Fungal identity mediates the impacts of multiple stressors on freshwater ecosystems.

Predicting how multiple anthropogenic stressors affect natural ecosystems is a major challenge in ecology. Freshwater ecosystems are threatened worldwide by multiple co-occurring stressors, which can affect aquatic biodiversity, ecosystem functioning and human wellbeing. In stream ecosystems, aquatic fungi play a crucial role in global biogeochemical cycles and food web dynamics, therefore, assessing the functional consequences of fungal biodiversity loss under multiple stressors is crucial. Here, a microcosm approach was used to investigate the effects of multiple stressors (increased temperature and nutrients, drying, and biodiversity loss) on three ecosystem processes: organic matter decomposition, fungal reproduction, and fungal biomass accrual. Net effects of stressors were antagonistic for organic matter decomposition, but additive for fungal reproduction and biomass accrual. Net effects of biodiversity were mainly positive for all processes, even under stress, demonstrating that diversity assures the maintenance of ecosystem processes. Fungal species displayed distinct contributions to each ecosystem process. Furthermore, species with negligible contributions under control conditions changed their role under stress, either enhancing or impairing the communities' performance, emphasizing the importance of fungal species identity. Our study highlights that distinct fungal species have different sensitivities to environmental variability and have different influence on the overall performance of the community. Therefore, preserving high fungal diversity is crucial to maintain fungal species with key ecosystem functions within aquatic communities in face of environmental change.

Microbial community history and leaf species shape bottom-up effects in a freshwater shredding amphipod.

Arable land use and the associated application of agrochemicals can affect local freshwater communities with consequences for the entire ecosystem. For instance, the structure and function of leaf-associated microbial communities can be affected by pesticides, such as fungicides. Additionally, the leaf species on which these microbial communities grow reflects another environmental filter for community structure. These factors and their interaction may jointly modify leaves' nutritional quality for higher trophic levels. To test this assumption, we studied the structure of leaf-associated microbial communities with distinct exposure histories (pristine [P] vs vineyard run off [V]) colonising two leaf species (black alder, European beech, and a mixture thereof). By offering these differently colonised leaves as food to males and females of the leaf-shredding amphipod Gammarus fossarum (Crustacea; Amphipoda) we assessed for potential bottom-up effects. The growth rate, feeding rate, faeces production and neutral lipid fatty acid profile of the amphipod served as response variable in a 2 × 3 × 2-factorial test design over 21d. A clear separation of community history (P vs V), leaf species and an interaction between the two factors was observed for the leaf-associated aquatic hyphomycete (i.e., fungal) community. Sensitive fungal species were reduced by up to 70 % in the V- compared to P-community. Gammarus' growth rate, feeding rate and faeces production were affected by the factor leaf species. Growth was negatively affected when Gammarus were fed with beech leaves only, whereas the impact of alder and the mixture of both leaf species was sex-specific. Overall, this study highlights that leaf species identity had a more substantial impact on gammarids relative to the microbial community itself. Furthermore, the sex-specificity of the observed effects (excluding fatty acid profile, which was only measured for male) questions the procedure of earlier studies, that is using either only one sex or not being able to differentiate between males and females. However, these results need additional verification to support a reliable extrapolation.

Leaf Species-Dependent Fungicide Effects on the Function and Abundance of Associated Microbial Communities.

Microbially-mediated leaf litter decomposition is a critical ecosystem function in running waters within forested areas, which can be affected by fungicides. However, fungicide effects on leaf litter decomposition have been investigated almost exclusively with black alder leaves, a leaf species with traits favourable to consumers (i.e., low recalcitrance and high nutrient content). At the same time, little is known about fungicide effects on microbial colonisation and decomposition of other leaf species with less favourable traits. In this 21 day lasting study, we explore the effects of increasing fungicide sum concentrations (0-3000 µg/L) on microbial colonisation and decomposition of three leaf species (black alder, Norway maple and European beech) differing in terms of recalcitrance and nutrient content. Leaf litter decomposition rate, leaf-associated fungal biomass and bacterial density were quantified to observe potential effects at the functional level. Beech, as the species with the least favourable leaf traits, showed a substantially lower decomposition rate (50%) in absence of fungicides than alder and maple. In the presence of high fungicide concentrations (300-3000 µg/L), beech showed a concentration-related decrease not only in microbial leaf litter decomposition but also fungal biomass. This suggests that favourable traits of leaf litter (as for alder and maple) enable leaf-associated microorganisms to acquire leaf-bound energy more easily to withstand potential effects induced by fungicide exposure. Our results indicate the need to deepen our understanding on how leaf species' traits interact with the impact of chemical stressors on the leaf decomposition activity of microbial communities.

