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.This work is supported by the project STREAMECO—Biodiversity and ecosystem functioning under climate change: from the gene to the stream: PTDC/CTA-AMB/31245/2017 funded by the Portuguese Foundation for Science and Technology (FCT) and by the “Contrato-Programa” UIDB/04050/2020 funded by national funds through the FCT I.P. Additional support from the National Science Foundation (NSF DEB-1655797) to V.G. is gratefully acknowledged

Low-to-moderate nitrogen and phosphorus concentrations accelerate microbially driven litter breakdown rates

Particulate organic matter (POM) processing is an important driver of aquatic ecosystem productivity that is sensitive to nutrient enrichment and drives ecosystem carbon (C) loss. Although studies of single concentrations of nitrogen (N) or phosphorus (P) have shown effects at relatively low concentrations, responses of litter breakdown rates along gradients of low‐to‐moderate N and P concentrations are needed to establish likely interdependent effects of dual N and P enrichment on baseline activity in stream ecosystems. We established 25 combinations of dissolved inorganic N (DIN; 55–545 μg/L) and soluble reactive P (SRP; 4–86 μg/L) concentrations with corresponding N:P molar ratios of 2–127 in experimental stream channels. We excluded macroinvertebrates, focusing on microbially driven breakdown of maple (Acer rubrum L.) and rhododendron (Rhododendron maximum L.) leaf litter. Breakdown rates, k, per day (d−1) and per degree‐day (dd−1), increased by up to 6× for maple and 12× for rhododendron over our N and P enrichment gradient compared to rates at low ambient N and P concentrations. The best models of k (d−1 and dd−1) included litter species identity and N and P concentrations; there was evidence for both additive and interactive effects of N and P. Models explaining variation in k dd−1 were supported by N and P for both maple and rhododendron ( = 0.67 and 0.33, respectively). Residuals in the relationship between k dd−1 and N concentration were largely explained by P, but residuals for k dd−1 and P concentration were less adequately explained by N. Breakdown rates were more closely related to nutrient concentrations than variables associated with measurements of two mechanistic parameters associated with C loss (fungal biomass and microbial respiration rate). We also determined the effects of nutrient addition on litter C : nutrient stoichiometry and found reductions in litter C:N and C:P along our experimental nutrient gradient. Our results indicate that microbially driven litter processing rates increase across low‐to‐moderate nutrient gradients that are now common throughout human‐modified landscapes

Detrital stoichiometry as a critical nexus for the effects of streamwater nutrients on leaf litter breakdown rates

Nitrogen (N) and phosphorus (P) concentrations are elevated in many freshwater systems, stimulating breakdown rates of terrestrially derived plant litter; however, the relative importance of N and P in driving litter breakdown via microbial and detritivore processing are not fully understood. Here, we determined breakdown rates of two litter species, Acer rubrum (maple) and Rhododendron maximum (rhododendron), before (PRE) and during two years (YR1, YR2) of experimental N and P additions to five streams, and quantified the relative importance of hypothesized factors contributing to breakdown. Treatment streams received a gradient of P additions (low to high soluble reactive phosphorus [SRP]; ~10–85 μg/L) crossed with a gradient of N additions (high to low dissolved inorganic nitrogen [DIN]; ~472–96 μg/L) to achieve target molar N:P ratios ranging from 128 to 2. Litter breakdown rates increased above pre‐treatment levels by an average of 1.1–2.2× for maple, and 2.7–4.9× for rhododendron in YR1 and YR2. We used path analysis to compare fungal biomass, shredder biomass, litter stoichiometry (nutrient content as C:N or C:P), discharge, and streamwater temperature as predictors of breakdown rates and compared models containing streamwater N, P or N + P and litter C:N or C:P using model selection criteria. Litter breakdown rates were predicted equally with either streamwater N or P (R2 = 0.57). In models with N or P, fungal biomass, litter stoichiometry, discharge, and shredder biomass predicted breakdown rates; litter stoichiometry and fungal biomass were most important for model fit. However, N and P effects may have occurred via subtly different pathways. Litter N content increased with fungal biomass (N‐driven effects) and litter P content increased with streamwater P availability (P‐driven effects), presumably via P storage in fungal biomass. In either case, the effects of N and P through these pathways were associated with higher shredder biomass and breakdown rates. Our results suggest that N and P stimulate litter breakdown rates via mechanisms in which litter stoichiometry is an important nexus for associated microbial and detritivore effects

