My recent overviews of the major groups of algae have been useful as a way of highlighting which families and orders I’ve neglected. That’s mostly because my posts are largely reactions to the circumstances I find myself in, rather than as a comprehensive overview of the world of freshwater algae. My travels, this week, have brought me to Lough Down, a little-known Irish lake to which many of us have journeyed over the past few weeks. Today, I thought I would peer at the littoral zone in the hope that I might find an alga from an order I have not previously written about.
I seem to be in luck: there has not been much rain recently and there is recently-exposed mud. When I look closely, I see tiny green spheres, each the size of a pin head, dotted across the mud surface. These are vesicles of the alga Botrydium and, despite their bright green colour, they actually belong to the Xanthophyceae (see “When a green alga is not necessarily a Green Alga …”). Below this pear-shaped vesicle there is a system of branched rhizoids which anchor the plant to the surface (see lowermost photograph). Surprisingly, the whole plant is a single cell, a similar situation to the one we encountered in Vaucheria, another representative of the Xanthophyceae (see “The pros and cons of cell walls”).
The vesicle itself is green, and a close look reveals the presence of many chloroplasts and also (less easy to see without special stains) nuclei. You can also see, on the image below, crystals of calcium carbonate which are deposited on the vesicle. However, if the marginal mud where Botrydium thrives is flooded again, the cell contents divide into a large number of spores, each with a single nucleus and two flagellae, which are liberated. On the other hand, if the pond continues to dry, then a different type of spore is produced, as the cell contents retracts into the rhizoids where it forms thick-walled spores, which can survive long periods of desiccation. Once the mud is dampened again, these spores germinate into motile spores.
A vesicle of Botrydium granulatum spotted with crystals of calcium carbonate. The photograph at the top of the post shows vesicles on the bed of a pond (both photos: Chris Carter).
Botrydium is a small genus, with just eleven species listed on AlgaeBase, of which just one, B. granulatum, is recorded from the UK and Ireland. It fills in one glaring gap in my coverage of the Xanthophyceae, leaving just two Orders still to feature. These are the ones containing the awkward little single cells and colonies that are difficult to identify and easy to confuse with similar-shaped forms in the Chlorophyta. I’ll get around to writing about these one day. Maybe I’ll find representatives from them on a future visit to Lough Down. Who knows how much time I will have to become acquainted with this fascinating lake?
A complete Botrydium granulatum plant, showing the vesicle on the left with a series of rhizoids extending out below. The lowermost rhizoids are obscured by soil particles (photo: Chris Carter). All images in this post are from material collected from Pitsford Water, Northamptonshire.
Some other highlights from this week:
Wrote this whilst listening to: J.S. Bach’s St Matthew’s Passion and Laura Marling’s new album, Song for Our Daughter. My systematic review of Dylan’s back catalogue has reached the incomparable Blonde on Blonde.
Cultural highlights: Portrait of a Lady on Fire. French arthouse film. I know, I know …
Currently reading: Drawing to the close of The Mirror and The Light .
Culinary highlight: Buying a bag of strong white flour after many abortive attempts. And a Simnel Cake, in celebration of Easter.