carbon dioxide (the chemistry behind this is explained in “Buffers for duffers”. It may also explain why Chromulina lives in surface films rather than submerged in the pond (the locations where we’ve it found are unlikely to have sufficiently low pH).

One extra twist to the story is that many chrysophytes are “mixotrophic”, meaning that they can switch between using photosynthesis as a means of getting the carbon they need to grow from inorganic sources, and “feeding” on other organisms.  Our Chrysocapsa epiphytica, in other words,  has parked itself beside a convenient supermarket of pre-packaged carbon in the form of decaying vegetation and associated bacteria which it then ingests by a process known as “phagotrophy”.

Phagotrophy is, in fact, a very ancient characteristic, insofar as the very first eukaryotic cells were the result of Cyanobacteria-type cells being ingested by larger heterotrophic cells and being retained as on-board “energy farms” rather than digested and treated as one-off vegetarian dinners.   However, the shift to a permanent role for chloroplasts within a eukaryotic cell involved a lot of rewiring of intercellular machinery, and effectively “switching off” the intercellular mechanisms involved in phagotrophy.   Retaining the ability to “feed” on bacteria alongside a capacity for photosynthesis is the cellular equivalent of a hybrid car: there is a lot more to cram under the bonnet.  Flexibility, in other words, comes at a cost.

On the other hand, phagotrophy does not just result in extra carbon for the Chrysocapsa cells in Cogra Moss.   In an oligotrophic tarn such as this, the extra nutrients that are obtained when the bacteria are absorbed will also be a useful boost.   Once again, though, you can see that, in environments where nutrients are more plentiful, the cost to the cell of maintaining the equipment required for phagotrophy outweighs the benefits.

I’m sure that a close inspection of the land around Cogra Moss would have revealed insectivorous plants such as Drosera(sundew) and we also recorded Utricularia minor, an aquatic insectivorous plant, in another tarn we visited whilst desmid-hunting (see “Lessons from School Knott Tarn”).  Chrysocapsa is, in many senses, a microscopic equivalent of these carnivorous plants.   OK, so it has a taste for bacteria rather than flesh but, somewhere out there, there must be a sub-editor in search of a headline …

References

Lund, J.W.G. (1949). New or rare British Chrysophyceae. 1.  New Phytologist48: 453-460.

Maberly, S. C., Ball, L. A., Raven, J. A., & Sültemeyer, D. (2009). Inorganic carbon acquisition by chrysophytes. Journal of Phycology 45: 1052-1061. https://doi.org/10.1111/j.1529-8817.2009.00734.x

Raven, J. A. (1997). Phagotrophy in phototrophs. Limnology and Oceanography 42: 198-205. https://doi.org/10.4319/lo.1997.42.1.0198

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1997). The growth response of Synura petersenii(Synurophyceae) to photon flux density, temperature, and pH. Phycologia 26: 233-243. https://doi.org/10.2216/i0031-8884-36-3-233.1

Saxby-Rouen, K. J., Leadbeater, B. S. C., & Reynolds, C. S. (1998). The relationship between the growth of Synura petersenii (Synurophyceae) and components of the dissolved inorganic carbon system. Phycologia 37: 467-477.  https://doi.org/10.2216/i0031-8884-37-6-467.1

Terrado, R., Pasulka, A. L., Lie, A. A. Y., Orphan, V. J., Heidelberg, K. B., & Caron, D. A. (2017). Autotrophic and heterotrophic acquisition of carbon and nitrogen by a mixotrophic chrysophyte established through stable isotope analysis. ISME Journal. https://doi.org/10.1038/ismej.2017.68