Fungal-fungal and fungal-bacterial interactions in aquatic decomposer communities: bacteria promote fungal diversity

Fungi produce a variety of extracellular enzymes, making recalcitrant substrates bioavailable. Thus, fungi are central for the decomposition of dead organic matter such as leaf litter. Despite their ecological importance, our understanding of relationships between fungal species diversity and ecosystem functioning is limited, especially with regard to aquatic habitats. Moreover, fungal interactions with other groups of microorganisms such as bacteria are rarely investigated. This lack of information may be attributed to methodological limitations in tracking the biomass of individual fungal species in communities, impeding a detailed assessment of deviations from the overall performance expected from the sum of individual species' performances, so-called net diversity effects (NDEs). We used fungal species-specific biomolecular tools to target fungal-fungal and fungal-bacterial interactions on submerged leaves using four cosmopolitan aquatic fungal species and a stream microbial community dominated by bacteria. In microcosms, we experimentally manipulated fungal diversity and bacterial absence/presence and assessed functional performances and fungal community composition after 14 d of incubation. Fungal community data were used to evaluate NDEs on leaf colonization. The individual fungal species were functionally distinct and fungal cultures were on average more efficient than the bacterial culture. In absence of bacteria, NDEs correlated with growth rate (negatively) and genetic divergence (positively), but were predominantly negative, suggesting that higher fungal diversity led to a lower colonization success (niche overlap). In both absence and presence of bacteria, the overall functional performances of the communities were largely defined by their composition (i.e., no interactions at the functional level). In the presence of bacteria, NDEs correlated with genetic divergence (positively) and were largely positive, suggesting higher fungal diversity stimulated colonization (niche complementarity). This stimulation may be driven by a bacteria-induced inhibition of fungal growth, alleviating competition among fungi. Resulting feedback loops eventually promote fungal coexistence and synergistic interactions. Nonetheless, overall functional performances are reduced compared to bacteria-free cultures. These findings highlight the necessity to conduct future studies, investigating biodiversity-ecosystem functioning relationships using artificial systems, without exclusion of key organisms naturally co-occurring in the compartment of interest. Otherwise, study outcomes might not reflect true ecological relationships and ultimately misguide conservation strategies

Making the Invisible Visible? Using Stable Isotope Analysis to Detect Indirect Toxicant Effects

Although stable isotope analysis (SIA) is widely used to address ecological research questions, its application in an ecotoxicological context has been limited. Recent studies have proposed an effect of chemical stressors on an organism's isotope signature, questioning the use of SIA in food webs impacted by toxicants. Against this background, the present study investigates 1) whether trophic enrichment factors (TEFs; i.e., the offset in stable isotope signatures of a consumer to its diet) are altered by the neonicotinoid thiacloprid, and 2) whether tracking toxicant effects on an organism's diet composition (i.e., indirect effect) with SIA fits direct observations of consumption. To address the former, the amphipod Gammarus fossarum (Koch) was exposed to three levels (0, 0.75, and 5 mu g L-1) of thiacloprid and fed with either black alder leaves or Baetis rhodani (Pictet) larvae over 6 weeks (n = 35). The thiacloprid-induced changes in TEFs that we found were statistically significant but small compared with other factors (e.g., resource quality, consumer, and physiological condition) and thus likely of minor importance. To address the latter issue, gammarids were exposed to two levels of thiacloprid (0 and 0.75 mu g L-1) and fed with either black alder leaves, live B. rhodani larvae, or both over 2 weeks (n = 10). Dietary proportions as suggested by SIA were indeed in agreement with those derived from direct observation of consumption. The present study consequently suggests that SIA is as a robust tool to detect indirect toxicant effects especially if TEFs are assessed in parallel. Environ Toxicol Chem 2022;00:1-9. (c) 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC