Heterotrophic Microbial Activity Research Articles

Uptake of Leucine, Chitin, and Iron by Prokaryotic Groups during Spring Phytoplankton Blooms Induced by Natural Iron Fertilization off Kerguelen Island (Southern Ocean)

Iron and carbon are essential for microbial heterotrophic activity, but the bioavailability of these elements is low in surface waters of the Southern Ocean. Whether the access to iron and carbon differs among phylogenetic groups of prokaryotes is barely known. To address this question we used iron (55FeCl3), and the carbon compounds chitin (3H-Diacetylchitobiose) and leucine (3H-leucine) as model substrates in combination with MICRO-CARD-FISH during spring phytoplankton blooms induced by natural iron fertilization off Kerguelen Island (KErguelen Ocean and Plateau compared Study 2 - KEOPS2; October-November 2011). The application of probes at broad phylogenetic levels indicated an overall similar community composition in surface waters at the 8 investigated sites. The relative contributions of the prokaryotic groups to abundance revealed a strong positive relationship with their respective contributions to the leucine-active community (p0.05; r=0.26). These results suggest preferential uptake of iron and chitin by some prokaryotic groups. SAR11 and Cytophaga-Flavobacterium-Bacteroides (CFB) were the dominant contributors to the leucine-active community, while CFB and Archaea had the highest contributions to the chitin-active community. By contrast, Gammaproteobacteria, including SAR86, and CFB revealed the highest contributions to the iron-active community. We found several correlations between the group-specific fractions of active cells for a given substrate and most of them included CFB, pointing to the potential importance of microbial interactions for iron and carbon cycling in the Southern Ocean.

Resource stoichiometry and the biogeochemical consequences of nitrogen deposition in a mixed deciduous forest.

Ecosystems often show differential sensitivity to chronic nitrogen (N) deposition; hence, a critical challenge is to improve our understanding of how and why site-specific factors mediate biogeochemical responses to N enrichment. We examined the extent to which N impacts on soil carbon (C) and N dynamics depend on microbial resource stoichiometry. We added N to forest plots dominated by ectomycorrhizal (ECM) trees, which have litter and soil pools rich in organic N and relatively wide C:N ratios, and adjacent forest plots dominated by arbuscular mycorrhizal (AM) trees, which have litter and soil pools rich in inorganic N and relatively narrow C:N ratios. While microbes in both plot types exhibited fairly strict biomass homeostasis, microbes in AM- and ECM-dominated plots differed in their physiological responses to N addition. Microbes in ECM plots responded to N enrichment by decreasing their investment in N-acquisition enzymes (relative to C-acquisition enzymes) and increasing N mineralization rates (relative to C mineralization rates), suggesting that N addition alleviated microbial N demand. In contrast, heterotrophic microbial activities in AM plots were unaffected by N addition, most likely as a result of N-induced increases in net nitrification (60% increase relative to control plots) and nitrate mobilization (e.g., sixfold increases in mobilization relative to control plots). Combined, our findings suggest the stoichiometric differences between AM and ECM soils are the primary drivers of the observed responses. Plant and microbial communities characterized by wide C:N are more susceptible to N-induced changes in decomposition and soil C dynamics, whereas communities characterized by narrow C:N are more susceptible to N-induced nitrate leaching losses. Hence, the biogeochemical consequences of N deposition in temperate forests may be driven by the stoichiometry of the dominant trees and their associated microbes.

Citrate and malonate increase microbial activity and alter microbial community composition in uncontaminated and diesel-contaminated soil microcosms

