• Energy Research

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.

Urbanization and industrial activities have contributed to widespread contamination by metals and polycyclic aromatic hydrocarbons, but the combined effects of these toxics on aquatic biota and processes are poorly understood. We examined the effects of cadmium (Cd) and phenanthrene on the activity and diversity of fungi associated with decomposing leaf litter in streams. Leaves of Alnus glutinosa were immersed for 10 days in an unpolluted low-order stream in northwest Portugal to allow microbial colonization. Leaves were then exposed in microcosms for 14 days to Cd (0.06-4.5 mg L(-1)) and phenanthrene (0.2 mg L(-1)) either alone or in mixture. A total of 19 aquatic hyphomycete species were found sporulating on leaves during the whole study. The dominant species was Articulospora tetracladia, followed by Alatospora pulchella, Clavatospora longibrachiata, and Tetrachaetum elegans. Exposure to Cd and phenanthrene decreased the contribution of A. tetracladia to the total conidial production, whereas it increased that of A. pulchella. Fungal diversity, assessed as denaturing gradient gel electrophoresis fingerprinting or conidial morphology, was decreased by the exposure to Cd and/or phenanthrene. Moreover, increased Cd concentrations decreased leaf decomposition and fungal reproduction but did not inhibit fungal biomass production. Exposure to phenanthrene potentiated the negative effects of Cd on fungal diversity and activity, suggesting that the co-occurrence of these stressors may pose additional risk to aquatic biodiversity and stream ecosystem functioning.

SummaryA consequence of drought in streams is the emersion of decomposing leaf litter, which may alter organic matter recycling. We assessed the effects of emersion on decomposition of black poplar leaves and associated microbes (microbial biomass, extracellular enzyme activities and microbial diversity) in two streams with distinct characteristics, in particular nutrients, temperature and oxygen levels. Leaf decomposition rates, fungal biomass and extracellular enzyme activities were lower in the most impacted stream (high nutrients and temperature, low oxygen). Also, the structure of fungal and bacterial communities differed between streams. Emersion strongly affected all microbial functional measures. Leaf decomposition, fungal biomass and extracellular enzyme activities were more sensitive at the most pristine site, while fungal reproduction and bacterial biomass production were more affected by emersion at the most impacted stream. Microbial community structure was strongly altered after emersion. Although similar effects on leaf‐associated microbes were found in both streams, functional responses to emersion differed probably as a consequence of different initial microbial communities with different sensitivity to the drying stress. Our study highlights the need of understanding the effects of drought in streams suffering from different environmental perturbations, since responses to emersion appear to depend on the environmental context.

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.

ABSTRACTIn 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−1soluble reactive phosphorus, 0.30 to 5.50 mg liter−1NO3-N, 2 to 103 μg liter−1NO2-N, and <4 to 7,100 μg liter−1NH4-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.

Microorganisms are key drivers of leaf litter decomposition; however, the mechanisms underlying the dynamics of different microbial groups are poorly understood. We investigated the effects of seasonal variation and invertebrates on fungal and bacterial dynamics, and on leaf litter decomposition. We followed the decomposition of Populus nigra litter in a Mediterranean stream through an annual cycle, using fine and coarse mesh bags. Irrespective of the season, microbial decomposition followed two stages. Initially, bacterial contribution to total microbial biomass was higher compared to later stages, and it was related to disaccharide and lignin degradation; in a later stage, bacteria were less important and were associated with hemicellulose and cellulose degradation, while fungi were related to lignin decomposition. The relevance of microbial groups in decomposition differed among seasons: fungi were more important in spring, whereas in summer, water quality changes seemed to favour bacteria and slowed down lignin and hemicellulose degradation. Invertebrates influenced litter-associated microbial assemblages (especially bacteria), stimulated enzyme efficiencies and reduced fungal biomass. We conclude that bacterial and fungal assemblages play distinctive roles in microbial decomposition and differ in their sensitivity to environmental changes, ultimately affecting litter decomposition, which might be particularly relevant in highly seasonal ecosystems, such as intermittent streams.