Comparison of Fungal Activities on Wood and Leaf Litter in Unaltered and Nutrient-Enriched Headwater Streams

Fungi are the dominant organisms decomposing leaf litter in streams and mediating energy transfer to other trophic levels. However, less is known about their role in decomposing submerged wood. This study provides the first estimates of fungal production on wood and compares the importance of fungi in the decomposition of submerged wood versus that of leaves at the ecosystem scale. We determined fungal biomass (ergosterol) and activity associated with randomly collected small wood (<40 mm diameter) and leaves in two southern Appalachian streams (reference and nutrient enriched) over an annual cycle. Fungal production (from rates of radiolabeled acetate incorporation into ergosterol) and microbial respiration on wood (per gram of detrital C) were about an order of magnitude lower than those on leaves. Microbial activity (per gram of C) was significantly higher in the nutrient-enriched stream. Despite a standing crop of wood two to three times higher than that of leaves in both streams, fungal production on an areal basis was lower on wood than on leaves (4.3 and 15.8 g C m⁻² year⁻¹ in the reference stream; 5.5 and 33.1 g C m⁻² year⁻¹ in the enriched stream). However, since the annual input of wood was five times lower than that of leaves, the proportion of organic matter input directly assimilated by fungi was comparable for these substrates (15.4 [wood] and 11.3% [leaves] in the reference stream; 20.0 [wood] and 20.2% [leaves] in the enriched stream). Despite a significantly lower fungal activity on wood than on leaves (per gram of detrital C), fungi can be equally important in processing both leaves and wood in streams.     

Fungal importance extends beyond litter decomposition in experimental early‐successional streams

Fungi are important decomposers of leaf litter in streams and may have knock‐on effects on other microbes and carbon cycling. To elucidate such potential effects, we designed an experiment in outdoor experimental channels simulating sand‐bottom streams in an early‐successional state. We hypothesized that the presence of fungi would enhance overall microbial activity, accompanied by shifts in the microbial communities associated not only with leaf litter but also with sediments. Fifteen experimental channels received sterile sandy sediment, minimal amounts of leaf litter, and one of four inocula containing either (i) fungi and bacteria, or (ii) bacteria only, or (iii) no microorganisms, or (iv) killed microorganisms. Subsequently, we let water from an early‐successional catchment circulate through the channels for 5 weeks. Whole‐stream metabolism and microbial respiration associated with leaf litter were higher in the channels inoculated with fungi, reflecting higher fungal activity on leaves. Bacterial communities on leaves were also significantly affected. Similarly, increases in net primary production, sediment microbial respiration and chlorophyll a content on the sediment surface were greatest in the channels receiving a fungal inoculum. These results point to a major role of fungal communities in stream ecosystems beyond the well‐established direct involvement in leaf litter decomposition.     

Comparison of ATP and Ergosterol as Indicators of Fungal Biomass Associated with Decomposing Leaves in Streams

ATP and ergosterol were compared as indicators of fungal biomass associated with leaves decomposing in laboratory microcosms and streams. In all studies, the sporulation rates of the fungi colonizing leaves were also determined to compare patterns of fungal reproductive activity with patterns of mycelial growth. During leaf degradation, ATP concentrations exhibited significant, positive correlations with ergosterol concentrations in the laboratory and when leaves had been air dried prior to being submerged in a stream. However, when freshly shed leaves were submerged in a stream, concentrations of ATP and ergosterol were negatively correlated during degradation. This appeared to be due to the persistence of leaf-derived ATP in freshly shed leaves during the first 1 to 2 weeks in the stream. Estimates of fungal biomass from ergosterol concentrations of leaf litter were one to three times those calculated from ATP concentrations. ATP, ergosterol, and sporulation data generally provided similar information about the fungi associated with decomposing leaves in streams during periods when fungi were growing. Ergosterol concentrations provide a more accurate indication of fungal biomass in situations in which other organisms make significant contributions to ATP pools.     

