More about Tabellaria

My statement in the previous post that “Tabellaria flocculosa is by far the most common species” [in the genus Tabellaria] is true except for one important point: Tabellaria flocculosa is not a species, in the biological sense.  We have good evidence from the UK, and my colleague Maria Kahlert has similar evidence from Sweden, that it is actually a complex of several species that no-one has yet taken time to dissect out and establish in their own right. 

The picture at the top of the post shows strains from eight different locations in Scotland collected for our library of molecular barcodes, with the black boxes delimiting three groups, based on differences in their rbcL gene.   All fit the description of T. flocculosa but the scale of the differences is such that they are highly likely to be different species.   However, it is hard to see morphological characteristics that set any of these groups apart.   It is possible that some features will be revealed by a more thorough study using scanning electron microscopy but that will not be particularly useful for routine identification using the light microscope.   Watch this space …

However, even if Tabellaria flocculosa is not a “species” in the strict biological sense, it is a “species” in a broader linguistic sense, insofar as it is a widely-used term that enables freshwater biologists to exchange information about the organisms that live in lakes and streams.   Biologists have a formal way of understanding species, with each and every Tabellaria cell being related to a physical “type specimen”.  However, philosophers and linguistics have a more abstract way of using the word “type” in relation to organisms, deriving from the work of Plato.   He argued that each and every ox was different but there must be something that unites all of these individual oxen.   This universal property could not necessarily be perceived directly with any human sense but must be understood with the mind.   The lay person does not need to link a beech tree back to a type specimen: they have the “type” lodged in their memory, enabling unambiguous identification, even from a distance.   

Not only is Tabellaria flocculosa recognisable as a distinct entity (using characteristics listed in the previous post), but it also conveys some useful information about the habitat.  If T. flocculosa is abundant in a sample (i.e. more than 10% of all cells), then you can be fairly sure that the habitat you are examining has relatively soft water (probably slightly acidic) and relatively low concentrations of inorganic nutrients (see graphs at the end of the post).   All this despite us knowing that it is not really a species in the formal biological sense at all.   There have been some attempts to split this species in the past, but none have stuck, at least in part, I suspect, to the proposed divisions not being sufficiently convincing – in either morphological or ecological terms – to usurp the longstanding Platonic “type” of T. flocculosa.   There are some hints in the work of both Brenda Knudson (see previous post) and John Koppen (see reference list) that there are morphologically distinct planktonic and benthic forms but, again, no-one has produced a convincing rationale for splitting these into separate species.

Linguists would argue that the words we use to describe the world around us are a consensus of usage, not an absolute, and also that meaning of words can change over time.   That’s certainly true for diatoms.  When I first started out, “Cocconeis placentula” was interpreted in much broader terms than it is now, and the same can be said for many other diatom species.  The same is true for genera: we all thought we understood what characterised the genus “Fragilaria”; now, we have a much more constrained definition, with many of its former constituents shifted to other genera.    So it is perfectly possible that, in 30 years time, we’ll have a different view of what “Tabellaria flocculosa” means.   

Distribution of Tabellaria flocculosa along pH and alkalinity gradients, based on analysis of the DARES dataset.   Vertical lines on the pH graph indicate thresholds for high (blue), good (green), moderate (orange) and poor (red) status.   Vertical lines on the alkalinity graph divide the scale into (from left) low, moderate, high and very high alkalinity water.  The arrow indicates the position of the present sample on the respective gradients.  

Distribution of Tabellaria flocculosa along nitrate-N and reactive P gradients, based on analysis of the DARES dataset.  

Relative abundance of Tabellaria flocculosa in lakes, regulated rivers and unregulated rivers in west Cumbria between 2019 and 2021.   

Notes

The graphs showing the distribution of Tabellaria flocculosa in relation to chemical variables are based on interrogation of a database of 6500 river samples collected as part of DARES project (Kelly et al., 2008, in reference list).  Vertical lines on the pH and reactive phosphorus graphs show UK environmental standards for conditions necessary to support good ecological status: blue = high status; green = good status, orange = moderate status and red = poor status.  Standards differ between water body types and thresholds for lowland high alkalinity rivers have been plotted here.  These indicate the maximum thresholds for particular ecological status classes for each variable and tighter standards will apply in many waters.

