Cláudia Pascoal

Contribution of Fungi and Bacteria to Leaf Litter Decomposition in a Polluted River

ABSTRACT The contribution of fungi and bacteria to the decomposition of alder leaves was examined at two reference and two polluted sites in the Ave River (northwestern Portugal). Leaf mass loss, microbial production from incorporation rates of radiolabeled compounds into biomolecules, fungal biomass from ergosterol concentration, sporulation rates, and diversity of aquatic hyphomycetes associated with decomposing leaves were determined. The concentrations of organic nutrients and of inorganic nitrogen and phosphorus in the stream water was elevated and increased at downstream sites. Leaf decomposition rates were high (0.013 day−1 < k < 0.042 day−1), and the highest value was estimated at the most downstream polluted site, where maximum values of microbial production and fungal biomass and sporulation were found. The slowest decomposition occurred at the other polluted site, where, along with the nutrient enrichment, the lowest current velocity and dissolved-oxygen concentration in water were observed. At this site, fungal production, biomass, and sporulation were depressed, suggesting that stimulation of fungal activity by increased nutrient concentrations might be offset by other factors. Although bacterial production was higher at polluted sites, fungi accounted for more than 94% of the total microbial net production. Fungal yield coefficients varied from 10.2 to 13.6%, while those of bacteria were less than 1%. The contribution of fungi to overall leaf carbon loss (29.0 to 38.8%) greatly exceeded that of bacteria (4.2 to 13.9%).

Aquatic hyphomycete diversity and identity affect leaf litter decomposition in microcosms

We conducted a microcosm experiment with monocultures and all possible combinations of four aquatic hyphomycete species, Articulospora tetracladia, Flagellospora curta, Geniculospora grandis and Heliscus submersus, to examine the potential effects of species richness on three functional aspects: leaf litter decomposition (leaf mass loss), fungal production (ergosterol buildup) and reproductive effort (released spores). Both species richness and identity significantly affected fungal biomass and conidial production (number and biomass of released spores), whereas only species identity had a significant effect on leaf mass loss. In mixed cultures, all measures of fungal functions were greater than expected from the weighted performances of participating species in monoculture. Mixed cultures outperformed the most active monoculture for biomass accumulation but not for leaf mass loss and conidial production. The three examined aspects of aquatic hyphomycete activity tended to increase with species richness, and a complementary effect was unequivocally demonstrated for fungal biomass. Our results also suggest that specific traits of certain species may have a greater influence on ecosystem functioning than species number.

Anthropogenic stress may affect aquatic hyphomycete diversity more than leaf decomposition in a low-order stream

Fungal diversity and microbial decomposition of leaf litter were examined in a low-order stream at a reference and an impacted site. The latter, 10 km downstream of the reference site, has high nutrient loads from domestic sewage and agriculture, and increased heavy metal levels in the stream water and sediments. At the polluted site aquatic hyphomycete diversity and sporulation were reduced, whereas fungal biomass and leaf decomposition rate were not. Articulospora tetracladia and Flagellospora sp. were the dominant species at the reference site, and Dimorphospora foliicola was dominant at the polluted site. Biomass of bacteria was higher at the polluted site, but only approached 10 % of fungal biomass, indicating that fungi remained the main microbial decomposers. In addition, the results suggest that aquatic hyphomycete communities may respond to stress according to the redundancy model, in which overall function remains stable because increased biomasses of tolerant species compensate for the loss of sensitive species.

Role of fungi, bacteria, and invertebrates in leaf litter breakdown in a polluted river

Abstract The effects of water-quality degradation caused by urbanization, agricultural, and industrial activities on leaf litter breakdown and associated communities of invertebrates and microorganisms were examined at 1 reference and 2 downstream polluted sites in the Ave River (northwestern Portugal). Conductivity, concentrations of NH4+-N, NO3−-N, and PO43−-P, and density of culturable microorganisms were high at the polluted sites. Rates of leaf breakdown also were high, and the highest value was found at the most-downstream, nutrient-enriched polluted site. However, the other polluted site had low current velocity and sedimentation, and nutrient enrichment did not lead to rapid leaf breakdown. Shredders were scarce or absent at all sampling sites, and low shredder density probably explained the lack of differences in leaf breakdown rates between fine-mesh and coarse-mesh bags. High fungal and bacterial production on leaves supported high leaf breakdown rates. Bacterial production was greater at both polluted sites than at the reference site, but it did not exceed 11% of the total microbial production. Fungal biomass and production were markedly different between polluted sites, with the highest values corresponding to the fastest leaf breakdown. Our findings indicate that fungi were the major decomposers in this polluted river. We encourage further research on the effects of multiple stressors on the activity of fungal decomposers to help us better understand the mechanisms underlying leaf litter breakdown in streams under stress.

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

ABSTRACT 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.

Leaf breakdown rates : a measure of water quality?

The breakdown rates of Alnus glutinosa leaves and the structure of macroinvertebrate communities were used to evaluate the impact of the village of Montalegre (Portugal) on the water quality of the Cavado river. Chemical and microbial analyses of stream water indicated a high organic load in the vicinity of the village. The abundance of macroinvertebrates associated with leaves increased along the pollution gradient, whereas richness of the community decreased. Biotic indices and multivariate analysis applied to aquatic macroinvertebrate communities discriminated polluted from non-polluted sites. Exponential breakdown rates of alder leaves were high (0.014 to 0.060 day -1 ) and the differences observed among sites suggested that nutrient enrichment stimulated leaf breakdown significantly. Leaf breakdown rates have not reflected improved biotic conditions as assessed by biotic indices at the most downstream site. These results suggest that both data from the structure and function of a stream are important for assessing water quality.