Microbial leaf degraders in boreal streams: bringing together stochastic and deterministic regulators of community composition

1. Leaves that fall into the water represent a new habitat for microorganisms to colonise in streams, providing an opportunity to study colonisation and the subsequent regulation of community structure. We explored community composition of bacteria and fungi on decomposing alder leaves in nine streams in central Sweden, and describe their relationship with environmental variables. Succession of the microbial community was studied in one of the streams for 118 days. Microbial community composition was examined by denaturing gradient gel electrophoresis on replicate samples of leaves from each stream. 2. During succession in one stream, maximum taxon richness was reached after 34 days for bacteria and 20 days for fungi respectively. Replicate samples within this stream differed between each other earlier in colonisation, while subsequently such variation among replicate communities was low and remained stable for several weeks. Replicate samples taken from all the nine streams after 34 days of succession showed striking similarities in microbial communities within‐streams, although communities differed more strongly between streams. 3. Canonical analysis of microbial communities and environmental variables revealed that water chemistry had a significant influence on community composition. This influence was superimposed on a statistical relationship between the properties of stream catchments and microbial community composition. 4. The catchment regulates microbial communities in two different ways. It harbours the species pool from which the in‐stream microbial community is drawn and it governs stream chemistry and the composition of organic substrates that further shape the communities. We suggest that there is a random element to colonisation early in succession, whereas other factors such as species interactions, stream chemistry and organic substrate properties, result in a more deterministic regulation of communities during later stages.     

Environmental degradation results in contrasting changes in the assembly processes of stream bacterial and fungal communities

Environmental degradation may have strong effects on community assembly processes. We examined the assembly of bacterial and fungal communities in anthropogenically altered and near‐pristine streams. Using pyrosequencing of bacterial and fungal DNA from decomposed alder Alnus incana leaves, we specifically examined if environmental degradation deterministically decreases or increases the compositional turnover of bacterial and fungal communities. Our results showed that near‐pristine streams and anthropogenically altered streams supported distinct fungal and bacterial communities. The mechanisms assembling these communities were different in near‐pristine and altered environments. Environmental disturbance homogenized bacterial communities, whereas fungal communities were more dissimilar in disturbed sites than in near‐pristine sites. Compositional variation of both bacteria and fungi was related to water chemistry variables in disturbed sites, further implying the influence of environmental degradation on community assembly. Bacterial and fungal communities in near‐pristine streams were weakly controlled by environmental factors, suggesting that the relative importance of niche‐based versus neutral processes in assembling microbial communities may strongly depend on the spatial scale and local environmental context. Our results thus suggest that environmental degradation may strongly affect the composition and β‐diversity of stream microbial communities colonizing leaf litter, and that the direction of the change can be different between bacteria and fungi. A better understanding of the environmental tolerances of microbes and the mechanisms assembling microbial communities in natural environmental settings is needed to predict how environmental alteration is likely to affect microbial communities.     

Application of Fungal and Bacterial Production Methodologies to Decomposing Leaves in Streams

As leaves enter woodland streams, they are colonized by both fungi and bacteria. To determine the contribution of each of these microbial groups to the decomposition process, comparisons of fungal and bacterial production are needed. Recently, a new method for estimating fungal production based on rates of [(sup14)C]acetate incorporation into ergosterol was described. Bacterial production in environmental samples has been determined from rates of [(sup3)H]leucine incorporation into protein. In this study, we evaluated conditions necessary to use these methods for estimating fungal and bacterial production associated with leaves decomposing in a stream. During incubation of leaf disks with radiolabeled substrates, aeration increased rates of fungal incorporation but decreased bacterial production. Incorporation of both radiolabeled substrates by microorganisms associated with leaf litter was linear over the time periods examined (2 h for bacteria and 4 h for fungi). Incorporation of radiolabeled substrates present at different concentrations indicated that 400 nM leucine and 5 mM acetate maximized uptake for bacteria and fungi, respectively. Growth rates and rates of acetate incorporation into ergosterol followed similar patterns when fungi were grown on leaf disks in the laboratory. Three species of stream fungi exhibited similar ratios of rates of biomass increase to rates of acetate incorporation into ergosterol, with a mean of 19.3 (mu)g of biomass per nmol of acetate incorporated. Both bacterial and fungal production increased exponentially with increasing temperature. In the stream that we examined, fungal carbon production was 11 to 26 times greater than bacterial carbon production on leaves colonized for 21 days.     