Phosphorus 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 P attached to particles that would have been removed by membrane filtration. The current UK phosphorus standards are site specific, using altitude and alkalinity as predictor variables.  This means that a range of thresholds applies, depending upon the geological preferences of the species in question.  The plots here show boundaries based on the average alkalinity (50 mg L^-1 CaCO₃) and altitude (75 m) in the whole dataset.  

There are no UK standards for nitrate-N; thresholds in this report are based on values derived using the same principles as those used to derive the P standards and give an indication of the tolerance of the species to elevated nitrogen concentrations (see “This is not a nitrate standard”).  However, they have no regulatory significance.

The photographs of Tabellaria flocculosa are from cultures isolated by Shinya Sato and David Mann during the UK diatom metabarcoding project (Kelly et al., 2020, in reference list) 

References

Kelly, M.G., Juggins, S., Guthrie, R., Pritchard, S., Jamieson, B.J., Rippey, B, Hirst, H & Yallop, M.L. (2008).   Assessment of ecological status in UK rivers  using diatoms.   Freshwater Biology 53: 403-422.

Kelly, M.G., Juggins, S., Mann, D.G., Sato, S., Glover, R., Boonham, N., Sapp, M., Lewis, E., Hany, U., Kille, P., Jones, T. & Walsh, K. (2020).  Development of a novel metric for evaluating diatom assemblages in rivers using DNA metabarcoding.   Ecological Indicators 118: 106725.

Koppen, J.D. (1978).  Distribution and aspects of the eclogy of the genus Tabellaria Ehr. (Bacillariophyceae) in the Northcentral United States.   American Midland Naturalist 99: 383-397.

Wrote this whilst listening to:   Brilliant folk-blues guitarist Gwenifer Raymond, another artist who performed at Green Man 2019, but whose set I missed.   And Bruce Springsteen’s Girls in their Summer Clothes, because it caught the mood of the weekend so well.

Cultural highlights:  The Israeli TV show Fauda now available on Netflix, about life on the West Bank.   

Currently reading:  still on American Wife by Curtis Sittenfeld.

Culinary highlight: An impromptu French bistro evening at home, with coq au vin followed by tarte au citron.

The bluffer’s guide to Tabellaria …

Every now and then I write a post about how to identify algae.  Not about how to name algae (the job of taxonomists) or what algae live in which habitats (the job of ecologists) but of the nebulous area between these two specialities where optical nerves are stimulated by the light patterns seen through a microscope’s eyepiece, triggering impulses in the brain that relate particular patterns and shapes to a Latin binomial.   This process is integral to almost all ecological data collection, but we rarely stand back to consider how this happens.   Recent posts on this subject include “Disagreeable distinctions …” and “Dispatches from Plato’s cave …” – the latter making the point that bioinformatic pipelines used in metabarcoding are akin to the thought processes used when we identify organisms using traditional approaches.   Then, in “Identification by association” I pointed out how the mind can be conditioned during the identification process, leading to possible biases in the outputs.

I want to stay with this idea of the mind being conditioned in this post, using the genus Tabellaria as a case study.  This is a small genus with just four representatives, with overlapping ecological preferences (found mostly in soft water with low nutrient concentrations).  It is quite common to find two or more representatives of the genus in a single sample although, in benthic habitats at least, Tabellaria flocculosa is by far the most common species.   The question I want to ask is whether analysts approach each new specimen in the sample as a completely new identification task, or whether they carry forward a cache of information that they have built up about a sample.   Most biologists would probably claim the former, but I suspect that the latter is more common.  Moreover, I don’t think that there is any disgrace in admitting this.  In fact, I think it is a strength under some circumstances.

What is happening under such situations is a form of Bayesian reasoning: as we accumulate experience from a sample (and, over time, from many similar samples), we realise that most of the valves of Tabellaria belong to T. flocculosa.   We approach the next valve of Tabellaria with this prior knowledge along with the information we need to confirm or deny this (the “model”).   Equipped with this, we then check the next valve using this model and emerge with “posterior knowledge” which, in this case will be either confirmation that the valve belongs to T. flocculosa or an alternative diagnosis of T. fenestrata, T. quadrisepta or T. ventricosa.   