Abstract. Petroleum hydrocarbons (PHCs) are among the most prevalent sources of environmental contamination. It has been hypothesized that plant root exudation of low molecular weight organic acid anions (carboxylates) may aid degradation of PHCs by stimulating heterotrophic microbial activity. To test their potential implication for bioremediation, we applied two commonly exuded carboxylates (citrate and malonate) to uncontaminated and diesel-contaminated microcosms (10 000 mg kg−1; aged 40 days) and determined their impact on the microbial community and PHC degradation. Every 48 h for 18 days, soil received 5 µmol g−1 of (i) citrate, (ii) malonate, (iii) citrate + malonate or (iv) water. Microbial activity was measured daily as the flux of CO2. After 18 days, changes in the microbial community were assessed by a community-level physiological profile (CLPP) and 16S rRNA bacterial community profiles determined by denaturing gradient gel electrophoresis (DGGE). Saturated PHCs remaining in the soil were assessed by gas chromatography–mass spectrometry (GC-MS). Cumulative soil respiration increased 4- to 6-fold with the addition of carboxylates, while diesel contamination resulted in a small, but similar, increase across all carboxylate treatments. The addition of carboxylates resulted in distinct changes to the microbial community in both contaminated and uncontaminated soils but only a small increase in the biodegradation of saturated PHCs as measured by the n-C17 : pristane biomarker. We conclude that while the addition of citrate and malonate had little direct effect on the biodegradation of saturated hydrocarbons present in diesel, their effect on the microbial community leads us to suggest further studies using a variety of soils and organic acids, and linked to in situ studies of plants, to investigate the role of carboxylates in microbial community dynamics.

Salinity and nutrient contents of tidal water affects soil respiration and carbon sequestration of high and low tidal flats of Jiuduansha wetlands in different ways

Soils were collected from low tidal flats and high tidal flats of Shang shoal located upstream and Xia shoal located downstream with different tidal water qualities, in the Jiuduansha wetland of the Yangtze River estuary. Soil respiration (SR) in situ and soil abiotic and microbial characteristics were studied to clarify the respective differences in the effects of tidal water salinity and nutrient levels on SR and soil carbon sequestration in low and high tidal flats. In low tidal flats, higher total nitrogen (TN) and lower salinity in the tidal water of Shang shoal resulted in higher TN and lower salinity in its soils compared with Xia shoal. These would benefit β-Proteobacteria and Anaerolineae in Shang shoal soil, which might have higher heterotrophic microbial activities and thus soil microbial respiration and SR. In low tidal flats, where soil moisture was high and the major carbon input was active organic carbon from tidal water, increasing TN was a more important factor than salinity and obviously enhanced soil microbial heterotrophic activities, soil microbial respiration and SR. While, in high tidal flats, higher salinity in Xia shoal due to higher salinity in tidal water compared with Shang shoal benefited γ-Proteobacteria which might enhance autotrophic microbial activity, and was detrimental to β-Proteobacteria in Xia shoal soil. These might have led to lower soil microbial respiration and thus SR in Xia shoal compared with Shang shoal. In high tidal flats, where soil moisture was relatively lower and the major carbon input was plant biomass that was difficult to degrade, soil salinity was the major factor restraining microbial activities, soil microbial respiration and SR.

Soil organic matter quality and microbial activities in spruce swamp forests affected by drainage and water regime restoration

AbstractThe effect of spruce swamp forest (SSF) drainage and water regime restoration on soil organic matter (SOM) quality and soil microbial heterotrophic activities was studied in pristine, drained and restored SSF in the Bohemian Forest, Czech Republic. Sequential chemical SOM fractionation using cold and hot water and hot acid was used to separate SOM fractions according to their mobility and potential lability/recalcitrance, and Fourier transform infrared spectra were used for SOM characterization. Soil physicochemical parameters and heterotrophic microbial activities were also determined. Drainage of SSF had significant long‐term effects (more than 50 yr) on plant communities and SOM quality. On drained sites, cover of sphagnum moss and sedge was much smaller than on pristine locations. A greater proportion of recalcitrant compounds and a smaller proportion of labile compounds were found in drained SSF as compared to pristine sites, which first led to an energy limitation and was followed by a decrease in microbial biomass and heterotrophic microbial activities (CO2 production, methanogenesis and methanotrophy). Restoration resulted in slow progressive changes in the vegetation cover, including the spread of sphagnum mosses, retreat of mosses typical of drier conditions and increased sedge cover compared with drained SSF. Moreover, soil physicochemical parameters (pH and bulk density), hot‐water‐extractable C and methanotrophic activity tended to evolve towards the pristine SSF and seem to be good indicators of the restoration process. No other SOM fractions changed significantly after restoration. Thus, to change significantly overall SOM quality and most microbial heterotrophic activities following restoration, more than 7 yr are required.