Effects of Zn,Fe and Mn on Leaf Litter Breakdown by Aquatic Fungi: a Microcosm Study

We investigated the effects of heavy metals on leaf litter decomposition in streams. Leaves were immersed (10 days) at a reference (R) and a metal‐impacted (I) site and exposed in microcosms with increased Zn, Mn or Fe content, and to stream water from site R or I. Fungal biomass was higher in microcosms with leaves colonized at I and water from R. Fungal sporulation was higher in microcosms with leaves and water from R. Concentrations of 4.9, 9.6 and 5 ppm of Zn, Mn and Fe decrease fungal sporulation. The number of fungal species (spore counts and DGGE fingerprints) was lower in leaves colonized at site I. Cluster analyses of DGGE showed that Fe was the metal that most altered the structure of fungal community. Our results suggest that metal pollution affect leaf‐associated fungi depending on metal identity and concentration, and effects appear to be less pronounced in metal‐adapted communities. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microbial Decomposer Communities Are Mainly Structured by Trophic Status in Circumneutral and Alkaline Streams

In streams, the release of nitrogen and phosphorus is reported to affect microbial communities and the ecological processes they govern. Moreover, the type of inorganic nitrogen (NO₃, NO₂, or NH₄) may differently impact microbial communities. We aimed to identify the environmental factors that structure aquatic microbial communities and drive leaf litter decomposition along a gradient of eutrophication. We selected five circumneutral (Portuguese) and five alkaline (French) streams differing in nutrient concentrations to monitor mass loss of alder leaves, bacterial and fungal diversity by PCR-denaturing gradient gel electrophoresis, fungal biomass and reproduction, and bacterial biomass during 11 weeks of leaf immersion. The concentrations of inorganic nutrients in the stream water ranged from 5 to 300 μg liter⁻¹ soluble reactive phosphorus, 0.30 to 5.50 mg liter⁻¹ NO₃-N, 2 to 103 μg liter⁻¹ NO₂-N, and <4 to 7,100 μg liter⁻¹ NH₄-N. Species richness was maximum in moderately anthropized (eutrophic) streams but decreased in the most anthropized (hypertrophic) streams. Different species assemblages were found in subsets of streams with different trophic statuses. In both geographic areas, the limiting nutrient, either nitrate or phosphate, stimulated the microbial activity in streams of intermediate trophic status. In the hypertrophic streams, fungal biomass and reproduction were significantly lower, and bacterial biomass dramatically decreased at the site with the highest ammonium concentration. The limiting nutrients that defined the trophic status were the main factor structuring fungal and bacterial communities, whatever the geographic area. A very high ammonium concentration in stream water most probably has negative impacts on microbial decomposer communities.There is evidence that increases in nitrate and phosphate concentrations stimulate microbial respiration and fungal and bacterial activity (biomass buildup, sporulation, and/or productivity) on plant litter, leading to faster leaf decomposition in freshwaters (16, 17, 26, 34). However, fungal demands of nitrate and phosphate are reported to be fulfilled at relatively low levels (1, 12), and further increases in these nutrients in the stream water do not necessarily result in enhanced fungal activity. Besides, the form in which inorganic nutrients are present in streams, their biological availability, and even their toxicity have different ecological consequences. In densely anthropized hypertrophic streams, high levels of nitrate and phosphate were associated with decreased fungal biomass and leaf breakdown, most probably because of the high concentrations of ammonium and ammonia (2). On the other hand, the positive effects of nutrients on biomass and productivity of leaf-associated fungi can be offset by other factors, such as low oxygen concentration and sedimentation, leading to retarded decomposition (26, 33, 34).Changes in inorganic nutrient concentrations in the stream water were reported to alter the structure of fungal communities on plant litter (16, 36). Nutrient additions to moderate levels increased the diversity of fungal communities in circumneutral soft-water Appalachian mountain streams (18) but not in a Mediterranean alkaline stream (1). Moreover, fungal diversity was lower in circumneutral eutrophic streams than in reference streams (10, 35). Fungal diversity has been assessed mostly through the morphological analysis of produced conidia, not taking into account nonsporulating fungi. This raises the question of whether the differential impacts of eutrophication on fungal diversity could be due partly to difficulties in measuring actual diversity. Besides, the study of bacterial diversity on decomposing leaves has been strongly restricted to a few cultivable bacteria (<1%). Molecular typing, such as denaturing gradient gel electrophoresis (DGGE) of a specific rRNA gene region, has proved useful for assessing diversity in both leaf-associated fungi and bacteria (7, 8, 9, 11, 30).We aimed to identify the environmental factors that drive the ecological processes in freshwaters impacted by eutrophication through examination of leaf litter decomposition and associated microbial communities along a gradient of nutrient enrichment. Specifically, we addressed the following two questions: (i) which are the environmental factors that mainly structure the fungal and bacterial communities and (ii) what are the relationships between concentrations of inorganic nutrients in the stream water, leaf litter decomposition, and the activity of associated microbes? We selected 10 stream sites spanning wide concentration ranges of dissolved inorganic nitrogen (NO₃-N, NO₂-N, NH₄-N, and NH₃-N) and soluble reactive phosphorus (SRP), including 5 in northwestern Portugal with circumneutral pH and 5 in southwestern France with an alkaline pH. With these two groups of stream sites, we assessed the structure of and diversity in both sporulating and nonsporulating fungal communities, using asexual spore morphology and DGGE fingerprints of the ITS2 region, and in bacterial communities, using DGGE fingerprints of the 16S rRNA gene region. Additionally, we examined leaf mass loss and microbial activity on decomposing leaves by determining bacterial and fungal biomass and fungal reproduction.     

