Two months after the visit I described in the previous post I was back at Castle Eden Dene. The trees were now in leaf and the floor of the forest was carpeted with wild garlic. The stream, however, had disappeared below the surface and, once again, I could walk along the channel without getting my feet damp.
Having found a rich crop of diatoms on my last visit when the stream was dry I was intrigued to see what was growing on the stones this time, so I used a toothbrush and some water that I had brought along to scrub a few and collected the dislodged material in my white tray. I was intrigued to see that the suspension that collected in my tray had a distinct green tinge and, when I got a drop of it under my microscope, found it to be dominated by small green cells. These were superficially similar to the cells of Desmococcus and Apatococcus that I found on the fence in my garden (see “Little Round Green Things …”) but this is a difficult group with not many clear morphological features with which to distinguish genera so I sent a sample off to Dave John for his opinion.
His view is also that groups such as this are almost impossible to identify unless you grow them in the laboratory or have access to DNA sequencing facilities. He commented that Desmococcus and Apatococcus both have distinctive 2- or 4-celled packets of cells, which were not common in the Castle Eden Dene sample. Likely candidates are the generaPleurastrumand Pseudopleurococcus, both of which are subaerial or terrestrial. Perhaps “Little Round Green Things” is as close as we need to go in this particular instance?
A distinctly-green suspension of the biofilm on stones at Castle Eden Dene in May 2019 (left) along with a magnified view showing some of the green cells which dominated the sample (right). Scale bar: 20 micrometres (1/5thof a millimetre).
A short distance further on I found some mats of entwined filaments on the tops of stones which also piqued my curiosity. Under the microscope, and with the addition of a drop of water to rehydrate them, these filaments revealed themselves to belong to Vaucheria (see “Who do you think you are?”). Technically speaking, Vaucheria is not filamentous but “siphonous”, meaning that there are no cross walls but, instead, the organism consists of branching tubes containing many separate nuclei and chloroplasts. The cell walls of Vaucheria, however, rupture easily releasing the chloroplasts and giving the appearance of an empty sausage skin. In this case, there are still quite a few chloroplasts but a healthy Vaucheria filament has a uniformly dense green appearance that none of those that I saw in Castle Eden Burn possessed.
There was more than just vegetative filaments of Vaucheria here: scattered amongst them were some larger, spheroid or jar-shaped cells, which are part of Vaucheria’s sexual reproduction apparatus. I’ve talked before in this blog about how sexual reproduction is relatively rare in the filamentous algae that we find in lakes and streams (see “The perplexing case of the celibate alga …”) and Vaucheria is another case in point. Put simply, many algae do not bother with sexual reproduction when conditions are favourable and they can grow through simple cell division. If you subjected a Vaucheria filament to Freudian analysis, it would probably tell you that one outcome of sexual reproduction was a 50% dilution of its unique genotype. So why bother if you don’t have to? On the other hand, sexual reproduction in these organisms usually results in a zygote with a thick wall that is capable of resisting tough conditions. The complete absence of water in Castle Eden Burn would be one such circumstance. To put it another way, when the going gets tough, the algae get frisky.
Mats of Vaucheria growing on a small boulder in Castle Eden Dene in May 2019. The picture frame in the left hand image is approximately 30 centimetres.
Cell walls of Vaucheria, with a few chloroplasts still present, from Castle Eden Burn, May 2019. Scale bar: 20 micrometres (= 1/50thof a millimetre).
Sexual reproduction involves an antheridium (the male organ) producing a large number of spermatozoids, each with a pair of flagella. If a spermatozoid encounters the oogonium (the female sexual organ), then it will fertilise the single egg that this contains. The zygote will then surround itself with a thick wall and undergoes a period of dormancy before geminating into a new vegetative filament. As I was taking the photographs below, a cloud of tiny spermatozoids was released, prompting me to call out “come quickly if you want to see an alga ejaculating” before remembering that we had visitors in the house who might think this a little weird. I even took a video. I’ll upload it to the Dark Web at some point. There must be a site for algae-themed pornography out there, if only I took the time to look…
An oogonium and an antheridium on Vaucheria filaments from Castle Eden Burn, May 2019. The one on the right was releasing spermatozoids (arrowed) at the time the photograph was taken. Scale bar: 20 micrometres (= 1/50thof a millimetre).