Seasonal variations overwhelm temperature effects on microbial processes in headwater streams: insights from a temperate thermal spring

Carbon cycling in headwater streams is mostly driven by the decomposition of allochthonous organic matter, and to a lesser extent by primary production. Quantifying the influence of temperature on these processes is therefore essential to better anticipate the consequences of global warming for stream ecological functioning. In this study, we measured alder litter microbial decomposition and associated fungal biomass and diversity, using leaf discs enclosed in fine-mesh bags along a natural geothermal temperature gradient, in both spring and winter. We monitored the chlorophyll-a accrual in biofilms growing on ceramic tiles. The temperature gradient, from upstream to downstream, ranged from 15.3 to 14.2 °C in spring and 18.2 to 13.2 °C in winter. Autotrophs and heterotrophs exhibited contrasting responses to temperature. The expected positive effect of temperature was actually observed for chlorophyll-a accrual only, while an apparent temperature-independence of litter decomposition rate was found. Moreover, temperature effects on heterotrophic and autotrophic organisms depended on the season, with higher litter decomposition rates, sporulation rates, fungal biomass and chlorophyll-a in spring, despite a lower mean water temperature than in winter. Together, these results suggest that the influence of temperature remained largely overrode by seasonal effects. This result is likely due to annual variations in light availability, and may involve indirect positive interactions between microbial primary producers and decomposers

Temperature and nutrient effects on the relative importance of brown and green pathways for stream ecosystem functioning: A mesocosm approach

In addition to global warming, aquatic ecosystems are currently facing multiple global changes among which include changes in nitrogen (N) loads. While several studies have investigated both temperature and N impacts on aquatic ecosystems independently, knowledge on their interactive effects remains scarce. In forested headwater streams, decomposition of leaf litter represents the main process ensuring the transfer of nutrients and energy to higher trophic levels, followed by autochthonous primary production, mainly ensured by phototrophic biofilms. The main aim of this study was to disentangle the independent and combined effects of temperature increase and nutrient availability on the relative importance of brown and green processes involved in stream functioning. We hypothesised that water temperature and nutrients would lead to a general increase in leaf‐litter decomposition and primary production, but that the intensity of these effects would be largely modulated by competitive interactions arising between microorganisms as well as by the top‐down control of microorganisms by macro‐invertebrates. Macro‐invertebrates would, in turn, be bottom‐up controlled by microbial resources quality. To test these hypotheses, we conducted a 56‐day experiment in artificial streams containing leaf litter, microbial decomposers and biofilm inoculum, and an assemblage of macro‐invertebrates. Two water inorganic N:phosphorus (P) ratios (33 and 100, molar ratios) and two temperatures (ambient, +2°C) were manipulated, each treatment being replicated three times. Fungal and biofilm growth as well as leaf‐litter decomposition and primary production were quantified. Top‐down impacts of invertebrate primary consumers on brown and green compartments were evaluated using exclosures while bottom‐up control was evaluated through the measurement of resource stoichiometry and fatty acid profiles, as well as quantification of macro‐invertebrate growth and survival. Contrary to expectations, microbial decomposition was not significantly stimulated by nutrient or temperature manipulations, while primary production was only improved under ambient temperature. In the + 2°C treatment with high N:P, greater biofilm biomass was associated with lower fungal development, which indicates competition for nutrients in these conditions. Temperature increased macro‐invertebrate growth and leaf‐litter consumption, but this effect was independent of any improvement of basal resource quality, suggesting that temperature mediated changes in consumer metabolism and activity was the main mechanism involved. Most of our hypotheses that were based on simplified laboratory observations have been rejected in our semi‐controlled mesocosms. Our study suggests that the complexity of biological communities might greatly affect the response of ecosystems to multiple stressors, and that interactions between organisms must be explicitly taken into account when investigating the impacts of global change on ecosystem functioning

Variable temperature effects between heterotrophic stream processes and organisms