High Diversity of Fungi may Mitigate the Impact of Pollution on Plant Litter Decomposition in Streams

We investigated how a community of microbial decomposers adapted to a reference site responds to a sudden decrease in the water quality. For that, we assessed the activity and diversity of fungi and bacteria on decomposing leaves that were transplanted from a reference (E1) to a polluted site (E2), and results were compared to those from decomposing leaves either at E1 or E2. The two sites had contrasting concentrations of organic and inorganic nutrients and heavy metals in the stream water. At E2, leaf decomposition rates, fungal biomass, and sporulation were reduced, while bacterial biomass was stimulated. Fungal diversity was four times lower at the polluted site. The structure of fungal community on leaves decomposing at E2 significantly differed from that decomposing at E1, as indicated by the principal response curves analysis. Articulospora tetracladia, Anguillospora filiformis, and Lunulospora curvula were dominant species on leaves decomposing at E1 and were the most negatively affected by the transfer to the polluted site. The transfer of leaves colonized at the reference site to the polluted site reduced fungal diversity and sporulation but not fungal biomass and leaf decomposition. Overall, results suggest that the high diversity on leaves from the upstream site might have mitigated the impact of anthropogenic stress on microbial decomposition of leaves transplanted to the polluted site.     

Shifts in leaf litter breakdown along a forest–pasture–urban gradient in Andean streams

Tropical montane ecosystems of the Andes are critically threatened by a rapid land‐use change which can potentially affect stream variables, aquatic communities, and ecosystem processes such as leaf litter breakdown. However, these effects have not been sufficiently investigated in the Andean region and at high altitude locations in general. Here, we studied the influence of land use (forest–pasture–urban) on stream physico‐chemical variables (e.g., water temperature, nutrient concentration, and pH), aquatic communities (macroinvertebrates and aquatic fungi) and leaf litter breakdown rates in Andean streams (southern Ecuador), and how variation in those stream physico‐chemical variables affect macroinvertebrates and fungi related to leaf litter breakdown. We found that pH, water temperature, and nutrient concentration increased along the land‐use gradient. Macroinvertebrate communities were significantly different between land uses. Shredder richness and abundance were lower in pasture than forest sites and totally absent in urban sites, and fungal richness and biomass were higher in forest sites than in pasture and urban sites. Leaf litter breakdown rates became slower as riparian land use changed from natural to anthropogenically disturbed conditions and were largely determined by pH, water temperature, phosphate concentration, fungal activity, and single species of leaf‐shredding invertebrates. Our findings provide evidence that leaf litter breakdown in Andean streams is sensitive to riparian land‐use change, with urban streams being the most affected. In addition, this study highlights the role of fungal biomass and shredder species (Phylloicus; Trichoptera and Anchytarsus; Coleoptera) on leaf litter breakdown in Andean streams and the contribution of aquatic fungi in supporting this ecosystem process when shredders are absent or present low abundance in streams affected by urbanization. Finally, we summarize important implications in terms of managing of native vegetation and riparian buffers to promote ecological integrity and functioning of tropical Andean stream ecosystems.     