Novelties in Microthyriaceae (Microthyriales): Two New Asexual Genera with Three New Species from Freshwater Habitats in Guizhou Province, China.

Microthyriaceae is typified by the sexual genus Microthyrium, with eight asexual genera. Three interesting isolates were collected during our investigation of freshwater fungi from the wetlands in Guizhou Province, southwest China. Three new asexual morphs are identified. Phylogenetic analyses using ITS and LSU gene regions revealed the placement of these isolates in Microthyriaceae (Microthyriales, Dothideomycetes). Based on the morphology and phylogenetic evidence, two new asexual genera, Paramirandina and Pseudocorniculariella, and three new species, Pa. aquatica, Pa. cymbiformis, and Ps. guizhouensis, are introduced. Descriptions and illustrations of the new taxa are provided, with a phylogenetic tree of Microthyriales and related taxa.

Opening Pandora's Box: Neglected Biochemical Potential of Permafrost-Associated Fungal Communities in a Warming Climate.

Permafrost, a vast storage reservoir of frozen organic matter, is rapidly thawing due to climate change, releasing previously preserved carbon into the environment. This phenomenon has significant consequences for microbial communities, including fungi, inhabiting permafrost-associated regions. In this review, we delve into the intricate interplay between permafrost thawing and fungal diversity and functionality with an emphasis on thermokarst lakes. We explore how the release of organic carbon from thawing permafrost alters the composition and activities of fungal communities, emphasizing the potential for shifts in taxonomic diversity and functional gene expression. We discuss the formation of thermokarst lakes, as an example of permafrost thaw-induced ecological disruptions and their impact on fungal communities. Furthermore, we analyze the repercussions of these changes, including effects on nutrient cycling, plant productivity, and greenhouse gas (GHG) emissions. By elucidating the multifaceted relationship between permafrost thaw and aquatic fungi, this review provides valuable insights into the ecological consequences of ongoing climate change in permafrost-affected regions.

Identificación taxonómica de hifomicetos acuáticos en la cuenca alta del río Bogotá (Colombia) a partir de muestras de espuma / Taxonomic identification of aquatic hyphomycetes in the upper Bogotá river basin (Colombia) from foam samples

Partiendo de que los hifomicetos u hongos ingoldianos son microorganismos presentes en los ambientes acuáticos como los ríos en los cuales intervienen en la descomposición del material orgánico devolviendo la energía a los niveles superiores de la cadena trófica, pero que a pesar de su importancia son pocos los estudios realizados referente a su diversidad y presencia en estos ecosistemas, el objetivo de este trabajo fue identificar los hifomicetos presentes en la cuenca alta del río Bogotá. Esto se realizó mediante la toma de muestra de espuma en los remansos de los ríos a las cuales se les realizó coloración con azul de lactofenol y observación con microscopio óptico para la identificación de los hongos ingoldianos a partir de la morfología de sus conidios con el uso de claves taxonómicas. Se identificaron un total de 10 géneros y 10 especies de las cuales 4 géneros y 7 especies son reportes nuevos para el territorio colombiano, lo cual demuestra la gran diversidad y desconocimiento que tenemos de estos microorganismos en nuestro país. (AU)

