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Impact of MAngrove exploitation of the Benthic ecosystem and assessment of Ecological quality objectives -Vietnam and Belgium cooperation
The second Master of Science in Nematology of IMABE project (2006-2007)
on 03-11-2007

In the framework of IMABE project, the second Msc. is Nguyen Van Thanh was completed the Postgraduate International Nematology Course (PINC 2006-2007) in Gent University, Belgium. The thesis defence was taken place on September 2007.

With title: Heavy metal toxicity for populations of  free-living estuarine nematodes.

Abstract:  Effects of anthropogenic pollution on ecosystems are important for their management and conservation. Many coastal ecosystems are contaminated with high loads of toxicants, including several heavy metals. Understanding and predicting effects of toxicants at the ecosystem level, however, requires a proper understanding of responses of individual populations of important taxa. Nematodes are typically the most abundant benthic metazoa and have a potential as bioindicator organisms. Some species are culturable and may be amenable to experimental ecotoxicological studies. This may allow their use in population dynamics studies looking for sublethal effects of toxicants rather than for the traditionally assessed acute toxicity effects.

In the framework of a programme aiming to elucidate current effects of historical anthropogenic pollution in coastal mangrove areas in Vietnam, we set out to establish a nematode bioassay for sublethal toxicity tests under controlled laboratory conditions. In order to improve the relevance of our work, we first established laboratory cultures of two nematode species, Diplolaimella dievengatensis and Diplolaimelloides delyi, from the Nam Dinh mangrove area in Vietnam.

We thoroughly checked the taxonomical identity of the newly isolated population using both traditional morphological and molecular methods, the latter including analysis of partial sequences of the nuclear SSU rDNA and the mitochondrial cytochrome oxidase subunit I gene. Based on morphological characteristics, our population could be identified as D. dievengatensis, although there was some – and in males substantial - overlap of several characters/measurements with D. thailandica. The SSU sequence data differed less than 1% from a D. dievengatensis population from The Netherlands, but the mitochondrial divergences between both populations were in the same order (20%) as divergences between D. dievengatensis and several representatives of the genus Diplolaimelloides. Conclusive species identification will therefore require multivariate analysis of morphological characters as well as analysis of additional molecular loci such as ribosomal ITS.

With the aim of optimizing culture conditions for the bioassay, we studied the life history and population development of the former species at different salinity (15 to 35 psu) and temperature (15 to 350C) conditions relevant for the area. Salinity had a limited effect on total population development and on most of the life history parameters studied, except at the highest value tested, which was 35 psu. At this salinity, the fitness of our D. dievengatensis population was significantly impaired compared to the other salinities. Salinities of 15 to 20 psu yielded the highest fitness, but for most parameters without significant differences with salinities up to 30 psu. Temperature had much more pronounced effects on the population development and on most life history parameters than salinity, with fastest development and highest fecundity at the highest temperatures tested here, 30 and 350C. Development at 150C was strongly retarded, and only very few F1 progeny matured to adulthood within the time frame of our experiment (24 days).

We then performed a first ecotoxicological assay with both populations of D. dievengatensis, the newly isolated one from Vietnam and the one originating from The Netherlands; with the aim to study (1) the toxicity of chromium, and (2) to compare the response of geographically separated populations stemming from environments with different abiotic and pollution characteristics. We studied effects of three different loadings of hexavalent chromium and compared with a Cr - free control. Both populations were highly sensitive to the highest two concentrations, but nevertheless clearly differed in their response: the Dutch D. dievengatensis population had lower mortality and higher fecundity at the secondhighest concentration of Cr than the Vietnamese population. Moreover, whereas population development of the Vietnamese population was significantly impaired at the lowest Cr concentration, the Dutch population performed equally at this Cr - level as in the Cr - free controls. The toxicological features found in the present assay provide a nice example of how similar species or even populations of the same species can differ in their responses to environmental factors. Our results also illustrate that nematodes are suitable models for toxicological assays assessing not only lethal but also sublethal concentrations of pollutants. All parameters studied here together show a promising potential of nematode bioassays for future ecotoxicological studies.

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