Temperature is known to stimulate metabolism with cascading effects on multiple biological processes. These effects may, however, vary across processes, types of organisms or levels of biological organisation. They can also vary with nutrient availability, with potentially stronger temperature effects when nutrients are not limiting. This context dependence of temperature effects on processes challenges our ability to anticipate their consequences on ecosystems in a changing world. In headwater streams, the decomposition of allochthonous leaf litter, driven by both microbial decomposers and invertebrates, is known to respond to both temperature and nutrient availability. These food webs are highly tractable and a useful model system to investigate the variations of temperature effects on processes across types of organisms (microbes versus invertebrates), resource availability levels (nutrient concentration), and levels of biological organisation (from individual to ecosystem). In a microcosm experiment, we measured the effects of temperature and nitrogen availability (four levels each) on respiration rates of litter-consuming microbes and invertebrates and their decomposition activity in different contexts of food web complexity. The latter included one treatment without invertebrate detritivore (microbial decomposers only), three single invertebrate taxa (Gammarus, Potamophylax, and Sericostoma) treatments, and one mixed invertebrate taxa treatment (three‐species altogether). Microbial processes increased nearly exponentially with temperature (Arrhenius model, activation energy (± 95% confidence interval) = 0.56 ± 0.53 and 1.00 ± 0.23 eV for litter decomposition and respiration), while invertebrate‐driven processes increased (activation energy from 0.47–1.15 eV) up to a maximal value at an intermediate temperature (c. 11–15°C depending on species and process), above which process rates decreased. By contrast, litter consumption in mixed invertebrate species treatments was not significantly influenced by temperature, because of a negative effect of species mixing occurring above 12°C. Nitrogen had a weaker influence, only slightly stimulating litter consumption by mixed‐species invertebrates, which limited the scope for synergies with temperature effects. Our results raise issues about how aquatic litter consumers meet their energy requirements at high temperature and suggest that a general consequence of warming could be loss of carbon through mineralisation in headwater stream food webs. In several aspects, our results deviate from expectations based on universal relationships between temperature and individual metabolism (e.g. metabolic theory of ecology), suggesting that we may need to develop less simplistic assumptions to predict the consequence of warming on ecosystem processes

Characterization of the ars Gene Cluster from Extremely Arsenic-Resistant Microbacterium sp. Strain A33▿ †

The arsenic resistance gene cluster of Microbacterium sp. A33 contains a novel pair of genes (arsTX) encoding a thioredoxin system that are cotranscribed with an unusual arsRC2 fusion gene, ACR3, and arsC1 in an operon divergent from arsC3. The whole ars gene cluster is required to complement an Escherichia coli ars mutant. ArsRC2 negatively regulates the expression of the pentacistronic operon. ArsC1 and ArsC3 are related to thioredoxin-dependent arsenate reductases; however, ArsC3 lacks the two distal catalytic cysteine residues of this class of enzymes

Gut Microbiota of Zebra Mussels (Dreissena polymorpha) as a Holobiont Concept- Significantly Reliable Method for Aquatic Environment Monitoring?

International audienceFrance's Water bodies are constantly exposed to emerging pollutants from urban areas. It requires constant monitoring of contamination. Zebra mussel (ZM) is an invasive species which is spread in European waters extensively. They are filter feeders that filter a large amount of water as its food source. They process and remove a high number of microorganisms (MO). ZM accumulate compounds from the water column and is an excellent indicator of the surrounding environment. The filtration process allows ZMs to absorb naturally occurring mixtures and pollutants. ZM's gut microbiota (GM) contains a high number of MO and creates a specific symbiosis with the host. It is closely connected to the outside environment and represents aquatic conditions. It shows the importance of holobiont symbiosis and its need for detailed understanding. This study investigates the GM of ZM as an indicator of emerging contaminants such as human pathogens and antibiotics resistance bacteria (ARB). The inquiry is how GM is affected by variations of diet, pathogens and contaminants appearing in aquatic environments and the overall understanding of the gut microbiota of ZMs and its essence in the accumulation of increasing quantities of pathogens and ARB.ZMs were collected in the field and let acclimated before following steps. After the acclimation period, the field investigation selected ZMs were transferred to appointed sites and put into specially created devices for collecting faeces. The rest of the ZMs were kept in laboratory conditions to further study their gastrointestinal microbiota. Further analysis will be performed by 16S rDNA sequencing and cultivation.The experiment combines laboratory-controlled conditions and field experiments. Obtaining data from different feeding conditions, filtration rates and the overall effect of the laboratory and field environment and possible accumulation of pathogens and ARB from the field is critical for further study. Testing of water bodies can be more reliable with ZMs accumulation of contaminants with comparison to regular grab water samples depending on many factors.This study emphasises the importance of the microbiota of filter feeders and their ability to accumulate compounds from the water