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1.
Both the absence of leaf shredding macroinvertebrates and low microbial activity are of major importance in determining slow and incomplete leaf decay in extremely acidic (pH<3.5) mining streams. These streams are affected by a heavy ochre deposition causing the formation of massive iron plaques on leaf surfaces that hinder microbial exploitation. An investigation was carried out to determine whether iron plaques and leaf conditioning status (acid conditioned with and without iron plaques, neutral conditioned, unconditioned) affect the feeding preference of the shredder Gammarus pulex (L.). Leaf respiration rates and fungal biomass (ergosterol contents) were measured to determine microbial colonization. Neutral conditioned leaves had significantly higher microbial colonization than acid conditioned leaves with iron plaques. Notwithstanding, leaves of both conditioning types were consumed at high rates by G. pulex. The microbial colonization had no influence on feeding preference in the experiment. It is presumed that iron adsorbed organic material caused the high palatability of leaves with iron plaques. The results indicate that the large deposits of leaves coated with iron plaques will be available to the stream food web when water quality will be restored to neutral as planed in scenarios for the future development of mining streams.  相似文献   

2.
Summary 1. Heterotrophic microorganisms are crucial for mineralising leaf litter and rendering it more palatable to leaf‐shredding invertebrates. A substantial part of leaf litter entering running waters may be buried in the streambed and thus be exposed to the constraining conditions prevailing in the hyporheic zone. The fate of this buried organic matter and particularly the role of microbial conditioning in this habitat remain largely unexplored. 2. The aim of this study was to determine how the location of leaf litter within the streambed (i.e. at the surface or buried), as well as the leaf litter burial history, may affect the leaf‐associated aquatic hyphomycete communities and therefore leaf consumption by invertebrate detritivores. We tested the hypotheses that (i) burial of leaf litter would result in lower decomposition rates associated with changes in microbial assemblages compared with leaf litter at the surface and (ii) altered microbial conditioning of buried leaf litter would lead to decreased quality and palatability to their consumers, translating into lower growth rates of detritivores. 3. These hypotheses were tested experimentally in a second‐order stream where leaf‐associated microbial communities, as well as leaf litter decomposition rates, elemental composition and toughness, were compared across controlled treatments differing by their location within the streambed. We examined the effects of the diverse conditioning treatments on decaying leaf palatability to consumers through feeding trials on three shredder taxa including a freshwater amphipod, of which we also determined the growth rate. 4. Microbial leaf litter decomposition, fungal biomass and sporulation rates were reduced when leaf litter was buried in the hyporheic zone. While the total species richness of fungal assemblages was similar among treatments, the composition of fungal assemblages was affected by leaf litter burial in sediment. 5. Leaf litter burial markedly affected the food quality (especially P content) of leaf material, probably due to the changes in microbial conditioning. Leaf litter palatability to shredders was highest for leaves exposed at the sediment surface and tended to be negatively related to leaf litter toughness and C/P ratio. In addition, burial of leaf litter led to lower amphipod growth rates, which were positively correlated with leaf litter P content. 6. These results emphasise the importance of leaf colonisation by aquatic fungi in the hyporheic zone of headwater streams, where fungal conditioning of leaf litter appears particularly critical for nutrient and energy transfer to higher trophic levels.  相似文献   

