Dry Valleys Biology
Life is incredibly resilient. We may be tempted to think that an environment as desolate and hostile as the Dry Valleys, being completely incompatible with human existence (without Antarctic gear, anyway), must also be devoid of all life. At first glance, the ice-free areas, and in particular the Dry Valleys, seem to be completely free of vegetation. Indeed, when Captain Scott passed through the Taylor Valley in 1903, he famously said “we have seen no living thing, not even a moss or lichen”, a statement modern mycologists and lichenologists working in Antarctica will be fast to reject.
Since Scott’s time, it’s been long known to scientists that eukaryotic organisms like springtails, mosses, and lichens can in fact be found in the Dry Valleys, although largely confined to their own preferred habitats. To become established in the Dry Valleys, these biota have had to overcome high levels of desiccation (lack of water), extreme cold, isolation and abrasion (sand-blasting) caused by the wind in one of the most extreme environments on Earth.
There are four main groups that make up the vegetation in the Dry Valleys; lichens, mosses, algae and cyanobacteria (blue-green algae). No flowering plants occur in continental Antarctica and certainly not in the Dry Valleys. All forms of vegetation that are present can dry up and be dormant in dry and cold times.
Lichens are a symbiosis (two or more organisms living together for most of their life cycle) and are made up mainly of a fungus and a photosynthetic alga or cyanobacterium. In the Ross Dependency all lichens contain algae for photosynthesis.
Lichens can withstand extreme conditions especially when dry, the orange lichen in the photograph has actually been sent into space and survived full exposure for two weeks with no ill effects. Lichens grow mainly on rocks where they get water on rare occasions from melting snow or snowfall. They are often found in cracks and hollows where the water supply is better.
Mosses are green plants that reproduce by spores instead of seeds and can withstand drying and extreme temperatures by becoming dormant. Conditions are so bad in the Dry Valleys that they never produce spores and distribute themselves by bits being blown about by the wind. Mosses mainly grow where there is regular water supply.
Algae are microscopic plants and they grow amongst the mosses and always in areas with a regular water supply. The green alga Spirogyra growing between stones in an area where there is a small stream every year.
These are primitive organisms that, as their name indicates, are actually large bacteria. They were one of the first photosynthetic organisms to become successful on Earth and were dominant for several billion years from 2 to 0.6 billion years ago. They are remarkably resistant to drying out and can be found anywhere in the Dry Valleys where water flows over the ground or, see later, as endolithic organisms inside rocks.
This photograph shows sheets of cyanobacteria growing in an area where water flows over the ground in summer.
Unusual adaptations and findings in the extreme environment:
Endolithic communities – life on Mars on Earth
The extreme dryness and the sandblasting in winter means that rock surfaces cannot often be colonised by lichens and mosses. However, in many places the lichens have actually moved inside the rocks where they enjoy a protected habitat that is actually warm in the daytime as the sun’s rays are absorbed by the rocks and so cold at night that they get dew formation that provides a water supply. This unusual life form is often thought to be the most likely to be found on Mars. Endolithics are the largest and most common vegetation type in the Dry Valleys.
Life is slow…
Even though the lichens and the mosses are often large in size, this does not mean that they grow rapidly. In fact, the exact opposite appears to be true. The lichens may be some of the oldest organisms on Earth.
Despite the environmental extremes present, five main groups of terrestrial invertebrates have managed to adapt and even thrive in the Dry Valleys. The following list and brief descriptions give a general overview of the terrestrial invertebrates found within the nzTABS study area of Garwood, Marshall and Miers Valleys.
Free living terrestrial arthropods:
In the physically harsh Antarctic environment both mites and springtails constitute the highest taxonomic level of living organisms and play key functional role atop the Antarctic Dry Valley food web.
Springtails and mites -Len Doel
With a circum global distribution there are over 6000 species of springtail found throughout the world. However, within the Ross Dependency of Antarctica there are only 10 species of springtail present, all with very limited and disjunct distributions. The dominant springtail found in the Dry Valleys, and the only species present in the nzTABS study area, is Gomphiocephalus hodgsoni. G. hodgsoni, the largest purely terrestrial animal in continental Antarctica, ranges from 1-3 mm in size. G. hodgsoni lives primarily under medium to large sized rocks near areas of high soil moisture (i.e. stream margins, snow patches) where it feeds on algae and lichens. In the winter, G. hodgsoni can survive temperatures down to -30C by evacuating its gut and producing glycerol (antifreeze) to prevent freezing while remaining in torpor.