Based on the fact that hyphomycetes or ingoldian fungi are microorganisms present in aquatic environments such as rivers in which they are involved in the decomposition of organic material, returning energy to the higher levels of the trophic chain, but despite their importance, few studies have been conducted on their diversity and presence in these ecosystems, the objective of this work was to identify the hyphomycetes present in the upper Bogotá river basin. This was done by taking foam samples in the backwaters of the rivers, staining them with lactophenol blue and observing them with optical microscopy to identify the ingoldian fungi based on the morphology of their conidia with the use of taxonomic keys. A total of 10 genera and 10 species were identified, of which 4 genera and 7 species are new reports for the Colombian territory, which demonstrates the great diversity and lack of knowledge that we have of these microorganisms in our country. (AU)

How do the Growth and Metabolic Activity of Aquatic fungi Geotrichum Candidum and Aspergillus Niger Respond to Nanoplastics?

In this study, exposure experiments were conducted to assess the effects of polystyrene nanoparticles (PS) and amine-modified polystyrene nanoparticles (APS) at environmental concentrations (1, 10, and 100 µg L- 1) on two fungal species (Geotrichum candidum and Aspergillus niger), isolated from leaf litter in streams, concerning their growth and metabolic activity. Results showed that PS at 1 and 10 µg L- 1 have hormesis effects on G. candidum growth. Compared with G. candidum, A. niger had higher sensitivity to nanoplastic exposure. Besides, the peroxidase and cellobiohydrolase activities of A. niger were significantly inhibited by nanoplastics (except 1 µg L- 1 PS), which would weaken its metabolic activity in carbon cycling. These results provided a new thought on how the growth and functions of aquatic fungi cope with the stress induced by nanoplastics. Overall, the study provided evidence for the different responses of aquatic fungi to nanoplastics in streams.

ITS rDNA Barcodes Clarify Molecular Diversity of Aquatic Hyphomycetes.

Aquatic hyphomycetes are key microbial decomposers of allochthonous organic matter in freshwater ecosystems. Although their importance in carbon flow and food webs in streams is widely recognized, there are still gaps in our understanding of their molecular diversity and distribution patterns. Our study utilized the growing database of ITS rDNA barcodes of aquatic hyphomycetes (1252 sequences) and aimed to (i) produce new barcodes for some lesser-known taxa; (ii) clarify the taxonomic placement of some taxa at the class or order level, based on molecular data; and (iii) provide insights into the biogeographical origins of some taxa. This study increased the number of aquatic hyphomycete species with available ITS barcodes from 119 (out of ~300 species described) to 136. Phylogenetically, the 136 species were distributed between 2 phyla, 6 classes, and 10 orders of fungi. Future studies should strive to increase the database of ITS sequences, especially focusing on species with unclear phylogenetic relationships (incertae sedis) and with few sequences available. The geographical distribution of species with available ITS sequences included 50 countries from five continents, but 6 countries had more than 20 species associated, showing a bias toward the northern hemisphere, likely due to sampling bias.

Diploid-dominant life cycles characterize the early evolution of Fungi.

Most of the described species in kingdom Fungi are contained in two phyla, the Ascomycota and the Basidiomycota (subkingdom Dikarya). As a result, our understanding of the biology of the kingdom is heavily influenced by traits observed in Dikarya, such as aerial spore dispersal and life cycles dominated by mitosis of haploid nuclei. We now appreciate that Fungi comprises numerous phylum-level lineages in addition to those of Dikarya, but the phylogeny and genetic characteristics of most of these lineages are poorly understood due to limited genome sampling. Here, we addressed major evolutionary trends in the non-Dikarya fungi by phylogenomic analysis of 69 newly generated draft genome sequences of the zoosporic (flagellated) lineages of true fungi. Our phylogeny indicated five lineages of zoosporic fungi and placed Blastocladiomycota, which has an alternation of haploid and diploid generations, as branching closer to the Dikarya than to the Chytridiomyceta. Our estimates of heterozygosity based on genome sequence data indicate that the zoosporic lineages plus the Zoopagomycota are frequently characterized by diploid-dominant life cycles. We mapped additional traits, such as ancestral cell-cycle regulators, cell-membrane- and cell-wall-associated genes, and the use of the amino acid selenocysteine on the phylogeny and found that these ancestral traits that are shared with Metazoa have been subject to extensive parallel loss across zoosporic lineages. Together, our results indicate a gradual transition in the genetics and cell biology of fungi from their ancestor and caution against assuming that traits measured in Dikarya are typical of other fungal lineages.

