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 (= 1/50th 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 (= 1/100th 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 (= 1/100th 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 (= 1/100th 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 (= 1/100th 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 (= 1/100th 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 (= 1/100thof 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 (= 1/100th 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 (= 1/100th 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 (= 1/100thof 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 8th in the queue, instead of 6th.

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 lineataRhoicosphenia 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 (= 1/50th 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.

Promising young algae …

Spring has arrived in Cassop Vale.  Leaves are appearing on many of the trees and the ground vegetation has the green flush of a new beginning.   More importantly, the herd of emo-fringed highland cows have been moved away, to give the plants more chance of flowering, and there is some warmth in the sun in the middle of the day.

From my point of view, the biggest change since I was last here is the appearance of an extensive floc of green algae covering much of the pond’s surface.   I had a hunch, from their appearance, that these would be predominately Spirogyra, but was not expecting the sight that greeted me when I put a small piece of a floc under the microscope. 

Flocs, predominately Spirogyra, in the margins of Cassop Pond, April 2021.

I find Spriogyra and its relatives quite regularly on my travels, but usually in the vegetative state.  It is relatively unusual to find them as they undergo sexual reproduction (see “Fifty shades of green …”).  But there was plenty of evidence of this process (termed “conjugation” in Cassop Pond’s green flocs.  There were plenty of vegetative filaments, each about 20 micrometres wide and with a single helical chloroplast.  But there were also many ellipsoidal zygotes apparent.   When I looked more closely, these were inside filaments which were linked to an adjacent filament by a narrow tube.   What started out as an early morning natural history trip has turned out to be the algal equivalent of Saturday night on Newcastle Quayside.   

For those of you unused to dating, Spirogyra style, here is a quick guide.   First, put on your best helical chloroplast (two or more, if you are daring), then head out to find a partner amongst the many other filaments in your particular floc.   Little is known about Spirogyra’s preferences, but we can assume that many species are not heterosexual, so don’t be shy: sidle up to any filament you fancy.   He/she/it might well play hard to get at first, so maybe you need to drop a hint.  Make sure your potential date gets a whiff of your aftershave (that’s what I assume “hormonal interactions between the paired filaments” means).  If he/she/it gets the hint, then you can indulge in a little mutual meiosis to get yourselves into the mood.    

Spirogyra from flocs in Cassop Pond, April 2021.   a. vegetative filament; b. two filaments undergoing sexual reproduction with zygotes in the lower filament.   Narrow filaments of Aphanizomenon gracile are also present.   Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Now we’ve got that all-important emotional (okay … hormonal) connection, it is time to get physical.   An embarrassing bulge appears on the side of your filament but, fortunately, a similar one should appear on the side of your date’s filament at about the same time.   Eventually, these fuse to form a tube that links you both together.  The correct term for this is the “copulation canal” which is as frank as it is alliterative (it could also be called a “tupping tube”, I guess?). The protoplast of both cells now contracts and one (the “boy”, for want of a better analogy) crawls, amoeba-like, through the tube and fuses with the “girl” protoplast to form a zygote.  That’s as far as our frisky filaments in Cassop Pond have got.  If our phycological peep-show continued for longer, we would see the green zygotes gradually become brown in colour as thick, resistant walls grew around them, and the cell contents were processed into starch and lipid-rich food reserves.   They would then sink to the bottom of the pond and rest, dormant, until conditions were ripe for its germination.

Features of Spirogyra conjugation: a. a vegetative cell in one of the two aligned filaments; b. conjugation canals developing between the aligned filaments; c. a zygote.  Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Why here, why now?   Nitrogen limitation has been quoted as one of the triggers for conjugation and the presence of a nitrogen-fixing cyanobacterium (Aphanizomenon gracile) plus nitrogen-fixing diatoms (Epithemia– see “Working their passage”) in the pond at the same time lends support to this hypothesis.  Also, the yellow-green appearance of the flocs is also a hint that they may be nitrogen-limited.   However, there are also reports of conjugation happening on a predictable annual pattern in some locations.  The two possibilities are not mutually exclusive, we should remember.  

Meanwhile, on dry land, there are plenty of other plants getting down to the complicated business of reproduction too.   We saw goat willow (Salix caprea) and hazel (Corylus aveana) as well as lesser celandine (Ficaria verna) in flower, and leaves of primroses yet to bloom.   You can read more about those here.   Just remember, when enjoying the sight of spring flowers, that the botanical bacchanalia takes place in less obvious ways in the water too.

Some other highlights from this week:

Wrote this whilst listening to:  Horses and Easter by the Patti Smith Group (see below).   And a 1977 BBC “Sight and Sound in Concert” recording of Jethro Tull, which I remembered seeing when it was first broadcast.

