Working their passage …

My February visit to Cassop Pond was a warmer experience than my January visit, though copious rain meant that the footpaths we followed to reach the reserve were extremely muddy.  I’ve been gazumped in my endeavour to bring you the phycological riches we encountered on this trip by Heather, who has already mentioned the two species of Trentepohlia that we noticed growing on the wall of an abandoned quarry in her post about the visit.  However, I still managed to find plenty to amuse me in the pond itself. 

On first glance, the pond still does not look very prepossessing.  The ice that had covered the pond during our January visit had gone, but there were still few signs of life above the water level.  It is only when we look below the surface that we find evidence of activity.   Standing at the edge of a marshy area, churned up by the herd of highland cattle who roam the reserve, I can see greenish flocs which, from a distance, they look like filamentous algae.  Up close, however, they are a tangle of narrow, branched strap-like fronds.   This is Riccia fluitans, one of just two free-floating liverworts found in Britain (the other one, Ricciocarpus natans, is also found here, but that’s a story for another day).   In my photograph, you’ll also see a few leaves of Lemna minor, duckweed.  That will become way more prolific later in the year.  Today, Riccia fluitans is the star of the show.

Flocs of the liverwort Riccia fluitans floating just below the surface in Cassop Pond, February 2021.
A close-up of the floc of Riccia fluitans in Cassop Pond.  Each frond is about a millimetre wide.

When we move in even closer and peer at Riccia fluitans through a microscope, the reason for its buoyancy becomes clear: there are a number of floatation chambers.  This, in turn, creates the impression that the cells that contain chloroplasts are forming sinuous bands along the thallus.   I had never thought of chloroplasts as being particularly heavy, but many aquatic plants are parsimonious in the way that they deploy their photosynthetic arsenal.  We saw this in the moss Sphagnum (see “Back to the bog”) and also, last year, in Potamogeton polygonifolius (see “The dark side of the leaf …”)

The thallus of Riccia fluitans viewed under a microscope, showing the mix of green photosynthetic cells and empty cells which help it to remain close to the surface. The thallus is about half a millimetre across.

One other characteristic that Riccia fluitans shares with Potamogeton polygonifolius (and, for that matter, Lemna minor: see “Cassop”) is that it carries passengers.   Cranking up the magnification on my microscope, I can just make out the outlines of several cells of the diatom Epithemia.  I’ll write more about the diatoms from Croft Kettle at a later date but Epithemia is an interesting find here because it is a diatom that is capable of nitrogen fixation (see “More about Croft Kettle”).  I’ve recorded Epithemia from Cassop Ponds in the past, but never noticed its affinity for Riccia fluitans before.   Rooted plants can draw on nitrogen in the sediments to meet their needs; however, free-floating plants depend on what can be acquired from the water.  We also know now that many aquatic organisms are quite leaky so an alga that has a trick like nitrogen fixation up its sleeve may be serving as a food bank for the host organism.   It might just be that the buoyancy that Riccia fluitans provides is a quid pro quo for these nutrients.   This looks like being a mutually beneficial relationship: this small corner of nature being less “red in tooth and claw” than “you scratch my back, I’ll scratch yours” …

Epithemia species just visible on the lower surface of Riccia fluitans from Cassop Pond.  The two cells at the centre of the image are each about a tenth of a millimetre long. 

Some other highlights from this week:

Wrote this whilst listening to: Kin Sonic by Congolese musicians Jupiter & Okwess.   

Cultural highlights:  The Martian, which is basically about growing potatoes in inhospitable environments.  “Fear my botany powers, Mars!”

Currently reading:  My Last Supper: One Meal, a Lifetime in the Making by Jay Rayner.  

Culinary highlight: Persian kababs in a meal box from Berenjak’s restaurant in London.  

5 thoughts on “Working their passage …

  1. Pingback: Structural engineering with diatoms … – microscopesandmonsters

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  5. Pingback: Duckweed delights … – microscopesandmonsters

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