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.

Fifty shades of green …

ennerdale_161005

Last week took me back at Ennerdale Water in the Lake District to see how the algae in the littoral zone had changed since my previous visit (see “Both sides now …”).   Back in July, we had found very few algae visible with the naked eye at most of the sites around the margin that we visited; three months on, the situation is very different, with obvious growths at many locations.  As Ennerdale is a remote lake with few human influences, any changes we see are likely to be the result of natural processes rather than “pollution”, so that makes the rapid increase in quantity of algae very intriguing.

One location was particularly intriguing: it was on the south west shore, where the steep scree-laden slope of Crag Fell enters the lake.  The littoral zone has some large stable boulders washed by waves blown down the lake from the high fells to the east.   The boulders had a covering of mosses on their upper surface and this moss, in turn, had been colonised by green algae.

Under the microscope, these growths were revealed to be the filamentous green alga Mougeotia, a relative of Spirogyra, which I have written about in a number of previous posts (it is often common in the River Ehen, for example, which flows out of Ennerdale: see “The River Ehen in February”).   The curious aspect of this particular population was that there were signs of sexual reproduction.   Mougeotia, along with Spirogyra and many other filamentous green algae, is usually observed in the vegetative state (see “The River Ehen in March” and “The perplexing case of the celibate alga”).

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Boulders in the splash zone of Ennerdale with growths of Mougeotia over mosses (left) and growing directly on the rock surface (right).   The top photograph shows a view from Kirkland across Ennerdale with Great Gable in the background.

mougeotia_conjugating_1

Filaments of the green alga Mougeotia in an early stage of conjugation, with papilla growing from the lower filament towards the upper one.   Scale bar: 20 micrometres (= 1/50th of a millimetre).  

Conjugation involves cells in two adjacent filaments developing outgrowths (“papilla”) that meet and fuse, creating a copulation canal between the two cells.   The cell contents (“protoplasts”) of both cells contract and then they both moves, amoeba-like, into the canal where they fuse  to form a zygote.

The image above suggests that the upper filament may be playing hard to get, rebuffing the amorous advances of the lower filament.   I don’t know enough about conjugation of these algae to know whether the enthusiasm for sex differs between filaments, but it is also possible that what I photographed is an artefact of filaments that may well have been establishing cosy relationships with neighbours before being dragged first from the lake and then onto a slide for my voyeuristic pleasure.   What may have been, in Ennerdale, a patchwork of stable relationships between filaments becomes, amidst the chaos of sampling and slide preparation, a picture of phycological bacchanal.

The lower picture shows a later stage of conjugation, with a zygote forming in the copulation canal.  The process takes place in three dimensions and it was difficult to obtain a crisp image, even using Helicon Focus stacking software but it gives an idea of what is taking place.  The zygote will, eventually, form a tough exterior wall and sink to the bottom of the lake where they will survive until conditions become favourable again.

mougeotia_conjugating_2

Filaments of Mougeotia at a later stage of conjugation: the cell contents are in the process of fusing to form a zygote.   Scale bar: 20 micrometres (= 1/50th of a millimetre).  

The question I have been asking myself is why this particular population has chosen to conjugate at this particular time and place.   I have visited the River Ehen regularly since 2012 and have found Mougeotia or relatives on almost every visit, yet this is the first time that I have seen conjugation.   There are various theories: low nitrogen concentrations have been suggested as something that promotes conjugation in Mougeotia’s relative Spirogyra, but this is unlikely to be a factor in a nutrient-poor lake such as Ennerdale.  A more likely explanation may be found in the graph below, which shows lake levels in Ennerdale over the past year.

ennerdale_lake_levels

Lake levels in Ennerdale Water (from www.riverlevels.uk, measured at NY 088 153, near the outflow to the River Ehen) for the year preceding our visit in October 2016.  

The alga had been growing, remember, in the splash zone.  If you look at the graph, you will see that the lake had recently been almost 30 centimetres higher than it was now and, indeed, had fluctuated quite a lot over the past month or two.   My suspicion is that falling lake levels, and the accompanying risk of drying out, may also have been a factor for initiating conjugation.  Another possibility is that this is a seasonal occurrence that I was fortunate enough to stumble upon, and there is some evidence that dormancy is related to temperature, possibly allowing the zygotes to overwinter in the bottom muds before the increased solar radiation in the spring initiates germination, followed by meiosis (reduction division) to produce the germlings from which next season’s filaments will grow.