Tabellaria flocculosa (“short forms”) from Smerla Water, August 2019.  Scale bar: 10 micrometres (= 1/100^th of a millimetre).  Photos: Lydia King.   The photograph at the top of the post shows live colonies of Tabellaria flocculosa.

How does this work with Tabellaria?  The genus is recognisable from its linear valves with central and terminal inflations, presence of septa and absence of a costa and a raphe.   That’s probably all the information you need when using most keys.  Now, assume that the individual is T. flocculosa unless there is evidence to the contrary.  What counterfactuals should we be looking for?

In valve view: 

• If the width is > 10 micrometres at the central inflation and the rimoportula is at the end rather than at the centre   …. T. ventricosa;
• If the axial area is narrow and linear throughout, and does not get broader towards the centre …. T. fenestrata;
• If there are distinct marginal spines … T. quadriseptata.

However, Tabellaria does not always present in valve view (particularly when viewing live material), so we also need some characteristics to help us when it is in girdle view:

• If there is a sharp kink in the septa just behind the point where they insert into the girdle band   …. T. fenestrata;
• If there are no more than four (occasionally five) septa and marginal spines are not obvious … T. fenestrata;
• If there are no more than four (occasionally five) septa and marginal spines are obvious … T. quadriseptata.

I’ve left colony formation out of this list because the literature on the extent to which this is a useful diagnostic criterion is contradictory.

Tabellaria species from Maa Water, Shetland Islands.   a.  zig-zag colonies of T. flocculosa which survived preparation.   b. – d.: three focal planes of a girdle view of T. fenestrata showing the kink in the septa close to the point of insertion (red arrow); e. T. ventricosa, with the terminal position of the rimoportula indicated by an arrow, in contrast to T. flocculosa (f. and g.) where it is close to the centre of the valve.   Scale bar: 10 µm.

Finally, if you are looking at an isolated girdle band (which is a frequent occurrence when Tabellaria are abundant in a sample), then all Tabellaria species except T. fenestrata have closed bands (i.e. a continuous ring of silica) whilst T. fenestrata’s girdle band is “open” (i.e. an incomplete ring of silica).   You should not include isolated girdle bands in a count, but this is evidence that T. fenestrata is present in a sample which should encourage you to look for other features from which you can make a reliable identification. 

One extra piece of advice I always give to beginners is that you should identify populations not individuals.   It may be that not every individual displays all the information you need to make a reliable identification, but that, through accumulating evidence from a range of specimens, in valve and girdle views, you can be confident of the population’s identity. Similarly, size ranges given in identification manuals are not always as reliable as they should be, and you should never reject a potential name solely because a specimen falls slightly outside a quoted range.  Taking measurements of a population to ensure that most specimens fall within these ranges again, gives you the confidence to include the few that exceed these measurements.   

Whilst taxononomy is a science, identification is a craft and, as such, something that individuals may develop and apply in different ways.   What works for me may not work for you so, I shall finish with a quotation George Orwell: “break any of these rules rather than say anything outright barbarous”

Reference

Knudson, B.M. (1952). The diatom genus Tabellaria. 1. Taxonomy and morphology.  Annals of Botany N.S. 16: 421-440.

Knudson, B.M. (1953a). The diatom genus Tabellaria. II. Taxonomy and morphology of the plankton varieties.  Annals of Botany N.S. 17: 131-155.

Knudson, B.M. (1953b). The diatom genus Tabellaria. III. Problems of infraspecific taxonomy and evolution in T. floccuosa.  Annals of Botany N.S. 17: 597-609.

Wrote this whilst listening to:   Otis Redding

Cultural highlights:   The film X+Y, sensitive portrayal of an autistic boy coming to terms with adolescence and coping with the world around him.

Currently reading:  American Wife by Curtis Sittenfeld.

Culinary highlight: spoilt for choice, but lunch at Square One in Great Dunmow, Essex (famous for nurturing Professional MasterChef winner Alex Webb) wins out over dinner at 56, our local Sichuanese restaurant in Durham.  Honourable mentions go to Wild Swan bakery in Wanstead and the Wanstead Tap.

