I have a theory, which I have touched on before in these posts, that the success in conveying the wonders of nature to non-biologists is easiest when the audience can relate what they see directly to their own experiences. You only have to watch a typical David Attenborough documentary to see this principle at work: it may feature sumptuous photography in glorious landscapes, but the events portrayed are not so different to a typical Friday evening at the Bigg Market in Newcastle. The BBC Natural History Unit would find plenty of courtship activities, territoriality and several kinds of violence here, much of it set around watering holes. Who needs a plane ticket to an exotic location?
As we lose that sense of empathy, so nature becomes “weird”. A few of us find fascination in the weird but we are a minority. Strangeness, however, brings problems, as I have commented before (see “Reflections from the trailing edge of science”) as stories cannot be conveyed using familiar metaphors drawn from our own experience. The example I used in that earlier post was the concept of “alternation of generations” in plants and my recent encounter with the red alga Lemanea a couple of weeks ago (see “Spaghetti Carbonara con Lemanea”) reminded me of a set of wonderful photographs by Chris Carter that illustrate this concept very well.
That post contained a photograph of Lemanea from the River Ehen in Cumbria which shows some of the wiry filaments growing on the stream bed. These filaments are, actually, hollow tubes of cells (see photograph in “The River Ehen in April”) along which there are a series of nodes. The nodes, in this case, bear sexual cells at certain times of the year (see “Lemanea in the River Ehen”).
A cross section of a filament of Lemanea from the River Rede, Northumberland (photo: Chris Carter).
Chris’ photographs shows how the Lemanea filaments are actually composed of a hollow tube of cells with an outer cortex. However, the centre of this tube is not completely empty, and the clusters of cells that we can see inside the tube are spore-producing organs called “carposporophytes”. At some point during the development of the carpospores, two cells fuse so that the carpospores is diploid (2n), rather than haploid (n). The carpospores are released when the Lemanea filament dies back in late Spring and these then germinate into a filamentous sporophyte (2n) phase, called the “chantransia”. At some point during the winter, these chantransia undergo meiosis, and the resultant haploid cells grow, still attached to the chantransia, into the next generation of gametophytes.
Transapical view of a Lemanea filament; the arrows show the sporophytes (“carposporophytes”) inside (photo: Chris Carter).
Finally, I have included Chris’ high magnification photograph of some of the cells of this carposporophyte plant, looking very similar to simple red algal genera such as Audouinella, which prompted my original series of posts on alternation of generations.
These photographs capture my fascination with the algae: apparently simple, easily overlooked, but actually presenting sophisticated, highly-evolved solutions to survival under tough circumstances. The constant current in rivers makes establishing and maintaining a population at one place hard enough, more so when a “population” actually consists of two discrete stages. This has led some to suggest that the complexities of the red alga life cycle may be a form of “bet hedging”, spreading the risk of mortality between the life stages. Having a large gametophyte phase, for example, gives the plants access to more light, making them more productive, but they are also exposed to the strong currents in the river, increasing their risk of loss due to scour. On the other hand, the smaller sporophytes (the “chantransia”) are protected from the ravages of the current because they live close to rock surfaces, within a “boundary layer” where current velocity falls off due to drag. It could be seen to be roughly parallel to the metamorphosis of butterflies and other insects, with phases of the life cycle optimised for different activities.
Lemanea faces a particular challenge: the gametophytes have “solved” (excuse the teleology) the challenge of living in very fast current speeds, where they have little competition from other plants and algae and, I would guess, little threat from grazing invertebrates. This gives the genus plenty of scope to thrive in fast-flowing upland rivers. There is normally a benefit to an organism of releasing spores and gametes into their immediate environment, as this encourages dispersal and cross-breeding. Were Lemanea to do this, the spores and gametes would be washed quickly downstream, away from their ideal habitat. The practice of keeping the carposporophyte inside the thallus, rather than on the outside, increases the chances of some of the carpospores finding their way to the rocks in the immediate vicinity of the gametophyte and, thereby, ensuring that the chantransia are well-placed to produce a new gametophyte generation the following year.
It is all very complicated. This is, I suspect, partly because systematic biologists have a fondness for obscure terminology that makes it hard for the non-initiate to follow the twists and turns of life cycles. But it also, I suspect, a consequence of dealing with habits and life cycles that are unfamiliar and, more importantly, cannot be distilled down to simple, anthropomorphic metaphors.
High magnification view of carpospores of Lemanea (photo: Chris Carter).
Sheath, R.G. (1984). The biology of freshwater red algae. Progress in Phycological Research 3: 89-157.