Evidence of micro and macroplastic toxicity along a stream detrital food-chain.

Freshwater ecosystems are subjected to plastic extensive pollution because they are the direct link between plastic wastes and marine ecosystems. The aim of this study was to assess the impacts of different sizes of polyethylene plastics (micro: µPs and macroplastics: PBs) on freshwater decomposers of plant litter. We exposed leaf associated microbial assemblages to µPs (0.5 or 1.5 g L-1) and discs of PBs as follows: green plastic bags (PB-G) alone or in mixtures with transparent plastic bags (PB-Mix). Then, we conducted a feeding preference experiment with the invertebrate shredder Limnephilus sp. to assess their capacity to distinguish leaf discs from PB discs of the same size (12 mm). Leaf decomposition, activities of fungal enzymes and sporulation were inhibited by µPs and PB-Mix, and shifts in fungal community composition were observed. The invertebrate shredders preferred to feed on leaves treated with µPs avoiding those exposed to PB-G/PB-Mix. Our results demonstrated that plastics can have a direct effect on stream-dwelling microbial decomposers and an indirect effect on higher trophic levels (shredders), highlighting that trophic transfer is a route of plastic exposure. The plastic properties (size, concentration, colour) appear to influence plastic toxicity to microbes and shredders, indicating the importance of considering physicochemical properties when assessing their risks to freshwater ecosystems.

Assembly of wood-inhabiting archaeal, bacterial and fungal communities along a salinity gradient: common taxa are broadly distributed but locally abundant in preferred habitats.

Wood decomposition in water is a key ecosystem process driven by diverse microbial taxa that likely differ in their affinities for freshwater, estuarine and marine habitats. How these decomposer communities assemble in situ or potentially colonize from other habitats remains poorly understood. At three watersheds on Coiba Island, Panama, we placed replicate sections of branch wood of a single tree species on land, and in freshwater, estuarine and marine habitats that constitute a downstream salinity gradient. We sequenced archaea, bacteria and fungi from wood samples collected after 3, 9 and 15 months to examine microbial community composition, and to examine habitat specificity and abundance patterns. We found that these microbial communities were broadly structured by similar factors, with a strong effect of salinity, but little effect of watershed identity on compositional variation. Moreover, common aquatic taxa were also present in wood incubated on land. Our results suggest that either taxa dispersed to both terrestrial and aquatic habitats, or microbes with broad habitat ranges were initially present in the wood as endophytes. Nonetheless, these habitat generalists varied greatly in abundance across habitats suggesting an important role for habitat filtering in maintaining distinct aquatic communities in freshwater, estuarine and marine habitats.

Antarctic Glacial Meltwater Impacts the Diversity of Fungal Parasites Associated With Benthic Diatoms in Shallow Coastal Zones.

Aquatic ecosystems are frequently overlooked as fungal habitats, although there is increasing evidence that their diversity and ecological importance are greater than previously considered. Aquatic fungi are critical and abundant components of nutrient cycling and food web dynamics, e.g., exerting top-down control on phytoplankton communities and forming symbioses with many marine microorganisms. However, their relevance for microphytobenthic communities is almost unexplored. In the light of global warming, polar regions face extreme changes in abiotic factors with a severe impact on biodiversity and ecosystem functioning. Therefore, this study aimed to describe, for the first time, fungal diversity in Antarctic benthic habitats along the salinity gradient and to determine the co-occurrence of fungal parasites with their algal hosts, which were dominated by benthic diatoms. Our results reveal that Ascomycota and Chytridiomycota are the most abundant fungal taxa in these habitats. We show that also in Antarctic waters, salinity has a major impact on shaping not just fungal but rather the whole eukaryotic community composition, with a diversity of aquatic fungi increasing as salinity decreases. Moreover, we determined correlations between putative fungal parasites and potential benthic diatom hosts, highlighting the need for further systematic analysis of fungal diversity along with studies on taxonomy and ecological roles of Chytridiomycota.

