The diversity and tolerance to osmotic stress of East Antarctic filamentous Cyanobacteria

Filamentous cyanobacteria are keystone species in Antarctic lake ecosystems; they are the basis of the
simple foodwebs, play a crucial role in biogeochemical cycling and form the structure of benthic microbial
mats which act as habitats for other prokaryotic and (micro-eukaryotic biota. Despite this, little is known
about their diversity, adaptation and survival strategies in the extreme Antarctic conditions. We studied the
uncultivated prokaryotic diversity using a 454 metagenomic analysis at the 16S rRNA level (V1-V3 region)
in Continental Antarctic lakes situated along a conductivity gradient (0.014-142.02 mS/cm). The quality
and length of the amplicons was analyzed with a custom-made Mothur pipeline and the resulting
sequences were mapped against the Greengenes database, which includes CyanoDB. Almost 27% of the
sequences could be assigned to the phylum of the cyanobacteria. The most abundant cyanobacteria in the
dataset belonged to the genera Microcoleus, Leptolyngbya, Pseudanabaena, Nodularia and Phormidum.
Some 16S rRNA types (at the 97% similarity level), such as sequences related to Leptolynbya antarctica,
were present in both freshwater and hypersaline lakes. In order to further investigate this distribution, we
isolated filaments of Leptolyngbya from seven lakes with conductivities ranging between 26.8 mS/cm and
0.038 mS/cm. The complete 16S rRNA and ITS genes of the isolates were subsequently sequenced.
We found several 16S types related to different lineages of filamentous cyanobacteria in the seven lakes
that were supported by ITS data. Two 16S types, belonging to a Leptolyngbya antarctica and Leptolyngbya
sp., were each present in two different freshwater lakes. Two different 16S types, both belonging to
Leptolynbya antarctica were present in a freshwater and hypersaline lake, which indicates a high
‘intraspecific’ molecular diversity. In order to better understand the adaptation and/or wide tolerance to
osmotic stress, we are currently performing ecophysiological experiments with these isolates aimed at
assessing the potential local adaptation of these strains to conductivity and desiccation.