Cassop Pond in May

A month on from the phycological debauchery that I wrote about in Promising Young Algae the Spirogyra flocs that covered quite a lot of the surface of Cassop Pond have disappeared.  With sexual reproduction over, the zygotes, I presume, have sunk to the bottom of the lake, where they will lay dormant until next Spring.   I searched around the margins of the pond, but only found a few wisps of Spirogyra hanging around some cattle hoofprints in the shallow water on the eastern side of the pond.   This, however, proved to be a different type of Spirogyra altogether, with broader and squatter cells than the main constituents of April’s flocs.  

The green tinge in this hoofprint is filaments of Spirogyra.   The picture at the top of the post shows Cassop Pond in May 2021: note the absence of flocs compared with the situation in April. 

Swimming around amongst the Spirogyra filaments were a number of very active green cells of Euglena.   Euglena is a genus that has not featured much in this blog over the years (see: “A visit to Loughrigg Fell”) but it is a genus closely associated with Cassop Vale, with 13 of the 36 species recorded from Britain and Ireland recorded from this location.  The story behind this richness is that there were no active experts in the Euglenophyta at the time when the Freshwater Algal Flora of the British Isles was being compiled.   Instead, a Polish expert, Konrad Wołowski, was invited to contribute and, to help him do this, he was taken on a tour of locations that Dave John and Brian Whitton, the editors, thought would be likely habitats.  Cassop Pond is well known to Brian and is conveniently located near Durham, so it was an obvious location.  As a result, it is probably the hot spot of recorded diversity for this genus in the UK but that is more due to the idiosyncrasies of biological recording than to anything about this location over many others that particularly favours Euglena. 

That said, cattle hoof prints are known to be a good location for Euglena and relatives, and I wrote about a relative of Euglena that I found in a puddle in Teesdale (see: “Puzzling puddles on the Pennine Way …”.  A hoofprint or a puddle is, from an alga’s point of view, a temporary pond and so long as you have a plan in place for when this dries up, it represents a potential habitat.   Cattle, as we have already seen, are allowed to graze on the reserve so there are plenty of damp hoofprints within which Euglena and relatives can thrive.  That’s also true of many other nature reserves around the country.  The one missing ingredient at all of those is naturalists within an inclination to search them out …

Spirogyra filaments from Cassop Pond, May 2021.  Scale bar: 20 micrometres (= 1/50^th of a millimetre).

Reference

Wołowki, K. (2010).  Euglenophyta.  pp. 181-239.   In: Freshwater Algal Flora of the British Isles (edited by John, D.M., Whitton, B.A. & Brook, A.J.). Cambridge University Press, Cambridge.

Euglena sp. from Cassop Pond, May 2021.  Scale bar: 10 micrometres (= 1/100^th of a millimetre).  

Some other highlights from this week:

Wrote this whilst listening to:   The soundtrack to The Pursuit of Love, available via BBC Sounds.   

Cultural highlights:   Emily Mortimer’s adaptation of Nancy Mitford’s The Pursuit of Love, available on the BBC iPlayer

Currently reading:  Pat Barker’s Noonday, the final part of the Life Class trilogy.

Culinary highlight: homemade digestive buiscuits.

The diatoms of Cassop Pond

We’ll stay at Cassop Pond for this next post, as I look back over the diatoms that I’ve found there.   So far, I have collected four samples although, due to the time it takes to prepare these for analysis, I’ve only got around to looking closely at three of these. Nonetheless, I’ve found a total of 98 species belonging to 39 genera in these three samples.  Here is a summary of the more abundant forms.

Araphid diatoms were particularly abundant in the sample I collected from reed stems in January.  Despite my comments in the post I wrote at the time (see: “A winter’s tale …”), the most abundant are Tabularia fasiculataand Ulnaria cf. acus., both of which grow singly or in small clusters, attached to the stem by a mucilage pad at one end.  The genus Tabularia is often described as a species of brackish and marine waters, but in my experience, it can be abundant in freshwater habitats where the water is quite hard.  