Keqinzhangia aquatica gen. et sp. nov. and Pseudocoronospora hainanense gen. et sp. nov., isolated from freshwater in southern China.

During an investigation of the diversity of aquatic hyphomycetes from southern China, two interesting isolates were collected. These two isolates were cultured and sequenced, and a BLAST search of their LSU sequences against data in GenBank revealed that the closest related taxa were in the genus Microthyrium. Phylogenetic analyses, based on the combined sequence data from the internal transcribed spacer (ITS) and large nuclear subunit ribosomal DNA (LSU), revealed that our isolates belong to the Microthyriaceae. Combined morphological characters allowed us to describe our isolates as two new genera and species in Microthyriaceae, named as: Keqinzhangia aquatica and Pseudocoronospora hainanense. The full descriptions, illustrations, and a phylogenetic tree showing the position of the two new genera were provided in this paper.

Evidence for Lignocellulose-Decomposing Enzymes in the Genome and Transcriptome of the Aquatic Hyphomycete Clavariopsis aquatica.

Fungi are ecologically outstanding decomposers of lignocellulose. Fungal lignocellulose degradation is prominent in saprotrophic Ascomycota and Basidiomycota of the subkingdom Dikarya. Despite ascomycetes dominating the Dikarya inventory of aquatic environments, genome and transcriptome data relating to enzymes involved in lignocellulose decay remain limited to terrestrial representatives of these phyla. We sequenced the genome of an exclusively aquatic ascomycete (the aquatic hyphomycete Clavariopsis aquatica), documented the presence of genes for the modification of lignocellulose and its constituents, and compared differential gene expression between C. aquatica cultivated on lignocellulosic and sugar-rich substrates. We identified potential peroxidases, laccases, and cytochrome P450 monooxygenases, several of which were differentially expressed when experimentally grown on different substrates. Additionally, we found indications for the regulation of pathways for cellulose and hemicellulose degradation. Our results suggest that C. aquatica is able to modify lignin to some extent, detoxify aromatic lignin constituents, or both. Such characteristics would be expected to facilitate the use of carbohydrate components of lignocellulose as carbon and energy sources.

Declining fungal diversity in Arctic freshwaters along a permafrost thaw gradient.

Climate change-driven permafrost thaw has a strong influence on pan-Arctic regions, via, for example, the formation of thermokarst ponds. These ponds are hotspots of microbial carbon cycling and greenhouse gas production, and efforts have been put on disentangling the role of bacteria and archaea in recycling the increasing amounts of carbon arriving to the ponds from degrading watersheds. However, despite the well-established role of fungi in carbon cycling in the terrestrial environments, the interactions between permafrost thaw and fungal communities in Arctic freshwaters have remained unknown. We integrated data from 60 ponds in Arctic hydro-ecosystems, representing a gradient of permafrost integrity and spanning over five regions, namely Alaska, Greenland, Canada, Sweden, and Western Siberia. The results revealed that differences in pH and organic matter quality and availability were linked to distinct fungal community compositions and that a large fraction of the community represented unknown fungal phyla. Results display a 16%-19% decrease in fungal diversity, assessed by beta diversity, across ponds in landscapes with more degraded permafrost. At the same time, sites with similar carbon quality shared more species, aligning a shift in species composition with the quality and availability of terrestrial dissolved organic matter. We demonstrate that the degradation of permafrost has a strong negative impact on aquatic fungal diversity, likely via interactions with the carbon pool released from ancient deposits. This is expected to have implications for carbon cycling and climate feedback loops in the rapidly warming Arctic.

Mycoplankton Biome Structure and Assemblage Processes Differ Along a Transect From the Elbe River Down to the River Plume and the Adjacent Marine Waters.

