Transcript in the microbial mats of antarctic lakes

DIVERSITY OF CYANOBACTERIA (BLUE GREEN ALGAE) IN THE
MICROBIAL MATS OF ANTARCTIC LAKES
A. TATON1, S. Grubisic1, D. Hodgson2, L. Hoffmann1, A. Wilmotte1
1 - Laboratory of Algology, Mycology and Experimental Systematic, Institute of Plant biology B22, University of Liège, B-4000 Liège, Belgium. 2 - B.A.S. High
Cross, Madingley Road, GB-CB3 0ET Cambridge, United Kingdom.
Introduction
Research on microbial biodiversity in Antarctica is still in a starting phase though it is a very promising area of research. Antarctica is characterised by its geographical isolation
and its extreme climate. This has led to the evolution of novel biochemical adaptations to severe low temperatures and hypersalinity in lakes (Fritsen & Priscu 1998), and possibly
also of endemic species. In addition, most of the continent has experienced little or no anthropogenic influence. This offers a unique opportunity to study diversity of pristine
biotopes.
The microbial mats appear as fibrous matrixes formed of interlocking filaments and produced by a microbial community generally associated to the sediments (Wharton et al.
1983) and generally dominated by cyanobacteria (Castenholz 1994). In the Antarctic lakes, the benthic microbial mats have accumulated since about ten of thousand of years on
the bottom of lakes. The absence of predation and disturbance by eukaryotes, as well as the low temperatures slow down the decomposition of the deep layers (Ellis-Evans 1996).
The primary production of these mats is higher that the planktonic one (Ellis-Evans 1996). Therefore they play a major role in the dynamics of these lakes but also for the
production of the continent.
List of samples and handling of studied samples
Floristic inventory
For the identification, the observations were realised in light microscopy and the measures were taken with an ocular micrometer.
A small saline lake (24.4 mS/cm) in
the Rauer Islands, near Larsemann
Hills.
The samples have been collected in 3 different parts of the continent.
In the Larsemann Hills, benthic microbial mats were collected from 73
lakes. We received also samples from the Dry Valleys (Lake Fryxell and
Hoare) and from Vestfold Hills (6 different lakes).
A physico-chemical characterisation of the lake water was carried
out and the lakes were selected to cover a wide range of chemical
environments (pH ranging from 5.5 to 9.4, salinity from 0.01 to 31 %)
to maximise the potential for microbial diversity in the sampling
program.
When we received the samples, they were processed immediately.
parts of the samples were fixed in 4% formaldehyde for microscopic
observation, transferred in liquid culture media and stored at – 20°C
for molecular study.
A surface sediment core containing
finely-layered microbial mats from
lake Nella in the Larsemann Hills.
Chroococcales
Aphanothece spp. (4 species)
Synechocystis sp.
Gloeocapsa spp. : Gl. cf. rupestris
Gl. cf. compacta
Gl. cf. alpina
Gl. cf. sanguinea
Chondrocystis cf. dermochroa
Chroococcus sp.
Pleurocapsa cf. aurantiaca
Chamaesiphon cf. subglobosus
Oscillatoriales
Pseudanabaena sp.
Schizothrix spp. : Sch. cf. tenuis
Sch. cf. braunii
or cf. heufleuri
Leptolyngbya spp. (6 species)
Oscillatoria spp. : O. cf. simplissima
O. sp.
Nostocales
Nostoc spp. (3 species)
Calothrix spp. (2 species)
Petalonema cf. involvens
Coleodesmium cf. scottianum
or cf. swazilandicum
Dichothrix sp.
10µm
Gloeocapsa sp. from lake
Gentner, Larsemann Hills.
20µm
Calothrix sp. From Firelight lake,
Bolingen island near Larsemann
Hills.
Culture and isolation
To isolate a maximum of strains, we have used different methods :
1 - using a dissecting needle under the binocular.
2 - homogenising the mats with a potter tube and spreading out 500 µl on solid media.
3 - for the mats rich in sediments, aliquoting in different culture media, vortexing and letting of the
mineral sediments at the bottom of the tubes, transfering of 1 ml of the upper phase in other tubes
with different liquid media or spreading out 200 µl on solid media.
To obtain clonal strains without fungi or eukaryotic microalgae, only one filament or some cells
were transferred on a media with cyclohexamide.
In total, 17 culture media were tested of which we have created 8 media with chemical
compositions similar to those observed in the original lakes.
In addition, three incubation temperatures were used: 5, 12, and 22°C.
20µm
Nostoc sp. From Big lake,
Larsemann Hills.
10µm
Coleodesmium sp. From lake n°
LH61, Larsemann Hills.
Ecology
1
3
Original samples
BG11 BG11o GANX GOX
2
1
1NP
3
Schizothrix sp. &
Leptolyngbya sp.
3NP
Leptolyngbya sp.
Vortex
500 ul
1 ml
200 ul
Calothrix sp.
In waters with a salinity lower than 0,1% as well as in hypersaline
waters, there is a low diversity (Leptolyngbya spp. and Oscillatoria spp.).
The highest diversity occurs in salinities ranging from 0,1 to 0,5%. In
these waters, we have observed all the genera. Whereas the genera
Calothrix, Synechocystis and Gloeocapsa seem to be limited to lakes with
weak salinity (from 0,1 to 0,5%), the genera Leptolyngbya, Oscillatoria,
Schizothrix, Nostoc and Aphanothece have a larger distribution. We
observed them in brackish waters and in waters with a salinity close to
the the sea water.
Molecular study
5°C, 12°C, 22°C
Examples of manipulations.
Obtention of clonal strains without
fungi or eukaryotic micro-algae.
A molecular approach will be carried out on selected samples to study
the genetic biodiversity of these mats in culture and in situ. The
techniques used will be based on the use of rDNA sequences and
include DGGE/TTGE and constructions of clone libraries.
We have presently obtained 37 cyanobacteria strains at 25°C, 20 strains at 12°C and 8 strains at
5°C. The strains are distributed in the following genera : Leptolyngbya, Schizothrix, Nostoc,
Calothrix, Petalonema, Gloeocapsa and Chamaesiphon and come from 20 different lakes.
Conclusion
These are the first steps of the study of the biodiversity of cyanophyceae in the microbial mats from Antarctic lakes. We have now a precise idea of this biodiversity, as it appears in
light microscopy. In addition, 65 strains have been isolated. With the help of the molecular tools, we will study the genetic biodiversity of these mats in culture and in situ. The
comparison of results obtained with these two approaches will allow us to define their advantages and pitfalls.
Bibliography
Castenholz R.W. (1994) - Microbial mat research: The recent past and new perspectives. In: Microbial Mats, Structure, Development and Environmental Significance (Stal L.J., Caumette P.
eds), pp. 3-18. Springer Verlag, Berlin.
Ellis-Evans J.C. (1996) - Microbial diversity and function in Antarctic freshwater ecosystems. Biodiversity and Conservation 5:1395-1431.
Fritsen C.H. & Priscu J.C. (1998) - Cyanobacterial assemblages in permanent ice-covers on Antarctic lakes: distribution, growth rate, and temperature response of photosynthesis. J. Phycol.
34:587-597.
Wharton R.A., Parker C.B. & Simmons Jr. G.M. (1983) - Distribution, species composition and morphology of algal mats in Antarctic dry valley lakes. Phycologia 22:355-365.
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