Linking Heterotrophic Microbial Activities with Particle Characteristics in Waters of the Mississippi River Delta in the Aftermath of Hurricane Isaac

Riverine runoff often triggers microbial responses in coastal marine environments, including phytoplankton blooms and enhanced bacterial biomass production that drive the transformation of dissolved and particulate organic matter on its way from land to the deep ocean. We measured concentrations and characteristics of particulate organic matter (POM), concentrations of dissolved organic carbon (DOC), and bacterial community abundance and activities in the water column at three sites near the Mississippi River Delta two weeks after Hurricane Isaac made landfall in late August 2012. River plumes had salinities of >30 PSU and high levels of DOC (210-380 µM), resulting from the storm surge that pushed large quantities of marine waters upstream. Relatively high concentrations of phytoplankton POM and low levels of microbial exopolymeric particles (TEP and CSP) suggested that storm-induced riverine discharge triggered the development of phytoplankton blooms that were in their initial stages at the time of sampling. Surface water POM had C/N ratios of 5-7 and strong protein-like fluorescence signals in the base-extracted POM (BEPOM) fraction at the two sites closer to the river mouth (Stns. TE and MSP). Freshly produced POM triggered a twofold increase in heterotrophic bacterial biomass production (3H-leucine incorporation) and a fourfold increase in bacterial peptide hydrolysis (activities of leucine-aminopeptidase). In contrast, elevated DOC concentrations coincided with only moderate bacterial community activity, suggesting that heterotrophic bacterial metabolism near the Mississippi River Delta in the aftermath of Hurricane Isaac was more closely linked with autochthonous primary production.

Principal component analysis and kinetic model adjustment to describe the community-level physiological profiles (CLPP) in the sediments of the river Minho estuary (Northern Portugal)

Principal component analysis and kinetic model adjustment to describe the community-level physiological profiles (CLPP) in the sediments of the river Minho estuary (Northern Portugal)

Linking heterotrophic microbial activities with particle characteristics in waters of the mississippi river delta in the aftermath of hurricane isaac

Linking heterotrophic microbial activities with particle characteristics in waters of the mississippi river delta in the aftermath of hurricane isaac

Excess of Organic Carbon in Mountain Spruce Forest Soils after Bark Beetle Outbreak Altered Microbial N Transformations and Mitigated N-Saturation.

Mountain forests in National park Bohemian Forest (Czech Republic) were affected by bark beetle attack and windthrows in 2004–2008, followed by an extensive tree dieback. We evaluated changes in the biochemistry of the uppermost soil horizons with the emphasis on carbon (C) and nitrogen (N) cycling in a near-natural spruce (Picea abies) mountain forest after the forest dieback, and compared it with an undisturbed control plot of similar age, climate, elevation, deposition, N-saturation level, and land use history. We hypothesised that the high litter input after forest dieback at the disturbed plot and its consequent decomposition might influence the availability of C for microorganisms, and consequently, N transformations in the soil. The concentrations of dissolved organic C (DOC) and N (DON) in soil water extracts rapidly increased at the disturbed plot for 3 yeas and then continually decreased. Net ammonification exhibited a similar trend as DOC and DON, indicating elevated mineralization. Despite the high ammonium concentrations found after the forest dieback (an increase from 0.5 mmol kg-1 to 2–3 mmol kg-1), net nitrification was stable and low during these 3 years. After the DOC depletion and decrease in microbial biomass 5 years after the forest dieback, net nitrification started to rise, and nitrate concentrations increased from 0.2–1 mmol kg-1 to 2–3 mmol kg-1. Our results emphasize the key role of the availability of organic C in microbial N transformations, which probably promoted microbial heterotrophic activity at the expense of slow-growing nitrifiers.