Effects of Zinc on Leaf Decomposition by Fungi in Streams: Studies in Microcosms

The effect of zinc on leaf decomposition by aquatic fungi was studied in microcosms. Alder leaf disks were precolonized for 15 days at the source of the Este River and exposed to different zinc concentrations during 25 days. Leaf mass loss, fungal biomass (based on ergosterol concentration), fungal production (rates of [1-14C]acetate incorporation into ergosterol), sporulation rates, and species richness of aquatic hyphomycetes were determined. At the source of the Este River decomposition of alder leaves was fast and 50% of the initial mass was lost in 25 days. A total of 18 aquatic hyphomycete species were recorded during 42 days of leaf immersion. Articulospora tetracladia was the dominant species, followed by Lunulospora curvula and two unidentified species with sigmoid conidia. Cluster analysis suggested that zinc concentration and exposure time affected the structure of aquatic hyphomycete assemblages, even though richness had not been severely affected. Both zinc concentration and exposure time significantly affected leaf mass loss, fungal production and sporulation, but not fungal biomass. Zinc exposure reduced leaf mass loss, inhibited fungal production and affected fungal reproduction by either stimulating or inhibiting sporulation rates. The results of this work suggested zinc pollution might depress leaf decomposition in streams due to changes in the structure and activity of aquatic fungi.

Intraspecific traits change biodiversity effects on ecosystem functioning under metal stress

Studies investigating the impacts of biodiversity loss on ecosystem processes have often reached different conclusions, probably because insufficient attention has been paid to some aspects including (1) which biodiversity measure (e.g., species number, species identity or trait) better explains ecosystem functioning, (2) the mechanisms underpinning biodiversity effects, and (3) how can environmental context modulates biodiversity effects. Here, we investigated how species number (one to three species) and traits of aquatic fungal decomposers (by replacement of a functional type from an unpolluted site by another from a metal-polluted site) affect fungal production (biomass acumulation) and plant litter decomposition in the presence and absence of metal stress. To examine the putative mechanisms that explain biodiversity effects, we determined the contribution of each fungal species to the total biomass produced in multicultures by real-time PCR. In the absence of metal, positive diversity effects were observed for fungal production and leaf decomposition as a result of species complementarity. Metal stress decreased diversity effects on leaf decomposition in assemblages containing the functional type from the unpolluted site, probably due to competitive interactions between fungi. However, dominance effect maintained positive diversity effects under metal stress in assemblages containing the functional type from the metal-polluted site. These findings emphasize the importance of intraspecific diversity in modulating diversity effects under metal stress, providing evidence that trait-based diversity measures should be incorporated when examining biodiversity effects.

Realized Fungal Diversity Increases Functional Stability of Leaf Litter Decomposition Under Zinc Stress

Freshwaters include some of the most impaired systems on Earth with high rates of species loss, underscoring the significance of investigating whether ecosystems with fewer species will be able to maintain ecological processes. The environmental context is expected to modulate the effects of declining diversity. We conducted microcosm experiments manipulating fungal inoculum diversity and zinc concentration to test the hypothesis that fungal diversity determines the susceptibility of leaf litter decomposition to Zn stress. Realized fungal diversity was estimated by counting released spores and by measuring species-specific biomasses via denaturing gradient gel electrophoresis. In the absence of Zn, positive diversity effects were found for leaf mass loss and fungal biomass through complementary interactions and due to the presence of key species. The variability of leaf decomposition decreased with increasing species number (portfolio effect), particularly under Zn stress. Results suggest that the effect of species loss on ecosystem stability may be exacerbated at higher stress levels.

Chapter 4 - Assessing the Contribution of Micro-Organisms and Macrofauna to Biodiversity–Ecosystem Functioning Relationships in Freshwater Microcosms

Summary A large body of research has revealed (often) positive biodiversity–ecosystem functioning (B–EF) relationships in manipulative experiments. The vast majority of such studies have focused on either micro- or macro-organisms, and none we are aware of have manipulated the diversity of both simultaneously under controlled laboratory conditions. We performed a microcosm experiment in which we manipulated species richness of aquatic fungi and invertebrates, two taxonomically distant sets of consumers that contribute to the same key ecosystem process in freshwaters, the decomposition of terrestrial leaf litter. We used a novel statistical design to maximize parsimony and analytical power in an experiment with three levels of species richness (seven mono-culture, 21 di-culture, and seven tri-culture treatments). Litter decomposition was measured as both leaf mass loss and the production of fine particulate organic matter (FPOM). We tested whether species richness affected these two processes or whether polycultures performed as predicted from their component mono-cultures. Further, we calculated assemblage metabolism in each microcosm to test whether the processes were driven by the metabolic demands of fungi and invertebrates. In general, across the 35 treatments, most species combinations performed in an additive fashion and we found no effect of species richness on either process. There was evidence of assemblage identity effects (i.e. certain species combinations not performing as expected), with instances of significant differences for species combinations that contained both caddis larvae and fungi. These assemblages performed worse than expected, which might have been due to dual vertical and horizontal interactions, with the possibility that although both consumed litter directly the former may also have grazed on the latter. Apart from these particular species combinations, overall performance of a species in polyculture was effectively the same as in mono-culture and reflected its metabolic demands. This suggests that even taxonomically distant consumers might exhibit a degree of functional redundancy for certain processes provided the remaining species can attain sufficient population biomass (and hence metabolic capacity) to compensate for the loss of other species, although whether such compensatory mechanisms operate in the field remains unknown. Further species contribute to a multitude of ecosystem processes and progressively more species are needed to sustain the sum of them. Our experiment highlights how, by taking metabolic demands into account, future B–EF studies could help to disentangle how species contribute to ecosystem processes both separately and in combination, and to help partition the effects of taxonomic and functional diversity.