Laboratory conditions can change the complexity and composition of the natural aquatic mycobiome on Alnus glutinosa leaf litter

Community structure is of major interest when aquatic fungi are studied, particularly in leaf decomposition experiments. Although such studies are often conducted as laboratory experiments with microbial communities taken from the field, it remains unclear to what extent natural fungal communities can be sustained under experimental conditions. Here, we used DNA metabarcoding to investigate the development of fungal communities on alder leaves both under laboratory and field conditions. Five leaf conditioning treatments were compared by colonizing leaves in a stream, exposing stream colonized leaves to a defined medium or filtered stream water and using stream colonized leaves to inoculate sterile leaves in the defined medium or stream water. Fewer species were found on leaves that were inoculated under laboratory conditions, whereas differences in fungal community composition were comparably low in the other treatments, irrespective of the chosen medium. Possible shifts in fungal communities should therefore be considered in laboratory experiments.     

Leaf breakdown in streams differing in catchment land use

1. The impact of changes in land use on stream ecosystem function is poorly understood. We studied leaf breakdown, a fundamental process of stream ecosystems, in streams that represent a range of catchment land use in the Piedmont physiographic province of the south‐eastern United States. 2. We placed bags of chalk maple (Acer barbatum) leaves in similar‐sized streams in 12 catchments of differing dominant land use: four forested, three agricultural, two suburban and three urban catchments. We measured leaf mass, invertebrate abundance and fungal biomass in leaf bags over time. 3. Leaves decayed significantly faster in agricultural (0.0465 day^?1) and urban (0.0474 day^?1) streams than in suburban (0.0173 day^?1) and forested (0.0100 day^?1) streams. Additionally, breakdown rates in the agricultural and urban streams were among the fastest reported for deciduous leaves in any stream. Nutrient concentrations in agricultural streams were significantly higher than in any other land‐use type. Fungal biomass associated with leaves was significantly lower in urban streams; while shredder abundance in leaf bags was significantly higher in forested and agricultural streams than in suburban and urban streams. Storm runoff was significantly higher in urban and suburban catchments that had higher impervious surface cover than forested or agricultural catchments. 4. We propose that processes accelerating leaf breakdown in agricultural and urban streams were not the same: faster breakdown in agricultural streams was due to increased biological activity as a result of nutrient enrichment, whereas faster breakdown in urban streams was a result of physical fragmentation resulting from higher storm runoff.     

The influence of water chemistry on the enzymatic degradation of leaves in streams

1. Aquatic hyphomycetes degrade leaf litter in both softwater and hardwater streams. During growth on leaves, these fungi secrete an array of extracellular polysaccharidases that are differentially affected by pH. Hydrolytic enzymes exhibit acidic pH optima, whereas pectin lyases have neutral to alkaline pH optima. 2. Enzyme activities associated with microbial communities colonizing yellow poplar (Liriodendron tulipifera) leaves submerged in an acidic (pH 6.3), softwater stream were compared with those occurring in an alkaline (pH 8.2), hardwater stream. In addition to pH differences, the hardwater stream had higher nutrient concentrations and higher temperatures than the softwater stream. Conditions in the hardwater stream favoured greater microbial growth, fungal activity, rates of leaf breakdown and softening. However, activities of hydrolytic enzymes (xylanase, endocellulase, galacturonanase) were lower in the hardwater stream than in the softwater stream. Consequently, activities of these hydrolytic enzymes were not good indicators of leaf breakdown in these streams. 3. In contrast, the activities of pectin lyase were higher in the hardwater stream than in the softwater stream, corresponding to the greater rates of leaf breakdown and softening that occurred in the hardwater stream. These results support previous findings that pectin lyase is closely associated with the softening and maceration of leaf detritus and suggest that pectin degradation is a key process in the initial stages of leaf breakdown.     