Araphid diatoms from Cassop Pond, 2021: a., b. Tabularia fasiculata; c. Ulnaria acus; d. Fragilaria tenera; e. Fragilaria cf. pectinalis; d. Fragilaria, unidentified girdle views; g. Diatoma tenuis; h. Tabellaria flocculosa.   Scale bar: 10 micrometres (= 1/100^th of a millimetre). 

Cocconeis species were particularly abundant in the sample from Riccia fluitans and Lemna minor collected in February.   This genus is often abundant as an epiphyte on other plants and algae and it is common to find more than one species in the same sample, which would suggest that there are some subtle aspects of their niches that we do not yet fully understand.   Two of these species were also encountered growing on rocks in Croasdale Beck in Cumbria (see “Curried diatoms?”).  Lemnicola hungarica was also present in the samples, albeit in low numbers.  This species is often epiphytic on Lemna minor (see: “The green mantle of the standing pond …”) and I suspect that a sample composed mostly of Lemna and with less Riccia fluitans might have a higher proportion.   Finally, I have included five different focal planes of a single valve of Eucocconeis flexella, just to show the complexity of valve structure in this diatom.   Note the S-shaped raphe on the upper valve.

Monoraphid diatoms from Cassop Pond, 2021.  a., b. Cocconeis euglypta; c. C. lineata; d.,e. C. pseudolineata; f. Achnanthidium caledonicum; g. A. eutrophilum; h. A. saprophilum; i., j., k.Planothidium lanceolatum (3 focal planes); l. Platessa oblongella; m., n., o., p., q, Lemnicola hungarica; r., s., t., u., v. Eucocconeis flexella.  Scale bar: 10 micrometres (= 1/100^th of a millimetre). 

One of the surprises of this sample was the relatively high proportion of Eunotia species that I found, particularly in the sample from February.  Eunotia is a species most often associated with soft water so I had not expected to find it to be frequent in a calcareous pond.   However, this sample was collected on the east side of the pond, where some spoil heaps form part of the shoreline.   Moreover, neither of the two species that were most abundant are particularly associated with very low pH.  However, these were a curiosity and there were also a few other species in the samples (e.g. Tabellaria flocculosa) which hinted that soft water might have some influence in the pond.   

Eunotia species from Cassop Pond, 2021.   a.,b. Eunotia bilunaris; c.,d.,e.,f. E. minor; g. E. incisa.   Scale bar: 10 micrometres (= 1/100^th of a millimetre). 

The role of Epithemia adnata in the pond was considered in Working their passage so we don’t need to say much more here except that this is another species that is far more abundant on plants than on rock surfaces in this pond.   By contrast, the genus Nitzschia was much more common the rocks at the north end of the pond than on the plants.   These rocks were covered with a fine layer of marl (fine calcite deposits that has precipitated out from the water).  The most abundant species here was Nitzschia palea.

Epithemia adnata from Cassop Pond, 2021.   Scale bar: 10 micrometres (= 1/100^th of a millimetre).

Nitzschia and Tryblionella species from Cassop Pond, 2021. a. Nitzschia paleacea; b. N. subtilis; c., d., e. N. palea; f. N. cf. archibaldii; g. N. capitellata; h. Tryblionella sp.  Scale bar: 10 micrometres (= 1/100^thof a millimetre).

Another diatom that was common on the rocks at the north end was Cymatopleura solea.  Whilst this was less abundant in terms of numbers than Nitzschia palea, it has much larger cells, so the overall contribution to biomass and photosynthesis is probably the same or even greater than that species.   When I tried to describe Cymatopleura solea, with its central  constriction along with transverse undulations across the valve surface, to a class a few years ago, one participant suggested that it was a “voluptuous” diatom.   The presence of this along with the Nitzschia suggests that motility is an attribute that favours diatoms in this habitat, in contrast to the two samples from plants, which were both dominated by non-motile species. 

Cymatopleura solea from Cassop Pond, 2021.   One valve photographed at three focal planes.   Scale bar: 10 micrometres (= 1/100^th of a millimetre).