3.
  • 1. Forested headwater streams are generally considered to be light-limited ecosystems where primary production is reduced, and the main source of energy and nutrients is composed of allochthonous detritus. We hypothesised that in these ecosystems, the development of primary producers might also be limited by (1) competition for nutrients with leaf-litter decomposers (e.g. bacteria and fungi), and (2) leaf-litter leachates or allelopathic compounds produced by aquatic fungi.
  • 2. To test these hypotheses, a 48-day mesocosm experiment was performed in 12 artificial streams containing stream water inoculated with epilithic biofilm suspensions collected from a forested headwater stream. Three different treatments were applied: control without leaf litter (C), microbially conditioned leaf litter added at the beginning of the experiment and left to decompose throughout the experiment (L), or leaf litter renewed three times during the experiment (RL).
  • 3. We predicted that (1) the presence of litter, through microbial nutrient immobilisation and allelopathy, would reduce primary production and that (2) this effect would be amplified by litter renewal. We also predicted that nutrient competition would mean that (3) leaf-litter decomposers will alter primary producer community composition and physiology. These predictions were tested by analysing biofilm development, physiology, stoichiometry, and benthic algal community structure. To distinguish between the effects of nutrient immobilisation and allelopathy, the biofilm responses to leaf-litter leachates collected after different microbial conditioning durations were also measured in a parallel laboratory experiment.
  • 4. Contrary to our expectations, by day 28, primary producer growth was higher in the mesocosms containing leaf litter (L and RL) despite the rapid decrease in dissolved nutrients when leaf litter was present. After 48 days, the lowest phototrophic biofilm development was observed when leaf litter was renewed (RL), whereas phototrophic biofilm development was similar in the C and L treatments. Biofilm stoichiometry indicated that this effect was most probably related to greater nitrogen limitation in the RL treatment. The presence of leaf litter also affected primary producers' photophysiology, which could be attributed to changes in taxonomic composition and to physiological adjustments of primary producers.
  • 5. Laboratory measurements showed that despite a strong inhibition of primary producer growth by unconditioned leaf-litter leachates, microbially conditioned leaf litter had either low or no effects on the development of primary producers.
  • 6. These results reveal that leaf-litter decomposers can have both positive and negative effects on primary producers underlining the need to consider microbial interactions when investigating the functioning of forested headwater streams.
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4.
1. Trophic fractionation was studied in short‐term laboratory feeding experiments with larvae of the deposit‐feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll®). 2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources. 3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed. 4. Microbial degradation of food particles resulted in an approximate doubling of the δ15N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ13C increased only marginally, from ?20.66 ± 0.11‰ to ?20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.  相似文献   

5.
  1. The transformation of leaf litter, a key process in aquatic systems, is known to be reduced with decreasing oxygen concentrations, mainly due to lower abundance of and/or less active shredding macroinvertebrates. Aquatic fungi and phagotrophic protists involved in leaf litter processing can tolerate low oxygen, but little is known about their role in leaf litter processing under these conditions.
  2. We aimed to unravel the importance of phagotrophic protists within microbially mediated leaf litter processing under contrasting oxic conditions. We hypothesised that respiration, abundance of aquatic bacteria and biomass of aquatic fungi, and thus leaf litter processing, are enhanced in the presence of phagotrophic protists, both under normoxic and low oxic conditions. Lower leaf processing was expected under low oxic than under normoxic conditions.
  3. In microcosms, oxygen concentration was adjusted to either normoxic or low. For a total of 105 days, leaf litter was cocultivated with three microbial communities: (i) a multispecies bacterial community enriched from stream leaf litter, (ii) the aquatic fungus Heliscus lugdunensis added to the bacterial community and (iii) the phagotrophic protist Glaucoma scintillans added to the bacteria–fungi community.
  4. Oxic condition had no significant effect on microbially mediated leaf mass loss. The leaf mass loss was faster in the presence of the aquatic fungus and further accelerated by adding the phagotrophic protist. After 105 days, leaf mass remaining approximated 64–79%, 57–68% and 55–61% of initial leaf mass in the bacteria, bacteria–fungi, and bacteria–fungi–phagotrophic protist communities, respectively.
  5. Under both oxic conditions, the lower leaf toughness indicated that the aquatic fungus had the potential to process leaf structural components more efficiently than bacteria alone. The combination of lower ergosterol concentrations and enhanced leaf mass loss indicated that phagotrophic protists stimulated the efficiency of leaf processing by the microbial community. Under normoxic conditions, leaf‐associated respiration increased when successively adding an aquatic fungus and a phagotrophic protist to bacterial communities, which matches the faster leaf litter processing in these treatments. Under low oxic conditions, respiration of all three microbial communities was comparable. Thus, enhanced leaf litter processing in treatments with aquatic bacteria, fungi and phagotrophic protists was presumably caused either by other metabolic pathways such as fermentation and anaerobic respiration, or by changes in community composition and growth efficiency of the microbial community.
  6. Microbial communities composed of aquatic bacteria, fungi and phagotrophic protists play a crucial role in leaf litter processing under a range of oxic conditions. Although processing of leaf litter by microbial communities alone is slower than in the presence of shredding invertebrates, the interaction of aquatic bacteria, fungi and phagotrophic protists can modulate leaf quality and subsequently carbon flow within the microbial food webs of aquatic ecosystems.
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6.
The bacterial community in the activated sludge of a local wastewater treatment plant was studied in an effort to understand and exploit the metabolic versatility of microorganisms for the efficient biological treatment of food waste. Microorganisms capable of and efficient in degrading domestic food waste were screened based on their ability to produce areas of clearing on selective media containing protein, fat, cellulose and starch. Nine microbial species belonging to the genera Flavobacterium, Pseudomonas, Micrococcus, Aeromonas, Xanthomonas, Vibrio and Sphingomonas were found to degrade all components of food waste. These bacteria were added to domestic wastewater and shown to cause a 60% reduction in the biochemical oxygen demand (BOD) level of wastewater compared to a control in which no microorganisms were added. The ability of the microbial consortium to degrade domestic wastewater as evidenced by the decrease in BOD levels suggests its potential for use in the biological treatment of food waste.  相似文献   