Biofiltration of Ammonia and GHGs from Swine Gestation Barn Pit Exhaust

<abstract> <b><sc>Abstract.</sc></b> Livestock barn emissions can affect public health, the environment, and quality of life. While these emissions can be mitigated using several methods, exhaust air treatment may be required in some situations. Biofiltration is one of the most cost-effective exhaust air treatment methods. In a biofilter, polluted air passes through a moist medium (e.g., compost) where the water-soluble gases are dissolved and then degraded by microorganisms into harmless or less harmful compounds. In this study, a downflow biofilter using a compost and wood chip medium was evaluated over summer, fall, and winter (August 2010 to January 2011) for its ability to mitigate emissions of ammonia (NH₃) and three greenhouse gases (GHGs): methane (CH₄), nitrous dioxide (N₂O), and carbon dioxide (CO₂). Biofilter medium properties were analyzed at the beginning and twice during the study. Changes in medium properties and CO₂ data indicated greater heterotrophic microbial activity during summer through fall and greater autotrophic activity during fall through winter. Regardless of empty bed residence time (EBRT) (5.3 to 26 s), NH₃ removal efficiency (RE) was about 90% with inlet concentrations of ≤1.1 mg m^-3. With higher NH₃ loading rates, the RE may differ from this study. In fall, CH₄ RE was 49% (EBRT = 26 s) but only 13% in summer (EBRT = 13 s). Nitrous oxide RE varied in a narrow range of 14% to 18% over the study. In summer, CO₂ removal was negligible but was 15% in fall and 34% in winter. While a compost based medium may be more effective for CH₄ and N₂O mitigation, a wood chip based medium would be more economical. Care should be taken when using a photoacoustic sensor for high-frequency and low-concentration NH₃ measurements.

Biofiltration of Ammonia and GHGs from Swine Gestation Barn Pit Exhaust

<abstract> <b><sc>Abstract.</sc></b> Livestock barn emissions can affect public health, the environment, and quality of life. While these emissions can be mitigated using several methods, exhaust air treatment may be required in some situations. Biofiltration is one of the most cost-effective exhaust air treatment methods. In a biofilter, polluted air passes through a moist medium (e.g., compost) where the water-soluble gases are dissolved and then degraded by microorganisms into harmless or less harmful compounds. In this study, a downflow biofilter using a compost and wood chip medium was evaluated over summer, fall, and winter (August 2010 to January 2011) for its ability to mitigate emissions of ammonia (NH₃) and three greenhouse gases (GHGs): methane (CH₄), nitrous dioxide (N₂O), and carbon dioxide (CO₂). Biofilter medium properties were analyzed at the beginning and twice during the study. Changes in medium properties and CO₂ data indicated greater heterotrophic microbial activity during summer through fall and greater autotrophic activity during fall through winter. Regardless of empty bed residence time (EBRT) (5.3 to 26 s), NH₃ removal efficiency (RE) was about 90% with inlet concentrations of ≤1.1 mg m^-3. With higher NH₃ loading rates, the RE may differ from this study. In fall, CH₄ RE was 49% (EBRT = 26 s) but only 13% in summer (EBRT = 13 s). Nitrous oxide RE varied in a narrow range of 14% to 18% over the study. In summer, CO₂ removal was negligible but was 15% in fall and 34% in winter. While a compost based medium may be more effective for CH₄ and N₂O mitigation, a wood chip based medium would be more economical. Care should be taken when using a photoacoustic sensor for high-frequency and low-concentration NH₃ measurements.

Microbial metabolism of transparent exopolymer particles during the summer months along a eutrophic estuary system.

This study explores the role of transparent exopolymer particles (TEP) as an additional carbon source for heterotrophic microbial activity in the eutrophic Qishon estuary. From the coastal station and upstream the estuary; TEP concentrations, β-glucosidase activity, bacterial production and abundance have gradually increased. TEP were often found as bio-aggregates, scaffolding algae, detritus matter and bacteria that likely formed “hotspots” for enhance microbial activity. To further demonstrate the link between TEP and heterotrophic bacterial activity, confined incubations with ambient and polysaccharide-enriched estuary water were carried out. Following polysaccharide addition, elevated (~50%) β-glucosidase activity rates were observed, leading to TEP hydrolysis. This newly formed bioavailable carbon resulted in significantly higher growth rates, with up to a 5-fold increase in heterotrophic bacterial biomass, comprising mostly high nucleic acid content bacteria. Taking together the findings from this research, we conclude that even in highly eutrophic environments heterotrophic bacteria may still be carbon limited. Further, TEP as a polysaccharide matrix can act as a metabolic surrogate, adding fresh bioavailable carbon through tight associations with bacteria in eutrophic ecosystems such as the Qishon estuary.

