There were flurries of snow as I pulled on my waders beside the River Ehen last week. This is, somewhat counter-intuitively, a good omen for fieldwork. Rain flows off the land and into the river whereas snow stays where it falls, at least until the thaw comes. The water was cold – so cold that I could barely decipher the notes that I made with my frozen hands – but it is high flows that are the real problem when working in rivers.
The algae living in the Ehen were much the same as on the last visit though my first impression is that the overall quantities were lower. In particular, I was fairly sure that the gelatinous growths of Draparnaldia (see earlier post) were less prolific. They looked different under the microscope too, with the filaments surrounded by swarms of tiny pear-shaped cells, all moving around. Each has two flagella for motility though these are too small to be visible here.
These tiny cells are zoospores, produced by Draparnaldia as a means of dispersal. I had not looked at the sample immediately on my return and the following day it had sat in a plastic bag in my fridge whilst I got on with other jobs. So almost two days elapsed between removing the alga from the stream and looking at it under the microscope. My experience is that Draparnaldia and relatives often form zoospores when subjected to such indignities and it gives us a useful insight into the biology of these algae.
I’ve previously I mentioned that algae such as Spirogyra rarely form reproductive organs in the field, which makes it hard to identify the exact species. Those of us who study freshwater algae accept this as a fact of life but it is at odds with much of what is written about the advantages of sexual reproduction to organisms (Matt Ridley’s otherwise excellent book The Red Queen is a case in point). The pessimistic view is that the best you can get from sex is a 50 per cent dilution of your DNA. So if all is good in your world, invest your energy into adding more biomass to what you already have. This is especially the case for an alga stuck to a rock in a fast-flowing stream. The zoospores I found were the alga responding to an unfavourable environment (my fridge) by emigrating. Or, at least, trying too. A tiny minority of these, in the wild, would settle on a surface and grow into new filaments genetically identical to the parent. True sexual reproduction does happen in Draparnaldia but, as is the case for Spirogyra, it is rare.
If you do want to see sexual reproduction in Spirogyra, the best thing to do is let a sample dry out slowly. In other words, simulate the worst possible conditions that the alga would expect to encounter. Why? Because the outcome of conjugation is a resistant zygospore that can sit out the hard times, waiting until the environment is again suitable to produce a new filament. Curiously, science writers such as Ridley don’t hint at this as one possible advantage of sexual reproduction. This is doubly curious because, as algae are amongst the most primitive of organisms, these are the situations where sex must have first evolved. We view sexual reproduction from the opposite end of the evolutionary scale and, thanks to Petrarch, Shakespeare, Freud and others, give it far more prominence than perhaps should be the case.