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.

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