Aquatic hyphomycetes: a potential source of polyunsaturated fatty acids in detritus-based stream food webs

International audienceTo date, aquatic hyphomycetes have mainly been considered as a source of carbohydrates, mineral elements and proteins for detritivores in detritus-based headwater streams. Yet, consumers are highly dependent on other molecules, such as essential fatty acids, to complete their biological cycle. These molecules might be limiting in detritus-based ecosystems since they are primarily synthesized by autotrophic organisms. We investigated: (i) if aquatic hyphomycetes are able to synthesize polyunsaturated fatty acids (PUFAs); and (ii) if the abundance of phosphorus, often considered as a limiting factor in headwater streams, could affect the proportions of PUFAs in aquatic hyphomycetes. All hyphomycete species tested synthesized high amounts of PUFAs (C18 omega 3 and omega 6) and the proportions of these essential fatty acids varied with P availability. This study sheds new light on the importance of aquatic hyphomycetes in the detritus-based stream food web, but also on potential co-variations between mineral nutrient limitation and the availability of essential fatty acids for consumers

Phosphorus availability modulates the toxic effect of silver on aquatic fungi and leaf litter decomposition

International audienceThe functioning of forested headwater streams is intimately linked to the decomposition of leaf litter by decomposers, mainly aquatic hyphomycetes, which enables the transfer of allochthonous carbon to higher trophic levels. Evaluation of this process is being increasingly used as an indicator of ecosystem health and ecological integrity. Yet, even though the individual impacts of contaminants and nutrient availability on decomposition have been well studied, the understanding of their combined effects remains limited. In the current study, we investigated whether the toxic effects of a reemerging contaminant, silver (Ag), on leaf litter decomposition could be partly overcome in situations where microorganisms were benefitting from high phosphorus (P) availability, the latter being a key chemical element that often limits detritus decomposition. We also investigated whether these interactive effects were mediated by changes in the structure of the aquatic hyphomycete community. To verify these hypotheses, leaf litter decomposition by a consortium of ten aquatic hyphomycete species was followed in a microcosm experiment combining five Ag contamination levels and three P concentrations. Indirect effects of Ag and P on the consumption of leaf litter by the detritivorous crustacean, Gammarus fossarum, were also evaluated. Ag significantly reduced decomposition but only at the highest concentration tested, independently of P level. By contrast, P and Ag interactively affected fungal biomass. Both P level and Ag concentrations shaped microbial communities without significantly affecting the overall species richness. Finally, the levels of P and Ag interacted significantly on G. fossarum feeding rates, high [Ag] reducing litter consumption and low P availability tending to intensify the feeding rate. Given the high level of contaminant needed to impair the decomposition process, it is unlikely that a direct effect of Ag on leaf litter decomposition could be observed in situ. However, subtle Ag effects in relation to nutrient levels in ecosystems could be expected. In particular, owing to higher consumption of low P leaf litter, shredding invertebrates could increase the ingestion of contaminated resources, which could, in turn, represent an important threat to headwater stream ecosystems

Behavioural and physiological responses of Gammarus fossarum (Crustacea Amphipoda) exposed to silver

International audienceThe study aims at investigating the effects of silver (Ag), a re-emerging contaminant, on physiological and behavioural responses in Gammarus fossarum. In a first experiment, G. fossarum Ag LC50s were evaluated during 96 h under semi-static mode of exposure. Juveniles appeared to be more sensitive to Ag (LC50(96 h): 1.01 mu g L-1) than ovigerous females (LC50(96 h): 1.9 mu g L-1) and adult males (LC50(96 h): 2.2 mu g L-1). In a second experiment, the physiological (osmo-/ionoregulation; antioxidant enzymes; lipid peroxidation (LPO)) and behavioural (locomotor activity and ventilation) responses of male G. fossarum exposed to Ag (0, 0.5, 1, 2, and 4 mu g L-1) were investigated. The mortality and Ag bioconcentration of gammarids exposed to Ag were significantly higher than controls. Concerning physiological responses, a 48 h-exposure to Ag had no impact on catalase activity but led to a significant decrease of haemolymph osmolality and [Na+]. On the contrary, LPO, Se-GPx and Na+/K+-ATPase activity were significantly increased. Behavioural responses, such as locomotor and ventilatory activities, were also significantly reduced in Ag exposed gammarids. After 96 h-exposure, especially to 0.5 mu g Ag L-1, most responses (ventilation, locomotor activity, haemolymph osmolality and [Na+]) were even more pronounced and haemolymph [Cl-] was significantly decreased but, contrary to observations after 48 h-exposure, Na+/K+-ATPase activity was significantly reduced. Our results demonstrate the drastic effects of realistic [Ag] concentration (0.5 mu g Ag L-1) on an ubiquitous and functionally important freshwater invertebrate (implied in detritus breakdown), but also strongly suggest an energetic reallocation to the detriment of locomotor activity and in favour of maintenance functions (i.e., osmoregulation and detoxification). These results highlight the risk represented by Ag and the need to perform integrated studies (at different scales, from individual to ecosystem)

Manipulation of Leaf Litter Stoichiometry

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