The most diverse genus encountered was Navicula, with 14 species, although these were never found in great numbers.  As befits a motile genus, these were most abundant in the sample from the rock although they were also found in small numbers in the samples from plant surfaces.  Other biraphid symmetrical species found included Caloneis, Sellaphora, Hippodonta, Fallacia and Neidium.   Two of the images are described as “Sellaphora pupula” but we know that this is an aggregate of several species, barely distinguishable with the light microscope.  Both of these images are likely to represent different species. A significant omission from my list is Mastogloia (see “Structural engineering with diatoms”).  The habitat seems right for this species, and I have found it in other ponds in the area (see “Return to Croft Kettle”) so I suspect that it may turn up at some point during the year.

Biraphid symmetrical diatoms from Cassop Pond, 2021: a. Navicula cryptotenelloides; b., c. N. trivialis; d. N. subalpina; e. Caloneis amphisbaeana; f. Hippodonta capitata; g., h. Sellaphora pupula ag.; i. S .saugerresii; j. Fallacia pygmaea; k., l., m. Neidium dubium (three focal planes).  Scale bar: 10 micrometres (= 1/100^th of a millimetre).

Finally, there were a few valves of Gomphonema, Cymbella, Amphora and Halamphora, but none present in significant quantities.  

To put the 96 diatoms I’ve recorded to date into perspective, Heather found 182 and 123 angiosperm species over the course of a year at two nearby nature reserves, both with similar geology to Cassop Vale.   That puts the diversity of the microscopic world into perspective.  Bear in mind, too, that the samples I’ve looked at to date were collected in the winter.  I fully expect the final count of diatoms to exceed that of angiosperms but we’ll have to wait and see.   Is an element of competition creeping into this natural history malarky?   Surely not …

Heteropolar and dorsiventral diatoms from Cassop Pond, 2021: a. Gomphonema cf. graciledictum; b. Gomphonema sp.; c. Cymbella affinis; d. Halamphora montana.  Scale bar: 10 micrometres (= 1/100^thof a millimetre).

References 

Mann, D.M., Thomas, S.J. & Evans, K.M. (2008).  A revision of the diatom genus Sellaphora: a first account of the larger species in the British Isles.  Fottea (Olumec) 8: 15-78.

Some other highlights from this week:

Wrote this whilst listening to:   Scottish singer-songwriter Karine Polwart, who I first encountered as the “hold” music on Triodos Bank’s customer service line.  The first and only time in my life I wish I was 8^th in the queue, instead of 6^th.

Cultural highlights:   Nomadland, winner of the Oscar for best film.   A Ken Loach vibe but set in the western USA rather than north east England.

Currently reading:  John le Carré’s Absolute Friends.

Culinary highlight: our first meal out for many months, at Whitechurch in Durham.  Apart from the cold and the damp, it was great.  

Pond politics …

We have not travelled away from Cassop Pond for this next post, as I try to summarise the earlier visits in a picture.  On the left-hand side, there is a stem of Phragmites australis, with epiphytic diatoms, dominated by Tabularia fasiculata (rather than the species I suggested on first examination – see “A Winter’s Tale”).   At the top right there is part of the thallus of the liverwort Riccia fluitans (see “Working their passage”) with different epiphytes: a combination of Cocconeis lineata, Rhoicosphenia abbreviata and Epithemia adnata. Then, towards the centre of the picture there is Lemna minor, with a floating leaf and a single root dangling below.   The leaf has some more Cocconeis on the underside, but also some Fragilaria (probably F. gracilis) on the root.   

Epithemia is a diatom often associated with nitrogen limitation and, interestingly, is one of a number of clues that Cassop Pond is nitrogen-limited for at least part of the year.   I also found some filaments of the cyanobacterium Aphanizomenon gracile, which can fix nitrogen via its distinctive heterocysts, and I also mentioned, in my previous post, that nitrogen limitation might be one of the triggers for conjugation in Spirogyra.   Interestingly, the Epithemia seems to be most abundant in the flocs of Riccia fluitans: a scarce resource, presumably, being even scarcer when there are plenty of other plant cells hoovering up any that is in the vicinity.   Why not also on Spirogyra?  Probably because the slimy mucilage that surrounds these filaments makes it difficult for an epiphyte to gain purchase.   The only time when epiphytes are abundant on Spriogyra and relatives is when the filaments are clearly unhealthly.

