Sticky water …

It seems strange to be writing about an alga that thrives in winter in the middle of one of Britain’s rare heatwaves but I came across two papers recently that shed some light on the ecology of Ulothrix zonata.  This is a species that has intrigued me for some time, having a very distinct preference for times of the year when our rivers are at their coldest and I have tried to unravel the reasons for this in some earlier posts (see “Bollihope Bhavacakra” and “The intricate ecology of green slime …”).

The lead authors on these papers are based at Irkutsk, beside Lake Baikal in Siberia, so are in a good position to contemplate the effects of cold conditions on algae.   Whereas I complain about plunging my hand into cold water in northern England to collect Ulothrix zonata, they had to Scuba dive under the ice of Lake Baikal in water only just above freezing point.  They have found two adaptations in the Lake Baikal populations of U. zonata to cold conditions.  The first of these is that the ratio of polyunsaturated to saturated fatty acids are higher in these populations than in those of other Ulothrix species.   This sounds as if it could be an algal equivalent of the “blubber” that insulates sea mammals but the truth is rather more mundane: it is part of a series of adaptations of the cell membrane that allows the organism to keep functioning despite the harsh conditions.

Living underwater is, in many ways, the easy option for a plant in Siberia, where the average outside temperatures in winter are less than -10 °C, and the record low is almost -50 °C.  Terrestrial plants adapt to such harsh environments simply by shutting down operations.   However, whilst the surface layers of Lake Baikal freezes, life below the ice can continue and there are several studies about the rich algal life within this enormous lake (which contains a fifth of the planet’s fresh water).   Enough sunlight can penetrate through the ice to sustain growth, albeit at a slow rate but, on the other hand, the cold water creates problems of its own.   In particular, the density of water increases as temperature drops, making it more viscous.  We might not notice that cold water is more gloopy than warm water but that is because how we experience viscosity is partly a function of our size.  What might be an insignificant change to a human can be a big deal to a microscopic alga.

The cell membrane is composed largely of lipids and, like margarine, these are soft in warm environments (such as frying pans) but hard in cold environments (such as refrigerators or Lake Baikal).  The problem for cells is that there are other molecules embedded in the lipid layers which help the cell obtain the raw materials it needs, and these will not be able to function if the lipids in the membrane are too rigid.   Molecules of saturated fats pack together more compactly than those of polyunsaturated fats which means that a membrane with lots of these is more rigid than one with a high ratio of polyunsaturates.   Consequently, if an organism is to thrive in cold environments then it is beneficial for it to have a high ratio of polyunsaturated to saturated fats in the lipid molecules.

Water is one of the molecules that submerged algae need to shift into their cells to keep their cellular machinery running as this is one of the raw materials of photosynthesis.   There is no shortage of water on the outside of the cell.  However, having a membrane composed largely of hydrophobic lipids means that this is not straightforward and one of the molecules that is embedded in the lipids belongs to a group of proteins called “aquaporins”.  These are shaped in such a way that there is a narrow channel in the centre (like the hole in a doughnut) through which water molecules can pass in single file.

Aquaporins are well known in animal, plant and bacterial cells but it is only recently that they have been found in algae too.   Aleksey Permyakov and colleagues showed that Ulothrix zonata populations from Lake Baikal and streams in the vicinity had more aquaporins in winter than the summer, which they interpreted as an adaptation that ensured a steady supply of water to the cell despite the higher viscosity of the water.  This is also the first time that algal cells have been shown to be able to regulate the amount of aquaporin in membranes in response to their environment.

These two observations together suggest how cold-tolerant algae may have to invest some of their hard-earned energy in modifying their membranes to help them thrive.  I suspect that this is part of a complex network of interactions here: survival in such extreme conditions is possible because the slow rate of growth in very cold water is offset by an even slower rate of grazing and other processes which remove algal biomass.  Diverting energy and resources to make more aquaporins, in turn, means that photosynthesis is not limited by a shortage of raw materials.   It is a fine balance but, if an organism can get this right, then there is an opportunity to thrive with relatively little competition from other organisms.  It is another reminder that ecology is a science that depends on a 365 day perspective and that we should not assume that a few fieldtrips when the weather is most clement will reveal all of its riches.

References

Osipova, S., Dudareva, L., Bondarenko, N., Nasarova, A., Sokolova, N., Obulinka, L., Glyzina, O. & Timoshkin, O. (2009).  Temporal variation in fatty acid composition of Ulothrix zonata (Chlorophyta) from ice and benthic communities of Lake Baikal.  Phycologia 48: 130-135.

Permyakov, A., Osipova, S., Bondarenko, N., Obolinka, L., Timoshkin, O., Boedekker, C., Geist, B. & Schäffner, A.R. (2016).  Proteins homologous to aquaporins of higher plants in the freshwater alga Ulothrix zonata (Ulotrichales, Chlorophyta).  European Journal of Phycology 51: 99-106.

The photograph at the top shows Ulothrix zonata growing on the bed of the River Wear at Wolsingham, Co. Durham in February 2009.