Fatty acid compositions of four benthic species along the salinity gradient of a human impacted and mangrove dominated tropical estuary (Can Gio, Vietnam)

Tropical estuaries are characterised by high spatial and temporal variability in the conditions they provide for living organisms, which in turn may affect their biochemical composition. Anthropogenic contaminants such as nutrients, antibiotics, pesticides and trace metals may also affect the biochemical composition of these coastal organisms, notably with regard to fatty acids (FA), of which optimal species-specific polyunsaturated FA ratios are required to maintain living species in healthy conditions. The objective of this study was to evidence changes in the FA profile of four ubiquist benthic species in relation to the salinity gradient of a human impacted and mangrove dominated tropical estuary (Can Gio, Southern Vietnam). Among the four benthic species studied, two exhibited spatial and seasonal changes in their FA composition: the varunid crab Metaplax elegans and the predatory mangrove murex snail Chicoreus capucinus. Proportions of the FA 16:1ω7, proportions of highly unsaturated FA and ratios of eicosapentanoic acid/arachidonic acid in the tissues of both species seemed related to upstream inputs. Different relative abundances of benthic microalgae affected both claw muscle and hepatopancreas FA composition of M. elegans, while the FA composition of C. capucinus shows that the influence of prey's diet might be transferred within trophic chains. Finally, some species, such as Nerita balteata and Metapenaeus ensis might have more adaptive capacities, because of their mobility, or because of their specific metabolic pathways, minimizing the potential impact of spatial and/or temporal changes in food resources quality. A starvation experiment involving N. balteata actually revealed the metabolic synthesis of non-methylene interrupted FA that may confer cell membranes a higher resistance to external stressors than common polyunsaturated FA. Our study highlights the future possibility of using the FA composition of sentinel organisms (i.e. feeding on specific food resources and/or especially subject to environmental stresses) as a bioindicator tool to assess mangrove ecosystems health.