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

• Essential fatty acids are thought to be mainly produced by autotrophic organisms.
• Detritus-based headwater streams might thus be strongly limited by these compounds.
• Aquatic hyphomycetes produces high proportions of polyunsaturated fatty acids.
• These proportions co-vary with phosphorus availability.
• This study sheds new light on the ecological importance of aquatic hyphomycetes.

To 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 ω3 and ω6) 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.