Like springtails, mites have a circum global distribution with over 45,000 known species. Of which only 29 mite species have been described from the continental Antarctic. The Antarctic mite Stereotydeus mollis is the dominant mite species present in the nzTABS study area. S. mollis rarely exceeds >.7 mm in size. Like springtails, S. mollis can be found on the underside of rocks, feed primarily on algae and lichens and can survive sub-freezing temperatures due to the production of special glycerols and carbohydrates in their haemolymph.
The more complex soil invertebrate fauna of the McMurdo Dry Valleys plays a key role in the recycling of nutrients within this already nutrient poor environment. These soil invertebrates employ a special type of life history termed anhydrobiosis (Greek for ‘life without water’) in response to the harsh environmental conditions of Antarctica. Soil invertebrates in anhydrobiosis literally “dry up” losing >99% of their total body water content. They can remain in anhydrobiosis, resisting freezing, for decades waiting for adequate soil moisture before resuming metabolic activity.
Nematodes (round worms) make up the bulk of the soil invertebrates in the Dry Valleys. Within the nzTABS study area 3 species of nematode are found: Scottnema lindsayae, Plectus murrayi and Eudorylaimus antarcticus. Nematodes live in water provided within the interstitial spaces between soil particles and range in size from <1 mm (S. lindsayae) to >2 mm (E. antarcticus). S. lindsayae is unique in that it makes up the bulk of nematode biomass in the Dry Valleys and prefers a much dryer and more saline soil environment compared to both E. antarcticus and P. murrayi.
Plectus murrayi -nematode
Rotifers like other soil invertebrates live in the water provided within the interstitial spaces between soil particles. About 2500 species of rotifers have been described from around the world with the majority coming from freshwater environments. Two species of soil rotifers occur within the nzTABS study area Philodina gregaria and Cephallodella catellina. Rotifers range in size from .1-.5 mm.
More than 6000 species of tardigrades have been described from around the world. Known as polyextremophiles, they have been found living in hot springs, the bottom of the ocean floor and beneath glacial ice. While in anhydrobiosis tardigrades have been reported to be able to withstand temperatures as low as -200C and as high as 150C while also being able to withstand pressures ranging from 0 to over 6000 atmospheres!!! The exact number of tardigrade species occurring within the nzTABS study area is of some debate due to a high possibility of many un-described cryptic species of Antarctic tardigrades. Tardigrades range in size from <.5mm to >.1mm.
Tardigrade -Uffe Nielsen
Bacteria and Archaea
With the exception of Cyanobacteria (aka blue-green algae) commonly found in Dry Valley lakes and ponds and certain endolithic (i.e., living inside rocks) communities, past microbiologists were not able to find evidence of widespread microbial life (specifically, Bacteria and Archaea) in the Dry Valleys. This led some to suggest that perhaps Bacteria and Archaea, like other organisms, are present in very low diversity and confined to very specific habitats in the Dry Valleys; a hugely controversial statement since organisms of these two Domains are widespread in all other environments (even glacier ice and crude oil!).
In recent years, microbiologists have discovered that contrary to previous assumptions, Bacteria are in fact present everywhere in the Dry Valleys, and the level of Bacterial diversity, although much lower than more temperate environments, is higher than anyone had previously expected. The new discoveries were achieved by employing modern DNA- and RNA-based molecular ecology techniques such as DGGE, tRFLP, and more recently, direct sequencing of 16S ribosomal RNA PCR amplicons (see side bar). These new molecular techniques, apart from being more sensitive than traditional culturing techniques, address two fundamental issues with studying Dry Valley microbiology using culturing techniques; first, less than 1% of all bacterial species are cultivatable (although microbiologists didn’t know this until about 20 years ago); second, Dry Valley Bacteria generally grow very, very slowly at temperatures typically of the Dry Valleys (i.e., 0ºC on a nice summer day) but often not at all at higher temperatures. Microbiologists are typically very patient people, but waiting for several years for a bacterial colony to form is too slow a process for even the best of us.
Armed with new knowledge and an ever expanding tool set, microbiologists are making rapid progress at understanding the unique microbiology of the Dry Valleys. Not only are we now able to determine what is present in the Dry Valley environment, but we also know what species are active (versus the species which are dormant or non-viable). Through recent advances in DNA sequencing and metagenomics, we will soon resolve what the microorganisms are doing in the Dry Valleys as well. Through a greater understanding of Dry Valley microbiology, we hope to both understand how these resilient organisms are able to thrive in this unique environment and how global climate change will affect this pristine ecosystem.