Invertebrate Grazing on Live Turtlegrass (Thalassia testudinum): A Common Interaction That May Facilitate Fungal Growth

In coastal wetlands and tropical reefs, snails can regulate foundation species by feeding on marsh grasses and hard corals. In many cases, their impacts are amplified because they facilitate microbial infection in grazer-induced wounds. Whether snails commonly graze live plants and facilitate microbial growth on plants in tropical seagrass systems is less explored. On a Belizean Caye, we examined patterns in snail-generated grazer scars on the abundant turtlegrass (Thalassia testudinum). Our initial survey showed the occurrence of snail-induced scarring on live turtlegrass blades was common, with 57% of live leaves scarred. Feeding trials demonstrated that two of five common snails (Tegula fasciata–smooth tegula and Smaragdia viridis–emerald nerite) grazed unepiphytized turtlegrass blades and that smooth tegula abundance had a positive relationship with scarring intensity. Subsequent surveys at three Caribbean sites (separated by >150 km) also showed a high occurrence of snail-induced scars on turtlegrass blades. Finally, simulated herbivory experiments and field observations of a turtlegrass bed in Florida, United States suggests that herbivore damage could facilitate fungal growth in live seagrass tissue through mechanical opening of tissue. Combined, these findings reveal that snail grazing on live turtlegrass blades in the Caribbean can be common. Based on these results, we hypothesize that small grazers could be exerting top-down control over turtlegrass growth directly via grazing and/or indirectly by facilitating microbial infection in live seagrass tissue. Further studies are needed to determine the generality and relative importance of direct and indirect effects of gastropod grazing on turtlegrass health

Litter Decomposition as an Indicator of Stream Ecosystem Functioning at Local-to-Continental Scales

RivFunction is a pan-European initiative that started in 2002 and was aimed at esta- blishing a novel functional-based approach to assessing the ecological status of rivers. Litter decomposition was chosen as the focal process because it plays a central role in stream ecosystems and is easy to study in the field. Impacts of two stressors that occur across the continent, nutrient pollution and modified riparian vegetation, were exam- ined at >200 paired sites in nine European ecoregions. In response to the former, decomposition was dramatically slowed at both extremes of a 1000-fold nutrient gra- dient, indicating nutrient limitation in unpolluted sites, highly variable responses across Europe in moderately impacted streams, and inhibition via associated toxic and addi- tional stressors in highly polluted streams. Riparian forest modification by clear cutting or replacement of natural vegetation by plantations (e.g. conifers, eucalyptus) or pasture produced similarly complex responses. Clear effects caused by specific riparian distur- bances were observed in regionally focused studies, but general trends across different types of riparian modifications were not apparent, in part possibly because of important indirect effects. Complementary field and laboratory experiments were undertaken to tease apart the mechanistic drivers of the continental scale field bioassays by addressing the influence of litter, fungal and detritivore diversity. These revealed generally weak and context-dependent effects on decomposition, suggesting high levels of redundancy (and hence potential insurance mechanisms that can mitigate a degree of species loss) within the food web. Reduced species richness consistently increased decomposition variability, if not the absolute rate. Further field studies were aimed at identifying impor- tant sources of this variability (e.g. litter quality, temporal variability) to help constrain ranges of predicted decomposition rates in different field situations. Thus, although many details still need to be resolved, litter decomposition holds considerable potential in some circumstances to capture impairment of stream ecosystem functioning. For instance, species traits associated with the body size and metabolic capacity of the con- sumers were often the main driver at local scales, and these were often translated into important determinants of otherwise apparently contingent effects at larger scales. Key insights gained from conducting continental scale studies included resolving the appar- ent paradox of inconsistent relationships between nutrients and decomposition rates, as the full complex multidimensional picture emerged from the large-scale dataset, of which only seemingly contradictory fragments had been seen previously