7.
Biodiversity is under pressure worldwide, with amphibians being particularly threatened. Stressors related to human activity, such as chemicals, are contributing to this decline. It remains, however, unclear whether chemicals exhibiting a fungicidal activity could indirectly affect tadpoles that depend on microbially conditioned leaf litter as food source. The indirect effect of fungicides (sum concentration of a fungicide mixture composed of azoxystrobin, carbendazim, cyprodinil, quinoxyfen, and tebuconazole: 100 µg/L) on tadpoles was assessed relative to leaf litter colonized by microbes in absence of fungicides (control) and a worst‐case scenario, that is leached leaf litter without microbial colonization. The quality of leaf litter as food for tadpoles of the European common frog (Rana temporaria) was characterized through neutral lipid fatty acid profiles and microbial sum parameters and verified by sublethal responses in tadpoles (i.e., feeding rate, feces production, growth, and fatty acid composition). Fungicides changed the nutritious quality of leaf litter likely through alterations in leaves’ neutral lipid fatty acid profiles (i.e., changes in some physiologically important highly unsaturated fatty acids reached more than 200%) in combination with a potential adsorption onto leaves during conditioning. These changes were reflected by differences in the development of tadpoles ultimately resulting in an earlier start of metamorphosis. Our data provide a first indication that fungicides potentially affect tadpole development indirectly through bottom‐up effects. This pathway is so far not addressed in fungicide environmental risk assessment and merits further attention.  相似文献   