Fe and C co-limitation of heterotrophic bacteria in the naturally fertilized region off the Kerguelen Islands

Abstract. It has been univocally shown that iron (Fe) is the primary limiting nutrient for phytoplankton metabolism in high-nutrient, low-chlorophyll (HNLC) waters, yet the question of how this trace metal affects heterotrophic microbial activity is far less understood. We investigated the role of Fe for bacterial heterotrophic production and growth at three contrasting sites in the naturally Fe-fertilized region east of the Kerguelen Islands and at one site in HNLC waters during the KEOPS2 (Kerguelen Ocean and Plateau Compared Study 2) cruise in spring 2011. We performed dark incubations of natural microbial communities amended either with iron (Fe, as FeCl3) or carbon (C, as trace-metal clean glucose), or a combination of both, and followed bacterial abundance and heterotrophic production for up to 7 days. Our results show that single and combined additions of Fe and C stimulated bulk and cell-specific bacterial production at the Fe-fertilized sites, while in HNLC waters only combined additions resulted in significant increases in these parameters. Bacterial abundance was enhanced in two out of the three experiments performed in Fe-fertilized waters but did not respond to Fe or C additions in HNLC waters. Our results provide evidence that both Fe and C are present at limiting concentrations for bacterial heterotrophic activity in the naturally fertilized region off the Kerguelen Islands in spring, while bacteria were co-limited by these elements in HNLC waters. These results shed new light on the role of Fe in bacterial heterotrophic metabolism in regions of the Southern Ocean that receive variable Fe inputs.

Light increases energy transfer efficiency in a boreal stream.

Periphyton communities of a boreal stream were exposed to different light and nutrient levels to estimate energy transfer efficiency from primary to secondary producers using labeling with inorganic 13C. In a one-day field experiment, periphyton grown in fast-flow conditions and dominated by opportunistic green algae were exposed to light levels corresponding to sub-saturating (forest shade) and saturating (open stream section) irradiances, and to N and P nutrient additions. In a two-week laboratory experiment, periphyton grown in low-flow conditions and dominated by slowly growing diatoms were incubated under two sub-saturating light and nutrient enrichment levels as well as grazed and non-grazed conditions. Light had significant positive effect on 13C uptake by periphyton. In the field experiment, P addition had a positive effect on 13C uptake but only at sub-saturating light levels, whereas in the laboratory experiment nutrient additions had no effect on the periphyton biomass, 13C uptake, biovolume and community composition. In the laboratory experiment, the grazer (caddisfly) effect on periphyton biomass specific 13C uptake and nutrient content was much stronger than the effects of light and nutrients. In particular, grazers significantly reduced periphyton biomass and increased biomass specific 13C uptake and C:nutrient ratios. The energy transfer efficiency, estimated as a ratio between 13C uptake by caddisfly and periphyton, was positively affected by light conditions, whereas the nutrient effect was not significant. We suggest that the observed effects on energy transfer were related to the increased diet contribution of highly palatable green algae, stimulated by higher light levels. Also, high heterotrophic microbial activity under low light levels would facilitate energy loss through respiration and decrease overall trophic transfer efficiency. These findings suggest that even a small increase in light intensity could result in community-wide effects on periphyton in boreal streams, with a subsequent increase in energy transfer and system productivity.

Carbon Dioxide and Nitrous Oxide Emissions Impacted by Bioenergy Sorghum Management

Modern bioenergy feedstocks, such as bioenergy sorghum [Sorghum bicolor (L.) Moench], are being developed to supply future cellulosic biofuel demands. How these cropping systems impact greenhouse gas (GHG) emission of CO2 and N2O from the soil is unknown and field research is necessary to elucidate the effects of agronomic management practices on soil trace gas emissions. We studied the effects of N fertilization (0 vs. 280 kg urea‐N ha−1), residue management (0 vs. 50% of sorghum biomass returned), crop sequence (corn [Zea mays L.]–sorghum vs. sorghum–sorghum), and their interactions on CO2 and N2O emissions from bioenergy production scenarios on a Weswood (fine‐silty, mixed, superactive, thermic Udifluventic Haplustept) silty clay loam soil in central Texas. Gas fluxes were measured approximately weekly throughout the 2010 and 2011 growing season and at a reduced rate during the fallow season with a photoacoustic gas analyzer integrated with a static chamber. Overall, CO2 and N2O fluxes were relatively higher than those observed by others in the United States despite drought conditions throughout much of 2010 and 2011. Highest emissions of both gases were observed during the growing season, often following a precipitation–irrigation event and shortly after N fertilization. Residue return increased cumulative CO2 emissions each year, probably due to increased heterotrophic microbial activity. Nitrogen addition significantly increased cumulative emissions of N2O both years but only impacted cumulative CO2 emissions in 2011. While crop rotation impacted biomass yield, it had no significant effect on cumulative CO2 or N2O emissions. Additional research is needed to identify the optimal N and residue application rates that provide high yields with minimal soil GHG emissions and aid in sustaining long‐term soil quality.