Cultural highlights:   The film Black Bear – a rather dark and challenging, but ultimately rewarding, film.

Currently reading:  Just Kids, by Patti Smith.  Best read with Horses and Easter as a soundtrack.  The geographer in me also reads it with a map of New York to hand, as it is a book with a very strong sense of place.

Culinary highlight:.our local Indian restaurant makes a rather good lamb shank, cooked in aromatic spices which, with basmati rice and a side order of bhindi, is just about unbeatable.

A winter’s tale

Inspired by Anne Jungblut’s public lecture at this year’s British Phycological Society meeting, where she talked about the algal life that thrives in Antarctica (available here), I decided that a mere thirty centimetres of snow was not going to deter me from some pond dipping at a local lake.   “I’m going out, I may be some time” I muttered to no-one in particular as I pulled on my boots, filled my pockets with energy bars, and headed into the snow-blasted waste that was County Durham.  

Fortunately, my companion on this expedition has a heritage that embraces the nationalities of both Scott and Amundsen, so I knew that I had a 100% chance of getting there and at least 50% chance of returning home. Undaunted, we made our way across snow-covered fields, through a Narnia-esque woodland and finally emerged at the edge of a Cassop Pond which was, perhaps unsurprisingly, frozen over, foiling my efforts to gain a sample by usual means.   We tracked cautiously around the margin, never quite sure where solid land was replaced by thin ice, until we found the outflow where there was an exposed channel and some dead read stems.   This would have to do as a sampling location so I dug out a sample bottle and plastic bag and pulled on a veterinarian’s disposable glove, designed to be shoved up into the warmth of a cow’s rectum but also a way of keeping your arm dry whilst sampling cold water in the middle of the winter.

A frozen Cassop Pond, January 2021.  

This was the moment, of course, when two acquaintances passed by and, recognising us, wanted to know why I was crouched beside a stream of freezing cold water wearing a bright orange veterinarian’s glove and fumbling around amongst decaying reed stems.   This, I am fairly sure, does not happen to real Polar explorers.   Amidst all the trials and tribulations of life in Antarctica, feeling a bit of a plonker is the least of your worries.

An effective way of collecting the algae off submerged plant leaves and stems is to place them in a plastic bag with a little stream or pond water and give them a vigorous shake.   This is what I did with a couple of handfuls of reed stems and the result was a brownish suspension in the bag which I poured into a sample bottle before allowing our inner Amundsen to guide us home.  Our return route passed a field of Highland cattle all of whom gazed placidly at me in a way that they almost certainly would not have done had they known that I had a veterinarian’s glove in my bag.   

Back home and warmed up, it was time to look at the murky suspension and see what it contained.  I’ve visited Cassop a lot over the years (the very first post on this blog, for example, described a visit, also in January) and had a good idea of what algae I might find.  Whether the unprepossessing substratum of dead reed leaves or the time of year or a combination, there was not quite the rich diversity I was expecting, but there were a lot of cells of Fragilaira (probably F. tenera) and a few other species too.   There were also plenty of tiny Lemna minor plants floating around at this time of year and, as my first ever post showed, these host a number of epiphytic diatoms.  

Algae and protozoa from Cassop Pond, January 2021: a. – e.: Fragilaria cf. tenera; f. Ulnaria ulna; g., h. valve and girdle views of Gomphonema acuminatum; i. possibly Tetrastrum; j. unidentified protozoan.  Scale bar: 20 micrometres (= 1/50th of a millimetre).   

This will be the first of several visits to Cassop pond this year.  Last year was the first time in several years that I did not have a regular focus for my posts (see “Reflections from Castle Eden Burn” for an overview of my 2019 explorations).  I had originally planned a series of visits to a location which, though easily reachable by car, would fall foul of the current lockdown restrictions.  Instead I’m looking at a pond that is walking distance from my house.   Maybe that is not such a problem: not everyone is lucky enough to have a National Nature Reserve on their doorstep and I have, in all honesty, not given this pond the attention it deserves over the years.

The richness of aquatic microscopic life in winter has been a recurring theme in this blog over the years but should that richness surprise us?   We approach the world of algae with mindsets that could be described as “Angiosperm Supremacists”, basing our assumptions on the entire plant kingdom on how we expect higher plants to respond.  Yet, in evolutionary terms, angiosperms are relative newcomers forced to exploit dry land because the best aquatic habitats had already been monopolised by algae.   Away from the protection that water’s high specific heat capacity offers a plant, survival in winter depends on being able to divert energy into a range of protective strategies in order to prevent damage due to freezing (see reference below).   There are algae that live in and on ice but, for most, the environment beneath the ice offers fewer challenges.  Indeed, as most of their grazers are poikilotherms, winter (all other things being equal) is not a bad time to be an alga.   

