Many people are surprised to hear that vegetation – mainly mosses and lichens – manages to grow on the Antarctica mainland. These relatively simple photosynthesising organisms have been the focus of 27 years of study by Waikato University’s Allan Green and his co-workers and students from New Zealand, Spain, Austria, Germany and Australia.
Lichens – which are actually algal/fungal symbionts – can be active even when it’s extremely cold. They’ve been found photosynthesising in temperatures as low as -17ºC. But mosses don’t have this ability. While they endure the bitterly cold winters by dehydrating and entering a state of dormancy, in summer they are active and, in this state, can be damaged by sub-zero temperatures of only -3 to -5ºC. How do they manage to survive?
It seems that, during the summer, Antarctic mosses are not actually exposed to temperatures that are significantly below freezing. Studying the mosses in Taylor Valley, in the Dry Valleys at 77º South, Allan’s team showed that, during December, the mosses had temperatures that were warmer than the air in both day and ‘night’ (when it is still light, but the sun is at such a low angle that it has only a slight warming effect).
The following data are from measurements taken over a 10-day period in December.
Light intensity (red), moss temperature (blue) and air temperature (green), December 14-24
These data show that, late each day, the temperature of the mosses falls rapidly as the light intensity (PPFD) declines, but stops once the mosses reach freezing point. The plants stay at that temperature all ‘night’ until the sun rises higher in the sky next morning.
What is happening here?
This is a latent heat phenomenon. As the water freezes, latent heat is given out, so that the water and the new ice stay at freezing point (0ºC) until all the water is frozen. This does not happen because the mosses grow in wet areas and freezing is not completed overnight. Long-term records show that this happens for mosses along the whole of the Ross Sea coastline, from 72º to 77º South.
The story of the mosses’ survival is even more complex than this. Because the mosses in these wet areas do not go below freezing during the summer months, they do not protect themselves against lower temperature. This means that even a temperature as high as -3ºC will stop their metabolism and halt photosynthesis. Later in the season as the available water starts to dry up and the protection offered by latent heat becomes less reliable, the mosses change their metabolism so that they are still active and undamaged down to -7ºC.
Nature of Science
Scientific knowledge is developed by a process of on-going inquiry, sometimes over many months, years, decades or even centuries.