Effects of consumers and nitrogen availability on heterotrophic microbial activity during leaf decomposition in headwater streams

Summary Microbial processes in streams may be influenced by bottom‐up factors, such as nutrient availability, and by top‐down factors, such as the activity of upper trophic levels. The latter pathway has been well described between autotrophs and grazers, but the effects of consumers on heterotrophic basal trophic levels may also be important. Consumers may influence microbial activity through consumption of microbial biomass and by altering nutrient availability via consumer nutrient recycling (CNR). We investigated the effects of two shredder taxa, Gammarus spp. and Tipula spp., on nitrogen (N) uptake during organic matter decomposition by leaf‐associated heterotrophic microbial assemblages under conditions of ambient and enriched N availability in stream mesocosms. Colonised red maple leaves were placed in mesocosms with low (&lt;20 μg L−1) or high (&gt;800 μg L−1) nitrate concentration in the presence of amphipods, tipulids or no shredders. To test the importance of consumption and CNR, leaf packs were constructed from both coarse and fine mesh, which respectively allowed or prevented shredders from consuming detritus. Nitrogen enrichment generally increased red maple breakdown rates, while shredder influence on leaf breakdown was taxon specific. Feeding by tipulids increased red maple breakdown rate compared with amphipods, although not significantly. While feeding by amphipods did not directly influence breakdown rates, amphipod nutrient recycling stimulated leaf breakdown rate by the same magnitude as the N‐enrichment treatment. Fungal biomass increased with N enrichment. However, this did not lead to changes in N uptake, which was extremely variable and did not respond significantly to either treatment. In general, N uptake (per leaf mass) was greatest under enriched N conditions when tipulids were present. Our results indicate that both top‐down and bottom‐up factors influence microbial activity in heterotrophic systems and suggest that consumers may play important roles in detrital processing, not only through direct consumption but also through nutrient recycling.

Priming in the microbial landscape: periphytic algal stimulation of litter-associated microbial decomposers.

Microbial communities associated with submerged detritus in aquatic ecosystems often comprise a diverse mixture of autotrophic and heterotrophic microbes, including algae, bacteria, protozoa, and fungi. Recent studies have documented increased rates of plant litter mass loss when periphytic algae are present. We conducted laboratory and field experiments to assess potential metabolic interactions between natural autotrophic and heterotrophic microbial communities inhabiting submerged decaying plant litter of Typha angustifolia and Schoenoplectus acutus. In the field, submerged plant litter was either exposed to natural sunlight or placed under experimental canopies that manipulated light availability and growth of periphytic algae. Litter was collected and returned to the laboratory, where algal photosynthesis was manipulated (light/dark incubation), while rates of bacterial and fungal growth and productivity were simultaneously quantified. Bacteria and fungi were rapidly stimulated by exposure to light, thus establishing the potential for algal priming of microbial heterotrophic decay activities. Experimental incubations of decaying litter with 14C- and 13C-bicarbonate established that inorganic C fixed by algal photosynthesis was rapidly transferred to and assimilated by heterotrophic microbial decomposers. Periphytic algal stimulation of microbial heterotrophs, especially fungal decomposers, is an important and largely unrecognized interaction within the detrital microbial landscape, which may transform our current conceptual understanding of microbial secondary production and organic matter decomposition in aquatic ecosystems.

