No abstract

Summary Terrestrial leaf litter inputs provide an essential energy source for many freshwater organisms. Processing of leaf litter involves several physicochemical and biological factors but always starts with the colonisation of leaves by aquatic hyphomycetes. Here, we document changes occurring on leaf tissues of three tree species, alder, hornbeam and oak, with contrasted leaf properties. Changes in the mechanical properties of leaves and in fungal growth were followed at 10, 25, 35, 45 and 55 days of immersion in natural winter conditions. We hypothesised that fungal growth will be faster and mechanical characteristics will decrease more rapidly in softer, nutrient‐rich, than in tougher leaves. In addition, we tested whether the consumption rate of conditioned leaves by the amphipod shredder Gammarus pulex was correlated with the mechanical properties of leaves and their ergosterol content (as a proxy for fungal biomass). Leaf toughness decreased (with different rates for the 3 litter species), whereas the leaf thickness remained stable. The mechanical properties of alder and hornbeam leaves changed similarly, in contrast to oak leaves. Ergosterol contents increased over time more rapidly for alder and hornbeam than for oak leaves for which the fungal growth was delayed. Ergosterol content, leaf toughness and leaf consumption rates by G. pulex were significantly correlated. Leaf consumption rates by G. pulex were higher for soft than for tough leaves. Our results suggest that the consumption of different types of leaves may be delayed according to the time required to acquire conditioning sufficient for shredder consumption. The presence of tough leaves in riparian vegetation may constitute a reservoir of trophic resources available for aquatic organisms at the end of the winter, when soft leaves have been entirely consumed. The diversity of riparian vegetation may hence contribute to sustaining the availability of food resources for adjacent aquatic ecosystems.

The mechanisms by which Anopheles gambiae mosquitoes survive the desiccating conditions of the dry season in Africa and are able to readily transmit malaria soon after the rains start remain largely unknown. The desiccation tolerance and resistance of female An. gambiae M and S reared in contrasting environmental conditions reflecting the onset of dry season ("ods") and the rainy season ("rs") was determined by monitoring their survival and body water loss in response to low relative humidity. Furthermore, we investigated the degree to which the physiology of 1-h and 24-h-old females is altered at "ods" by examining and comparing their quantitative metabotypes and proteotypes with conspecifics exposed to "rs" conditions. Results showed that distinct biochemical rearrangements occurred soon after emergence in female mosquitoes that enhance survival and limit water loss under dry conditions. In particular, three amino acids (phenylalanine, tyrosine, and valine) playing a pivotal role in cuticle permeability decreased significantly from the 1-h to 24-h-old females, regardless of the experimental conditions. However, these amino acids were present in higher amounts in 1-h-old female An. gambiae M reared under "ods" whereas no such seasonal difference was reported in S ones. Together with the 1.28- to 2.84-fold increased expression of cuticular proteins 70 and 117, our data suggests that cuticle composition, rigidity and permeability were adjusted at "ods". Increased expression of enzymes involved in glycogenolytic and proteolytic processes were found in both forms at "ods". Moreover, 1-h-old S forms were characterised by elevated amounts of glycogen phosphorylase, isocitrate dehydrogenase, and citrate synthase, suggesting an increase of energetic demand in these females at "ods".