8.
The role of micro-organisms in the ecological connectivity of running waters   总被引:12,自引:1,他引:11  
1. Riparian zones hold a central place in the hydrological cycle, owing to the prevalence of surface and groundwater interactions. In riparian transition zones, the quality of exfiltrating water is heavily influenced by microbial activities within the bed sediments. This paper reviews the role of micro-organisms in biogeochemical cycling in the riparian-hyporheic ecotone. 2. The production of organic substances, such as cellulose and lignin, by riparian vegetation is an important factor influencing the pathways of microbial processing in the riparian zone. For example, anaerobic sediment patches, created by entrainment of allochthonous organic matter, are focal sites of microbial denitrification. 3. The biophysical structure of the riparian zone largely influences in-stream microbial transformations through the retention of organic matter. Particulate and dissolved organic matter (POM and DOM) is retained effectively in the hyporheic zone, which drives biofilm development and associated microbial activity. 4. The structure of the riparian zone, the mechanisms of POM retention, the hydrological linkages to the stream and the intensity of key biogeochemical processes vary greatly along the river continuum and in relation to the geomorphic setting. However, the present state of knowledge of organic matter metabolism in the hyporheic zone suggests that lateral ecological connectivity is a basic attribute of lotic ecosystems. 5. Due to their efficiency in transforming POM into heterotrophic microbial biomass, attached biofilms form an abundant food resource for an array of predators and grazers in the interstitial environments of rivers and streams. The interstitial microbial loop, and the intensity of microbial production within the bed sediments, may be a primary driver of the celebrated high productivity and biodiversity of the riparian zone. 6. New molecular methods based on the analysis of the low molecular weight RNA (LMW RNA) allow unprecedented insights into the community structure of natural bacterial assemblages and also allow identification and study of specific strains hitherto largely unknown. 7. Research is needed on the development and evaluation of sampling methods for interstitial micro-organisms, on the characterization of biofilm structure, on the analysis of the biodegradable matter in the riparian-hyporheic ecotone, on the regulation mechanisms exerted on microbiota by interstitial predators and grazers, and on measures of microbial respiration and other key activities that influence biogeochemical cycles in running waters. 8. Past experiences from large-scale alterations of riparian zones by humans, such as the River Rhine in central Europe, undeniably demonstrate the detrimental consequences of disconnecting rivers from their riparian zones. A river management approach that uses the natural services of micro-organisms within intact riparian zones could substantially reduce the costs of clean, sustainable water supplies for humans.  相似文献   

9.
The functioning and structure of terrestrial ecosystems are shaped and maintained by plant–decomposer interactions. The food and habitat of animal populations are biogenic and are mainly of plant origin (plant litter) in terrestrial ecosystems. Primary resources of the food-habitat template for the organization of soil animals are provided by the primary production of plants, and are then modified through decomposition processes by microbial populations. In the microbial decomposition system, the efficiency of carbon utilization by microbial decomposers characterizes the decomposition processes between tropical and temperate forest ecosystems. Tropical forests show poor development of soil reservoir systems because of the high efficiency of lignin decomposition by microbial populations. The decomposition processes of leaf litter are described briefly for the understanding of organization of soil animal communities in tropical and temperate forests. A comparison of decomposition processes shows qualitative differences in decomposition between temperate and tropical forests. The composition of functional groups of soil animals is well explained by the decomposition processes in both forests.  相似文献   

10.
Arndt  Hartmut 《Hydrobiologia》1993,255(1):231-246
Recent investigations have shown that processes within the planktonic microbial web are of great significance for the functioning of limnetic ecosystems. However, the general importance of protozoans and bacteria as food sources for rotifers, a major component of planktonic habitats, has seldom been evaluated. Results of feeding experiments and the analysis of the food size spectrum of rotifers suggest that larger bacteria, heterotrophic flagellates and small ciliates should be a common part of the food of most rotifer species. About 10–40 per cent of rotifers' food can consist of heterotrophic organisms of the microbial web. Field experiments have indicated that rotifer grazing should generally play a minor role in bacteria consumption compared to feeding by coexisting protozoans. However, according to recent experiments regarding food selection, rotifers should be efficient predators on protozoans. Laboratory experiments have revealed that even nanophagous rotifers can feed on ciliates. Preliminary microcosm and chemostat experiments have indicated that rotifers, due to their relatively low community grazing rates compared to the growth rates of bacteria and protozoans, should generally not be able (in contrast to some cladocerans) to suppress the microbial web via grazing, though they may structure it. Filter-feeding nanophagous rotifers (e.g. brachionids) seem to be significant feeders on the smaller organisms of the microbial web (bacteria, flagellates, small ciliates), whereas grasping species (e.g. synchaetids and asplanchnids) seem to be efficient predators on larger organisms (esp. ciliates). Another important role of rotifers is their feedback effect on the microbial web. Rotifers provide degraded algae, bacteria and protozoans to the microbial web and may promote microbial activity. Additional experimental work is necessary for a better understanding of the function of rotifers in aquatic ecosystems.  相似文献   

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