Fungal growth, production, and sporulation during leaf decomposition in two streams.

I examined the activity of fungi associated with yellow poplar (Liriodendron tulipifera) and white oak (Quercus alba) leaves in two streams that differed in pH and alkalinity (a hard water stream [pH 8.0] and a soft water stream [pH 6.7]) and contained low concentrations of dissolved nitrogen (<35 microg liter(-1)) and phosphorus (<3 microg liter(-1)). The leaves of each species decomposed faster in the hard water stream (decomposition rates, 0.010 and 0.007 day(-1) for yellow poplar and oak, respectively) than in the soft water stream (decomposition rates, 0.005 and 0.004 day(-1) for yellow poplar and oak, respectively). However, within each stream, the rates of decomposition of the leaves of the two species were not significantly different. During the decomposition of leaves, the fungal biomasses determined from ergosterol concentrations, the production rates determined from rates of incorporation of [(14)C]acetate into ergosterol, and the sporulation rates associated with leaves were dynamic, typically increasing to maxima and then declining. The maximum rates of fungal production and sporulation associated with yellow poplar leaves were greater than the corresponding rates associated with white oak leaves in the hard water stream but not in the soft water stream. The maximum rates of fungal production associated with the leaves of the two species were higher in the hard water stream (5.8 mg g(-1) day(-1) on yellow poplar leaves and 3.1 mg g(-1) day(-1) on oak leaves) than in the soft water stream (1.6 mg g(-1) day(-1) on yellow poplar leaves and 0.9 mg g(-1) day(-1) on oak leaves), suggesting that effects of water chemistry other than the N and P concentrations, such as pH or alkalinity, may be important in regulating fungal activity in streams. In contrast, the amount of fungal biomass (as determined from ergosterol concentrations) on yellow poplar leaves was greater in the soft water stream (12.8% of detrital mass) than in the hard water stream (9.6% of detrital mass). This appeared to be due to the decreased amount of fungal biomass that was converted to conidia and released from the leaf detritus in the soft water stream.     

Fungal communities on decaying leaves in streams: a comparison of two leaf species

Decomposition of leaf litter is a microbial mediated process that helps to transfer energy and nutrients from leaves to higher trophic levels in woodland streams. Generally, aquatic hyphomycetes are viewed as the major fungal group responsible for leaf litter decomposition. In this study, traditional microscopic examination (based on identification of released conidia) and phylogenetic analysis of 18S rRNA genes from cultivated fungi were used to compare fungal community composition on decomposing leaves of two species (sugar maple and white oak) from a NE Ohio stream. No significant differences were found in sporulation rates between maple and oak leaves and both had similar species diversity. From the 18S rRNA gene sequence data, identification was achieved for 12 isolates and taxonomic affiliation of 12 of the remaining 14 isolates could be obtained. A neighbor-joining tree (with bootstrap values) was constructed to examine the taxonomic distribution of the isolates relative to sequences of known operational taxonomic units (OTUs). Surprisingly, only 2 of the isolates obtained were aquatic hyphomycetes based on phylogenetic analysis. Overall, there were no differences between the two leaf types and a higher diversity was observed via culturing and subsequent 18S rRNA gene sequencing than by conidia staining. These differences resulted from the fact that traditional microscopy provides estimates of aquatic hyphomycete diversity while the other approach revealed the presence of both aquatic hyphomycete and non-aquatic hyphomycete taxa. The presence of this broad array of taxa suggests that the role of aquatic hyphomycetes relative to other fungi be re-evaluated. Even though the functional role of these non-aquatic hyphomycetes taxa is unknown, their presence and diversity demonstrates the need to delve further into fungal community structure on decomposing leaves.     