Continental-Scale Effects of Nutrient Pollution on Stream Ecosystem Functioning

Excessive nutrient loading is a major threat to aquatic ecosystems worldwide that leads to profound changes in aquatic biodiversity and biogeochemical processes. Systematic quantitative assessment of functional ecosystem measures for river networks is, however, lacking, especially at continental scales. Here, we narrow this gap by means of a pan-European field experiment on a fundamental ecosystem process—leaf-litter breakdown—in 100 streams across a greater than 1000-fold nutrient gradient. Dramatically slowed breakdown at both extremes of the gradient indicated strong nutrient limitation in unaffected systems, potential for strong stimulation in moderately altered systems, and inhibition in highly polluted streams. This large-scale response pattern emphasizes the need to complement established structural approaches (such as water chemistry, hydrogeomorphology, and biological diversity metrics) with functional measures (such as litter-breakdown rate, whole-system metabolism, and nutrient spiraling) for assessing ecosystem health

Evolutionary Relationships Between Aquatic Anamorphs and Teleomorphs: Tricladium and Varicosporium

Tricladium, with 21 accepted species, is the largest genus of aquatic hyphomycetes. It encompasses species with dematiaceous as well as mucedinaceous colonies. Conidiogenesis is thalloblastic; conidiogenous cells proliferate percurrently or sympodially. Conidia have typically two alternate primary lateral branches. Fontanospora and Variocladium are segregates of Tricladium, differing by conidial branching. Varicosporium comprises nine species, one not well known. Conidiogenesis is blastic or thalloblastic, conidiogenous cells proliferate sympodially or are determinate; conidia regularly produce primary and secondary branches and often fragment into part conidia. Molecular analyses on the 28S rDNA of 86 isolates, including 16 species of Tricladium, five species of Varicosporium, two species of Fontanospora and one species of Variocladium, place these hyphomycetes within Helotiales. Tricladium is polyphyletic and placed in six clades; Varicosporium is polyphyletic and placed in three clades; Fontanospora is polyphyletic within a single clade. Variocladium is placed with poor support as a sister taxon to Varicosporium giganteum, Hymenoscyphus scutula and Torrendiella eucalypti. (C) 2009 The British Mycological Society. Published by Elsevier Ltd. All rights reserved

Ecosystem and physiological scales of microbial responses to nutrients in a detritus-based stream: Results of a 5-year continuous enrichment

Our study examined the response of leaf detritus–associated microorganisms (both bacteria and fungi) to a 5-yr continuous nutrient enrichment of a forested headwater stream. Leaf litter dominates detritus inputs to such streams and, on a system-wide scale, serves as the key substrate for microbial colonization. We determined physiological responses as microbial biomass and activity expressed per unit mass of leaves and system-level responses by quantifying leaf litter standing crop monthly and expressing responses per unit area of streambed. Physiological (mass-specific) trends differed from system-level (area-specific) trends. Physiological responses to enrichment were generally positive. With the exception of bacterial biomass, nutrients increased all metrics expressed per unit mass leaf litter in the treatment stream relative to the reference (fungal biomass and production, bacterial production, microbial respiration). This positive physiological response to nutrient enrichment was associated with lower leaf litter standing crop in the treatment stream, resulting in less substrate for microbial colonization. Consequently, during most years on a system-level scale, only fungal production and microbial respiration were positively affected by nutrients, whereas fungal biomass was negatively affected. Thus, from a whole-stream perspective, nutrients led to a lower quantity of leaf detritus with greater variation, resulting in net reductions of associated fungal biomass and greater intra-annual variability in both fungal biomass an