The Example of Discovery research in Ecology
Tracking down water pollution
through DNA of algae
Diatoms between 0.01 and 0.02 mm,
consisting of a single cell surrounded by an artificially colored silica
skeleton. The alga in green is present in clean environments, while the orange
one lives in more polluted water.
Credit: Laure Apothéloz-Perret-Gentil,
UNIGE
Diatoms
are a group of unicellular algae particularly sensitive to changes that affect
their aquatic environment. This is why they are used as bioindicators for the
biological monitoring of water quality. However, their microscopic
identification in river samples requires a lot of time and skills. Biologists
from the University of Geneva (UNIGE), Switzerland, have succeeded in
establishing a water quality index based solely on the DNA sequences of the
diatoms present in the samples, without needing to identify each species
visually. This study, published in the journal Molecular Ecology Resources,
presents a revolutionary tool to process a very large number of samples in
parallel, allowing wide coverage of the monitored sites in a reduced time and at
a lower cost.
The
degree of pollution of rivers resulting from human activities is assessed using
different biotic indices. The latter reflect the ecological status of a river
based on the quantity and diversity of organisms selected as bioindicators, due
to their ecological preferences and tolerance to pollution. This is the case of
diatoms, algae consisting of a single cell surrounded by a silica skeleton,
recommended by the European Union and Switzerland as one of the ideal
bioindicators for rivers and lakes.
The
quality of our rivers is determined using the Swiss diatom index (DI-CH), whose
value defines the ecological status. "The morphological identification of
the different species present in each sample, however, no longer meets the
needs of rapid and reliable bioassessment measures introduced to protect
aquatic environments. This is why we have tried to develop a new method,"
says Jan Pawlowski, professor at the Department of Genetics and Evolution of
the UNIGE Faculty of Science.
DNA
sequences as bioindicators
In
collaboration with the Geneva Water Ecology Service (SECOE) and the PhycoEco
environmental office in La Chaux-de-Fonds, Switzerland, the researchers
analyzed the 90 or so samples taken in different rivers in Switzerland and
determined their ecological status using the DI-CH. They have thus established
a reference system in order to validate the molecular index under development.
The latter is based on the DNA sequences characteristic of all the diatom
species which may be present in these samples.
"The
whole range of DNA sequences revealed in each sample corresponds to a specific
DI-CH quality index. Furthermore, each sequence identified has a different
distribution and is detected in variable amounts from one sample to another. By
integrating all these data, we were able to calculate an ecological value for
each sequence, without having to identify the species to which it
belongs," explains Laure Apothéloz-Perret-Gentil, a member of the Geneva
group and the first author of the study.
A
tailor-made molecular index for the environment
This
approach makes it possible to determine the quality of water using all of these
ecological values. "Our assessment was correct for almost 80% of the
samples, which is very encouraging. Increasing the number and diversity of
samples will allow us to calibrate our method for future routine, large-scale
analyzes," indicates Jan Pawlowski.
The
synchronous processing of a large number of samples in record time and at a
reduced cost is not the only advantage of this new tool. The molecular index
developed by the biologists from UNIGE could easily be adapted to other groups
of unicellular bioindicators: a major asset for monitoring various types of
aquatic ecosystems.
Story
Source:
Materials provided by University of Geneva. Note: Content may be
edited for style and length.
Journal
Reference:
- Laure Apothéloz-Perret-Gentil, Arielle Cordonier, François Straub, Jennifer Iseli, Philippe Esling, Jan Pawlowski. Taxonomy-free molecular diatom index for high-throughput eDNA biomonitoring. Molecular Ecology Resources, 2017; DOI: 10.1111/1755-0998.12668
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