Structural and functional responses of leaf‐associated fungal communities to chemical pollution in streams

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Warming water and leaf litter quality but not plant origin drive decomposition and fungal diversity in an experiment

The fungi associated with leaf litter play a key role in decomposition and can be affected both by the warming water and the invasion of non-native species in riparian vegetation. Warming water and invasion of non-native riparian species on stream fungal communities have been studied mainly in temperate ecosystems. We tested the effects of warming water and non-native plant Psidium guajava on leaf litter decomposition, conidia density, species richness and beta diversity of tropical stream fungi. Thus, we carried out an experiment using the current mean temperature of streams from northwestern Paraná in South Brazil (22 °C) and two temperatures above the current mean temperature (26 °C and 29 °C). We also used the leaves of a non-native plant (P. guajava), and two native plants (one of similar nutritional quality, and the other of higher nutritional quality than the non-native species) occurring in Neotropical streams riparian vegetation. Warming water accelerated leaf litter decomposition and reduced conidia density and fungal richness in native and non-native plants. However, species composition and beta diversity were not affected by water temperature. Our study showed that warming affects the fungi of streams, the main microorganisms responsible for decomposition and that the nutritional quality of the leaves may be more important than the origin of riparian plant species. Despite this, further investigations should be conducted on the interaction of P. guajava with the flow of nutrients in these environments and how it can affect other ecosystem processes and the food chain. Efforts to study the effects of water warming and biological invasion on the attributes and distribution of fungi in streams are vital, making them a tool for the conservation of riparian ecosystems.     

Effects of Nutrient Enrichment on Decomposition and Fungal Colonization of Sweet Chestnut Leaves in an Iberian Stream (Central Portugal)

This study assessed the effect of nutrient enrichment on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and rates of fungal sporulation of sweet chestnut (Castanea sativa Miller) leaves in a 3rd order stream (Central Portugal), with medium to high background values of nutrients. Coarse and fine mesh leaf bags were attached to nutrient diffusing substrata containing NaNO₃, KH₂PO₄, both nutrients, or no additions. Leaf breakdown rates were similar in the four treatments and in the two mesh sizes (k=−0.0155 to −0.0219 day⁻¹). Phosphorus content of P or N + P enriched leaves was higher than in the other treatments after 28 days, but there were no differences in N concentrations. Ergosterol concentrations associated with decomposing leaves were similar among treatments. The peak sporulation rates of aquatic hyphomycetes were stimulated by the addition of N + P and N but not by P alone. Results from the experiment provide evidence that leaf breakdown in the study stream, as a model for streams with naturally medium to high level of nutrients, was not nutrient-limited, and that fungal reproductive activity was limited by dissolved N but not by dissolved P in stream water.     

A Cross-System Comparison of Bacterial and Fungal Biomass in Detritus Pools of Headwater Streams

The absolute amount of microbial biomass and relative contribution of fungi and bacteria are expected to vary among types of organic matter (OM) within a stream and will vary among streams because of differences in organic matter quality and quantity. Common types of benthic detritus [leaves, small wood, and fine benthic organic matter (FBOM)] were sampled in 9 small (1st-3rd order) streams selected to represent a range of important controlling factors such as surrounding vegetation, detritus standing stocks, and water chemistry. Direct counts of bacteria and measurements of ergosterol (a fungal sterol) were used to describe variation in bacterial and fungal biomass. There were significant differences in bacterial abundance among types of organic matter with higher densities per unit mass of organic matter on fine particles relative to either leaves or wood surfaces. In contrast, ergosterol concentrations were significantly greater on leaves and wood, confirming the predominance of fungal biomass in these larger size classes. In general, bacterial abundance per unit organic matter was less variable than fungal biomass, suggesting bacteria will be a more predictable component of stream microbial communities. For 7 of the 9 streams, the standing stock of fine benthic organic matter was large enough that habitat-weighted reach-scale bacterial biomass was equal to or greater than fungal biomass. The quantities of leaves and small wood varied among streams such that the relative contribution of reach-scale fungal biomass ranged from 10% to as much as 90% of microbial biomass. Ergosterol concentrations were positively associated with substrate C:N ratio while bacterial abundance was negatively correlated with C:N. Both these relationships are confounded by particle size, i.e., leaves and wood had higher C:N than fine benthic organic matter. There was a weak positive relationship between bacterial abundance and streamwater soluble reactive phosphorus concentration, but no apparent pattern between either bacteria or fungi and streamwater dissolved inorganic nitrogen. The variation in microbial biomass per unit organic matter and the relative abundance of different types of organic matter contributed equally to driving differences in total microbial biomass at the reach scale.     

Future increase in temperature may stimulate litter decomposition in temperate mountain streams: evidence from a stream manipulation experiment

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