Reference

Knight, M.R. & Knight, P.H. (2012).  Low temperature perception leading to gene expression and cold tolerance in higher plants.  New Phytologist 195: 737-751.

Some other highlights from this week:

Wrote this whilst listening to: John Martyn and, in particular, a 1978 Rock Goes To College set on YouTube that I remember watching when it was first aired. [https://www.youtube.com/watch?v=GYvncpoeV5Q&t=2132s]

Cultural highlights:  A great new film called Sylvie’s Love, set in jazz-era New York.

Currently reading:  The Science Delusion by Rupert Sheldrake

Culinary highlight:  Recipe from The Guardian last weekend: a chocolate and marmalade tart.

The intricate life of a colonial alga …

Cassop_Pond_July19

The annual Algal Training Course in Durham always has a field trip out to Cassop Pond, a small pond at the foot of the Permian Limestone escarpment in County Durham that has featured in a few of my posts over the years (see “A return to Cassop”).  This year, the group came back with some samples from the pond’s margins bearing a suspension of green dots just visible to the naked eye which, when examined under the microscope, turned out to be the colonial green alga Volvox aureus.  These are spherical, with the cells at the periphery, joined together by thin strands of protoplasm. The smaller colonies were scooting about, propelled by the pairs of flagellae borne by each of the cells that constitute the colony, whilst the larger ones (mostly “pregnant” with one or more daughter colonies) were sessile.

Volvox_Cassop_July19_#5

Volvox aureus colonies just visible to the naked eye in a drop of water from Cassop Pond, July 2019.   The drop is 13 millimetres across.

Volvox_Cassop_July19_#3

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

Volvox_Cassop_July19_#1

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

Watching a Volvox colony swimming around under the microscope is a beguiling experience, but its movement is not random.  Consider: there may be a 1000 or more cells in the larger colonies, each with two flagellae.  If all beat their flagellae at random, the colony would not get anywhere, as the force in one direction would be cancelled out by forces in all other directions.   But Volvox colonies do actually move with intent.   Look closely at the individual cells in the photos below and you will see that each has a red-coloured eye spot (the light-detecting organelle actually lies beneath the red layer, which acts as a filter).   People with more patience than me have noticed that the eye spots in different parts of the colony differ in size, suggesting a level of organisation that may not be immediately apparent.  We also know that the daughter colonies tend to form at the posterior end of the colony (assuming “posterior” and “anterior” in a spherical colony are defined by the direction of travel) and also that only a small number of cells (larger than the others) are responsible for the division that produces these.

In theory, a spherical object is going to offer less resistance and so sink faster than an object of the same size that had a greater surface area : volume ratio. This should mean that they are not able to stay in the light-rich surface layers where they can photosynthesise and grow.   In practice, Volvox colonies are able to adjust their position by using their flagella but this requires them to pump some of the energy they have obtained from photosynthesis into the flagella’s motors.  Another advantage in Volvox’s favour is a relatively low density of the colony as a whole.  The individual cells are separated by strands of protoplasm which creates a lattice through which water can penetrate, so the overall density of the colony is closer to that of the surrounding water than would be the case if the cells were tightly packed.

Volvox is most often found in the summer in relatively nutrient rich lakes, where nutrients are sufficiently plentiful to support a rich crop of algae.  A motile colony that is not too dense is well-placed to adjust its position to stay in the surface layers and harvest the sunlight.  Moreover, the size of the colony probably means that it is too big for the filter-feeding zooplankton that grazes on the algae.   At the same time, however, Volvox begins to experience some of the problems associated with multicellular life (see references in “The pros and cons of cell walls …”).   As large multicellular organisms ourselves, a nuanced discussion about the pros and cons of multicellularity may seem to only have one possible outcome.   However, Volvox inhabits a world where plenty of single-celled organisms thrive and where a colonial lifestyle offers a small competitive advantage.  It means that it is quite happy drifting around at the time of year when many of us would like nothing better than to don swimming trunks and soak up some sun in a local pool.   Study algae for too long and you end up realising that only losers need to evolve.

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Cells from a Volvox aureus colony from Cassop Pond, July 2019. You can see the red eye-spots in some of the cells in the left-hand image (bright-field) whilst the protoplasmic strands joining cells together can be seen in the right-hand image (phase contrast).   Scale bar: 10 micrometres (= 1/100thof a millimetre). 

References

Canter-Lund, H. & Lund, J.W.G. (1995).  Freshwater Algae: Their Microscopic World Explored.  Biopress, Bristol.

Reynolds, C.S. (1984). The Ecology of Freshwater Phytoplankton. Cambridge University Press, Cambridge.