We can think of this in terms of the cost-balance sheets of the respective organism.   Spirogyra’s business model is focussed on maximising photosynthesis and, as such, it diverts some of its budget to produce mucilage.  That means that there are no pesky epiphytes to stand between the sunlight and its chloroplasts.   Riccia fluitans has a different approach: it sees epiphytes not as a “cost” but as a “benefit”: maybe the diatoms growing on the surface stop some sunlight getting to the liverwort’s photosynthetic cells but quite a few of these diatoms fix nitrogen and, as their cells are prone to “leakage”, some of the surplus nitrogen will be there to help the liverwort grow.   The diatoms provide a “subsidy” to the liverwort, to use ecological jargon.  Spirogyra is one of those right-wing algae that probably talks glibly about “trickle down economics” but, in practise, it is going all out for itself.   Don’t get me started on trickle-down economics.

Dinobryon sertularia, a living colony from Cassop photographed at four different focal planes.  Scale bar: 20 micrometres (= 1/50^th of a millimetre).  

I also came across Dinobryon sertularia during my recent trips to Cassop Pond.  This is usually described as planktonic, although I found it growing in the brown film surrounding Phragmites stems at the pond’s margin.   The cells of this alga live in vase-shaped cases (termed a “lorica”) which are usually united to form colonies.  Each cell has two flagella – both clearly visible and busily thrashing around enough to make any attempt to produce a crisply-focussed image impossible.   You can see an excellent image by Hilda Canter-Lund here, almost certainly taken from fixed, rather than living, material.  Dinobryon is a member of the Chrysophyceae, which we last encountered in “The Little Tarn of Horrors”.  As explained in that post, many Chrysophyceae (including species of Dinobryon) are “phagotrophic” – capable of gaining energy and nutrients from bacteria and other particles they ingest.  The Dinobryon colony that I viewed was likely using its flagellae to create turbulence in the water that would waft bacteria in the direction of its gullet, as much as it was using them to move.   That’s another sign, perhaps, that Cassop Pond is, if not as nutrient-poor as Cogra Moss (where our previous encounter with Chrysophyceae took place), at least an imbalance in nutrients in the water that means that some “dietary supplements” will not go amiss.  

Four months into my visits to Cassop Pond and I am beginning to see the dynamics of the pond unfolding.   We’ve learnt about some of the “nouns” that occupy the pond but also, through these, are beginning to learn a little more about the “verbs”: the activities and functions that bind the other organisms into a living ecosystem.   We often think of ecosystems in terms of “survival of the fittest” but the picture that is emerging in Cassop Pond – and in countless other ecological studies – is that there are a lot of subsides and mutually-beneficial interactions between the organisms. Cassop Pond, like many of the villages around it, still leans to the left…

Reference

Caron, D.A., Sanders, R.W., Lim, E.L., Marrasé, C., Amarl, L.A., Whitney S., Aoki, R.B. & Porters, K.G. (1993). Light-dependent phagotrophy in the freshwater mixotrophic chrysophyte Dinobryon cylindricum .  Microbial Ecology 25: 93–111. 

Some other highlights from this week:

Wrote this whilst listening to:   Crosby, Stills and Nash’s 2009 set at Glastonbury via YouTube, which brought back some happy memories.   They played the day after Neil Young so I can stretch a point and say that I saw Crosby, Stills, Nash and Young that weekend.

Cultural highlights:   We watched both winners of the Oscars for best documentaries this week.  Both are good but we particularly recommend My Octopus Teacher, filmed in shallow waters off the South African coast and encapsulating the leitmotif of this blog: repeated visits to the same location yields unexpected insights into natural history.  The film also deserves the Oscar for Best Performance by Kelp in a Supporting Role, if such a category existed.  

Currently reading:  The Well-Gardened Mind, by Sue Stuart-Smith: a book about the therapeutic benefits of nature and gardening in the modern world.

Culinary highlight: Cauliflower steaks with a harissa sauce.  And Queen of Puddings.