Monday, July 12, 2010

Pond scum

Three weeks of warm weather had left my pond covered with large slimy masses of 'blanket weed' or 'pond scum', the filamentous green algae that tend to plague ponds that have too much nitrogen in the water. When I'd fished most of it out I took a look at a few filaments under the microscope and - like so many living organisms - it revealed structures of great beauty when it was magnified a few hundred times. Inside each cell in the filament the chloroplasts were arranged like strings of green pearls. Various filamentous algae have chloroplasts in different conformations and the most familiar is the spiral chloroplast in Spirogyra..... but this is a different genus.....

The series of fine rings that you can see around the bottom of the upper cell on the left here, just above its junction with the cell below it, identify this alga as a species of Oedogonium. A ring forms each time a cell of this genus divides, so this cell appears to have divided three times.

In amongst the algal filaments there were also desmids - this crescent moon-shaped example is Closterium. The clear areas at the tips of the 'moon' are vacuoles, that contain insoluble crystals of calcium sulphate - a diagnostic feature of this genus.

The most interesting alga in my pond, however, was this one - Coleochaete. It may look like just a pad of simple cells (with some of them apparently dead) but this is an organism of great evolutionary significance. Modern molecular biological studies, and comparative investigations into the ways in which cells divide in this species and in land plants, indicate that Coleochaete shares a common ancestor with present day land plants - mosses, liverworts, ferns, conifers and flowering plants. At some point - maybe half a billion years ago - algae like this, perhaps living in a warm pool of nutrient-rich water like my garden pond, started to colonise the mud and begin the long series of evolutionary changes that led to the development of today's terrestrial vegetation.

A discovery like this makes the chore of cleaning out the garden pond a whole lot more interesting.......... 


  1. Quite beautiful! great post :)

  2. I still remember my first time ever looking at the world of 'pond scum' as a kid...

    ...I never left =D

    What's wonderful about this world is that the more you learn, the more you have to look for, the more you find, and the more questions you get, etc.

    Did you see any ciliates on the algae? There is often a forest of sessile stuff growing on the algae themselves, including small stuff that appears to be some sort of bacteria (visible in DIC w 40x obj)

  3. Thanks Greeneko, it's amazing what you can find in your own backyard...

  4. Hi Psi, couldn't agree with you more - it's addictive. The real pleasure of science is in the doing of it, in the sense of personal discovery that, I suspect, has much the same effect on the brain as endorphins have for marathon runners. And it is a marathon: one thing always leads to another; now I have to find out what those rings around Oedogonium are all about. And I need to shoot some video of those dancing crystals in the Closterium vacuoles........ So much to see! I've been lecturing on the evolutionary relationship between Coleochaete, Charaphytes and embryophytes for fifteen years and it never occurred to me that Coleochaete was right there in my garden pond. Yep, it was seething with ciliates too (and testate rhizopods and rotifers) and you've just reminded me how much life I must have destroyed when I composted all the excess algae. There's some deep philosophical point in there somewhere.

  5. Pond scum has never looked so lovely, or been so interesting...great post!

  6. Thanks falltoclimb, low-life can be fascinaing. I'm following the posts from your trip with great interest (and envy)...

  7. Stunning pictures, they look like very delicate jewellery.

  8. Hi Toffeeapple,... know what you mean .... I was thinking strings of green pearls..


Note: Only a member of this blog may post a comment.