My most recent trip to Ennerdale Water was on a wonderful windless winter day, offering perfect reflections of the snow-dusted peaks beyond the lake. It was a cold day but I was well wrapped-up and could enjoy both the long-distance views and the close-ups of nature around the lake’s margins. One of the small streams that I crossed as I skirted the perimeter of the lake had patches of green algae growing on its submerged stones and even a quick examination showed it to be coarser than the green algae that covered most of the larger stones on the lake bed itself, as well on those in the River Ehen, just below the outfall. When I managed to get specimens under my microscope I saw that the algae on the lake bed was Spirogyra (which I have seen here before; see “A lake of two halves”) whilst that in the inflow stream was Oedogonium.
I’ve written about Oedogonium before, and lamented the problems we face when we try to identify the species within this large genus (see “The perplexing case of the celibate alga”). Ironically, a couple of weeks after I wrote this, I encountered a population of Oedogonium in another Cumbrian stream that did have sexual organs (see “Love and sex in a tufa-forming stream”). However, this was the exception that proves the rule, as I have not seen a sexually-mature population of Oedogonium since. The population I found beside Ennerdale was not sexually mature either but it did show a different, but equally effective, means of going forth and multiplying.
In the left hand diagram below we see a vegetative cell from an Oedogonium filament that has split open, allowing a vesicle to be extruded within which a single zoospore has formed. This has a ring of flagella at one end, resembling a monk’s tonsure (you can just see these flagella in the photograph). The other two photographs show the monk’s bald pate, though the fringe of flagella is not very clear. The transparent vesicle swells and eventually ruptures, releasing the zoospore, which swim around for an hour or so, before settling on a new substratum and growing into new filaments.
Zoospores of Oedogonium from a stream flowing into Ennerdale Water, January 2018. Scale bar: 25 micrometres (= 1/40th of a millimetre).
In my material, the new filaments were mostly attached to mature Oedogonium filaments; however, this is probably partly an artefact and, in the field, they would almost certainly also settle on rocks and other surfaces too. You can see, in the diagram below, how the “bald” end of the zoospore has started to differentiate into a holdfast that will secure the cell to the substrate whilst, over time, the other end will start to divide to produce the first cells of the new filament. The whole process is described in a series of papers by Jeremy Pickett-Heaps (see reference list below).
Why did I see zoospore formation in this particular sample? I don’t know for sure but it may be because I let a longer than usual time elapse between collecting and examining the sample. This one had sat around in a cool box and fridge for four days, whereas I usually manage to check them within 24 hours. Neglect can be a useful tool in the phycologist’s arsenal, as many freshwater algae see no need to indulge in anything more taxing than routine cell division for as long as the habitat keeps them replenished with whatever light, nutrients and other resources that they need. Only when this is no longer the case do the algae start to channel resources into survival strategies.
Oedogonium zoospores germinating into new filaments, both epiphytic on mature filaments. From a stream flowing into Ennerdale Water, January 2018. . Scale bar: 25 micrometres (= 1/40th of a millimetre).
Although I used the phrase “go forth and multiply” in an earlier paragraph, these Oedogonium cells are actually “going forth” rather than “multiplying” as the process we are watching only produces a single new cell. However, were this zoospore to be released in a stream rather than a sample bottle, then there is a good chance that it would have been washed downstream and that a few of the many zoospores might have settled on a suitable habitat away from the constraints of their former home. Asexual reproduction is a dispersal mechanism that results in the spread of genetically-identical copies of the parent cell. For a sessile organism, this strategy allows a single genotype to move on from less-favourable locations and to exploit the potential of nearby locations.
The word “reproduction” is misleading as the mixing of genetic material that we associate with sex doesn’t take place. The end product is a clone of a successful Oedogonium filament growing somewhere else. However, taking the “sex” out of “asexual” removes a huge potential for innuendo, and readers who have battled this far through a post on nondescript green filaments deserve a reward. So let’s finish with Woody Allen’s definition of masturbation as “sex with someone you love” and suggesting that the cytological huffing and puffing involved in zoospore production may not have the romance of sex but nor does it lead to any of the complications which result from sex either. The alga gets offspring that are 100% identical to itself, just slightly further downstream and there is no risk of mixing with inferior genotypes. That’s about as “selfish” as the “selfish gene” can get.
Pickett-Heaps, J. (1971). Reproduction by zoospores in Oedogonium. I. Zoosporogenesis. Protoplasma 72: 275-314.
Pickett-Heaps, J. (1971). Reproduction by zoospores in Oedogonium. II. Emergence of the zoospore and the motile phase. Protoplasma 74: 149-167.
Pickett-Heaps, J. (1972). Reproduction by zoospores in Oedogonium. III. Differentiation of the germling. Protoplasma 74: 169-173.
Pickett-Heaps, J. (1972). Reproduction by zoospores in Oedogonium. IV. Cell division in the germling and the possible evolution of the wall rings. Protoplasma 74: 195-212.
See also “The River Ehen in March” for some further perspectives on asexual reproduction in algae.
View from near our sampling site on Croasdale Beck, looking towards Ennerdale Bridge, January 2018.