Rivers are transport systems and supply adjacent ecosystems with nutrients. They also serve human well-being, for example as a source of food. Microorganism biodiversity is an important parameter for the ecological balance of river ecosystems. Despite the knowledge that fungi are key players in freshwater nutrient cycling and food webs, data on planktonic fungi of streams with higher stream order are scarce. This study aims to fill this knowledge gap by a fungi-specific 18S ribosomal RNA (rRNA) gene tag sequencing approach, investigating mycoplankton diversity in the Elbe River along a transect from shallow freshwater, to the estuary and river plume down to the adjacent marine waters (sections of seventh stream order number). Using multivariate analyses and the quantitative process estimates (QPEs) method, questions (i) of how mycoplankton communities as part of the river continuum change along the transect, (ii) what factors, spatial and environmental, play a role, and (iii) what assembly processes, such as selection or dispersion, operate along the transect, were addressed. The partitioning of mycoplankton communities into three significant distant biomes was mainly driven by local environmental conditions that were partly under spatial control. The assembly processes underlying the biomes also differed significantly. Thus, variable selection dominated the upstream sections, while undominated processes like ecological drift dominated the sections close to the river mouth and beyond. Dispersal played a minor role. The results suggest that the ecological versatility of the mycoplankton communities changes along the transect as response, for example, to a drastic change from an autotrophic to a heterotrophic system caused by an abrupt increase in the river depth. Furthermore, a significant salinity-dependent occurrence of diverse basal fungal groups was observed, with no clade found exclusively in marine waters. These results provide an important framework to help understand patterns of riverine mycoplankton communities and serve as basis for a further in-depth work so that fungi, as an important ecological organism group, can be integrated into models of, e.g., usage-balance considerations of rivers.

Mixture effects of a fungicide and an antibiotic: Assessment and prediction using a decomposer-detritivore system.

Antimicrobials, such as fungicides and antibiotics, pose a risk for microbial decomposers (i.e., bacteria and aquatic fungi) and invertebrate detritivores (i.e., shredders) that play a pivotal role in the ecosystem function of leaf litter breakdown. Although waterborne toxicity and diet-related effects (i.e., dietary exposure and microorganism-mediated alterations in food quality for shredders) of fungicides and antibiotics on decomposer-detritivore systems have been increasingly documented, their joint effect is unknown. We therefore assessed waterborne and dietary effects of an antimicrobial mixture consisting of the fungicide azoxystrobin (AZO) and the antibiotic ciprofloxacin (CIP) on microbial decomposers and the shredder Gammarus fossarum using a tiered approach. We compared effect sizes measured in the present study with model predictions (i.e., independent action) based on published data. During a 7-day feeding activity assay quantifying waterborne toxicity in G. fossarum, the leaf consumption of gammarids was reduced by ∼60 % compared to the control when subjected to the mixture at concentrations of each component causing a 20 % reduction in the same response variable when applied individually. Moreover, the selective feeding of gammarids during the food choice assay indicated alterations in food quality induced by the antimicrobial mixture. The food selection and, in addition, the decrease in microbial leaf decomposition is likely linked to changes in leaf-associated bacteria and fungi. During a long-term assay, energy processing, growth and energy reserves of gammarids were increased in presence of 15 and 500 µg/L of AZO and CIP, respectively, through the dietary pathway. These physiological responses were probably driven by CIP-induced alterations in the gut microbiome or immune system of gammarids. In general, model predictions matched observed effects caused by waterborne exposure on the leaf consumption, energy processing and growth of gammarids during short- and long-term assays, respectively. However, when complex horizontal (bacteria and aquatic fungi) and vertical (leaf-associated microorganisms and shredders) interactions were involved, model predictions partly over- or underestimated mixture effects. Therefore, the present study identifies uncertainties of mixture effect predictions for complex biological systems calling for studies targeting the underlying processes and mechanisms.

Five Novel Freshwater Ascomycetes Indicate High Undiscovered Diversity in Lotic Habitats in Thailand.