Interactive effects of temperature and soil moisture on fungal-mediated wood decomposition and extracellular enzyme activity

Heterotrophic soil microbes regulate the rate-limiting step in soil organic matter decomposition via the production of hydrolytic and oxidative extracellular enzymes. The influence of climate change on heterotrophic microbial activity remains poorly understood, not least in terms of the differential sensitivity of microbial functional groups to warming and altered precipitation regimes. Cord-forming basidiomycete fungi dominate primary decomposition in temperate woodlands. We investigate the interactive influence of elevated temperature (3 °C), wetting and drying (6–9% increase and 5–6% decrease of soil moisture, respectively) on saprotrophic basidiomycete-mediated beech (Fagus sylvatica) wood decomposition, as well as on hydrolytic (β-glucosidase, cellobiohydrolase, β-xylosidase, N-acetyl-glucosaminidase, acid phosphatase and leucine aminopeptidase) and oxidative (peroxidase and phenoloxidase) enzyme potential activities, in woodland soil mesocosms. While drying decreased beech wood decomposition rate, warming resulted in an increased rate and compensated for the negative effect of drying. Moisture regulated the extracellular enzyme pool; all enzymes except leucine aminopeptidase had significantly greater potential activity under wetting than drying. Again, elevated temperature consistently compensated for the negative effect of drying, but did not increase extracellular enzyme potential activity, alone or in combination with wetting. This reflects microbial production (fungal biomass was not increased under these conditions) rather than in situ effects on enzyme kinetics. N-acetyl-glucosaminidase and acid phosphatase displayed differential responses to temperature and moisture in systems dominated by different fungi. Decomposer communities appear to be more functionally resilient to the combined effects of elevated temperature and altered moisture than is suggested based on the manipulation of single abiotic variables.

Control of river stage on the reactive chemistry of the hyporheic zone

AbstractWe examined the influence of river stage on subsurface hydrology and pore water chemistry within the hyporheic zone of a groundwater‐fed river during the summer baseflow period of 2011. We found river stage and geomorphologic environment to control chemical patterns in the hyporheic zone. At a high river stage, the flux of upwelling water in the shallow sediments (&gt;20 cm) decreased at sample sites in the upper section of our study reach and increased substantially at sites in the lower section. This differential response is attributed to the contrasting geomorphology of these subreaches that affects the rate of the rise and fall of a river stage relative to the subsurface head. At sites where streamward vertical flux decreased, concentration profiles of a conservative environmental tracer suggest surface water infiltration into the riverbed below depths recorded at a low river stage. An increase in vertical flux at sites in the lower subreach is attributed to the movement of lateral subsurface waters originating from the adjacent floodplain. This lateral‐moving water preserved or decreased the vertical extent of the hyporheic mixing zone observed at a low river stage. Downwelling surface water appeared to be responsible for elevated dissolved organic carbon (DOC) and manganese (Mn) concentrations in shallow sediments (0–20 cm); however, lateral subsurface flows were probably important for elevated concentrations of these solutes at deeper levels. Results suggest that DOC delivered to hyporheic sediments during a high river stage from surface water and lateral subsurface sources could enhance heterotrophic microbial activities. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Experimental evaluation of the warming effect on viral, bacterial and protistan communities in two contrasting Arctic systems

The effect of Arctic warming, which is 3 times faster than the global average, on microbial communities was evaluated experimentally to determine how increasing temperatures affect bacterial and viral abundance and production, protist community composition, and bacterial loss rates (bacterivory and lysis) in 2 contrasting Arctic marine systems. In July 2009, we collected samples from open Arctic waters in the Barents Sea and Atlantic-influenced waters in Isfjorden, Svalbard Islands (Fjord waters). The samples were used in 2 microcosm experiments at 7 temper- atures, ranging from 1.0 to 10.0°C. In the open Arctic microbial community, collected at <1.0°C, bacterial and viral abundances, bacterial production and grazing rates due to protists increased significantly above 5.5°C, and remained at high values at even higher experimental temperatures. The abundance of protists, such as some heterotrophic pico/nanoflagellates, as well as some cili- ates, also increased with warming. In contrast, the biomass of phototrophs decreased above 5.5°C. The water temperature in Fjord waters was 6.2°C at the time of sampling, and the microbial com- munity showed smaller variations than the Arctic community. These results indicate that increases in temperature stimulate heterotrophic microbial biomass and activity compared to that of phototrophs, which has important implications for carbon and nutrient cycling in the system. In addition, open Arctic communities were more vulnerable to warming than those already adapted to the warmer Fjord waters influenced by Atlantic seawater.