An investigation of freshwater fungi in Thailand resulted in the collection of one new monotypic genus, Neoxylomyces, and a novel species each in Camposporium, Brunneofusispora, Rattania, Neoxylomyces, and Phaeoacremonium. Camposporium dulciaquae resembles C. septatum in conidial morphology and number of septa but differs in conidial sizes. Brunneofusispora hyalina is similar to B. sinensis in conidiogenesis and conidial shape but differs in the sizes of conidiomata and conidiogenous cells. Rattania aquatica is the second species in Rattania, while Phaeoacremonium thailandense is the third species recorded from freshwater habitats. A new genus, Neoxylomyces, typified by N. multiseptatus, is similar to Xylomyces giganteus, but differs in the number of septa, chlamydospore measurements, and absence of a mucilaginous coating around the chlamydospores. These novel taxa form an independent lineage distinct from other species based on multi-loci phylogenetic analyses. Descriptions, illustrations, and notes are provided for each taxon. These new freshwater ascomycetes add to the increasing number of fungi known from Thailand and it is now evident that there are numerous novel taxa awaiting to be described as new freshwater habitats are explored. An update of newly discovered taxa in the widely studied freshwater habitats of Thailand over the last five years is also provided.

Transcriptomics reveals the action mechanisms and cellular targets of citrate-coated silver nanoparticles in a ubiquitous aquatic fungus.

Silver nanoparticles (AgNPs) are among the major groups of contaminants of emerging concern for aquatic ecosystems. The massive application of AgNPs relies on the antimicrobial properties of Ag, raising concerns about their potential risk to ecologically important freshwater microbes and the processes they drive. Moreover, it is still uncertain whether the effects of AgNPs are driven by the same mechanisms underlying those of Ag ions (Ag+). We employed transcriptomics to better understand AgNP toxicity and disentangle the role of Ag+ in the overall toxicity towards aquatic fungi. To that end, the worldwide-distributed aquatic fungus Articulospora tetracladia, that plays a central role in organic matter turnover in freshwaters, was selected and exposed for 3 days to citrate-coated AgNPs (∼20 nm) and Ag+ at concentrations inhibiting 20% of growth (EC20). Responses revealed 258 up- and 162 down-regulated genes upon exposure to AgNPs and 448 up- and 84 down-regulated genes under exposure to Ag+. Different gene expression patterns were found after exposure to each silver form, suggesting distinct mechanisms of action. Gene ontology (GO) analyses showed that the major cellular targets likely affected by both silver forms were the biological membranes. GO-based biological processes indicated that AgNPs up-regulated the genes involved in transport, nucleobase metabolism and energy production, but down-regulated those associated with redox and carbohydrate metabolism. Ag+ up-regulated the genes involved in carbohydrate and steroid metabolism, whereas genes involved in localization and transport were down-regulated. Our results showed, for the first time, distinct profiles of gene expression in aquatic fungi exposed to AgNPs and Ag+, supporting different modes of toxicity of each silver form. Also, our results suggest that Ag+ had a negligible role in the toxicity induced by AgNPs. Finally, our study highlights the power of transcriptomics in portraying the stress induced by different silver forms in organisms.

Sensitivity differences among five species of aquatic fungi and fungus-like organisms for seven fungicides with various modes of action.

Five species of aquatic fungi and fungus-like organisms were used for toxicity assays with seven fungicides to determine the differences in species sensitivity. A microplate toxicity assay with adenosine triphosphate luminescence detection was used as an efficient and economical high-throughput assay. The obtained toxicity data were standardized based on the species sensitivity distribution method. Species sensitivity differed among the fungicides: Rhizophydium brooksianum was most sensitive to hydroxyisoxazole, isoprothiolane, and ferimzone; Chytriomyces hyalinus was most sensitive to tricyclazole; Sporobolomyces roseus was most sensitive to ipconazole; Aphanomyces stellatus was most sensitive to orysastrobin and kasugamycin. Tetracladium setigerum was not the most sensitive species to any of the tested fungicides. The ranges of EC50s to fungal species were lower than to other aquatic organisms (primary producers, invertebrates, and vertebrates) for hydroxyisoxazole, kasugamycin, isoprothiolane, ipconazole, and ferimzone. These results suggest the usefulness of a battery of fungal species to assess the ecological effects of fungicides.