Synthetic and biological surfactant effects on freshwater biofilm community composition and metabolic activity

Publication history: Accepted - 8 September 2022; Published - 19 September 2022.Surfactants are used to control microbial biofilms in industrial and medical settings. Their known toxicity on aquatic biota, and their longevity in the environment, has encouraged research on biodegradable alternatives such as rhamnolipids. While previous research has investigated the effects of biological surfactants on single species biofilms, there remains a lack of information regarding the effects of synthetic and biological surfactants in freshwater ecosystems. We conducted a mesocosm experiment to test how the surfactant sodium dodecyl sulfate (SDS) and the biological surfactant rhamnolipid altered community composition and metabolic activity of freshwater biofilms. Biofilms were cultured in the flumes using lake water from Lake Lunz in Austria, under high (300 ppm) and low (150 ppm) concentrations of either surfactant over a four-week period. Our results show that both surfactants significantly affected microbial diversity. Up to 36% of microbial operational taxonomic units were lost after surfactant exposure. Rhamnolipid exposure also increased the production of the extracellular enzymes, leucine aminopeptidase, and glucosidase, while SDS exposure reduced leucine aminopeptidase and glucosidase. This study demonstrates that exposure of freshwater biofilms to chemical and biological surfactants caused a reduction of microbial diversity and changes in biofilm metabolism, exemplified by shifts in extracellular enzyme activities.SG is funded by an Ulster University Vice Chancellors Doctoral Research Fellowship, and received additional support through an Ulster University Broadening Horizons Travel Bursary. Analytical costs were partly supported by the HYDRO-DIVERSITY project funded by the Environmental Systems Sciences Program of the Austrian Academy of Sciences (ÖAW) to JS, and core funding of the AFBI Aquatic Chemistry Laboratory (WH)

Evaluating stream CO2 outgassing via drifting and anchored flux chambers in a controlled flume experiment

Carbon dioxide (CO2) emissions from running waters represent a key component of the global carbon cycle. However, quantifying CO2 fluxes across air-water boundaries remains challenging due to practical difficulties in the estimation of reach-scale standardized gas exchange velocities (k(600)) and water equilibrium concentrations. Whereas craft-made floating chambers supplied by internal CO2 sensors represent a promising technique to estimate CO2 fluxes from rivers, the existing literature lacks rigorous comparisons among differently designed chambers and deployment techniques. Moreover, as of now the uncertainty of k(600) estimates from chamber data has not been evaluated. Here, these issues were addressed by analysing the results of a flume experiment carried out in the Summer of 2019 in the Lunzer:::Rinnen - Experimental Facility (Austria). During the experiment, 100 runs were performed using two different chamber designs (namely, a standard chamber and a flexible foil chamber with an external floating system and a flexible sealing) and two different deployment modes (drifting and anchored). The runs were performed using various combinations of discharge and channel slope, leading to variable turbulent kinetic energy dissipation rates (1.5 x 10(-3) epsilon < 1 x 10(-1) m(2) s(-3)). Estimates of gas exchange velocities were in line with the existing literature (4 < k(600) < 32 m(2) s(-3)), with a general increase in k(600) for larger turbulent kinetic energy dissipation rates. The flexible foil chamber gave consistent k600 patterns in response to changes in the slope and/or the flow rate. Moreover, acoustic Doppler velocimeter measurements indicated a limited increase in the turbulence induced by the flexible foil chamber on the flow field (22 % increase in 8, leading to a theoretical 5 % increase in k(600)). The uncertainty in the estimate of gas exchange velocities was then estimated using a generalized likelihood uncertainty estimation (GLUE) procedure. Overall, uncertainty in k(600) was moderate to high, with enhanced uncertainty in high-energy set-ups. For the anchored mode, the standard deviations of k 6 00 were between 1.6 and 8.2 m d(-1), whereas significantly higher values were obtained in drifting mode. Interestingly, for the standard chamber the uncertainty was larger (+ 20 %) as compared to the flexible foil chamber. Our study suggests that a flexible foil design and the anchored deployment might be useful techniques to enhance the robustness and the accuracy of CO2 measurements in low-order streams. Furthermore, the study demonstrates the value of analytical and numerical tools in the identification of accurate estimations for gas exchange velocities. These findings have important implications for improving estimates of greenhouse gas emissions and reaeration rates in running waters

Experimental evidence reveals impact of drought periods on dissolved organic matter quality and ecosystem metabolism in subalpine streams

Subalpine streams are predicted to experience lower summer discharge following climate change and water extractions. In this study, we aimed to understand how drought periods impact dissolved organic matter (DOM) processing and ecosystem metabolism of subalpine streams. We mimicked a gradient of drought conditions in stream‐side flumes and evaluated implications of drought on DOM composition, gross primary production, and ecosystem respiration. Our experiment demonstrated a production and release of DOM from biofilms and leaf litter decomposition at low discharges, increasing dissolved organic carbon concentrations in stream water by up to 50%. Absorbance and fluorescence properties suggested that the released DOM was labile for microbial degradation. Dissolved organic carbon mass balances revealed a high contribution of internal processes to the carbon budget during low flow conditions. The flumes with low discharge were transient sinks of atmospheric CO2 during the first 2 weeks of drought. After this autotrophic phase, the metabolic balance of these flumes turned heterotrophic, suggesting a nutrient limitation for primary production, while respiration remained high. Overall our experimental findings suggest that droughts in subalpine streams will enhance internal carbon cycling by transiently increasing primary production and more permanently respiration as the drought persists. We propose that the duration of a drought period combined with inorganic nutrient availability are key variables that determine if more carbon is respired in situ or exported downstream

Synthetic and biological surfactant effects on freshwater biofilm community composition and metabolic activity

Publication history: Accepted - 8 September 2022; Published - 19 September 2022.Surfactants are used to control microbial biofilms in industrial and medical settings. Their known toxicity on aquatic biota, and their longevity in the environment, has encouraged research on biodegradable alternatives such as rhamnolipids. While previous research has investigated the effects of biological surfactants on single species biofilms, there remains a lack of information regarding the effects of synthetic and biological surfactants in freshwater ecosystems. We conducted a mesocosm experiment to test how the surfactant sodium dodecyl sulfate (SDS) and the biological surfactant rhamnolipid altered community composition and metabolic activity of freshwater biofilms. Biofilms were cultured in the flumes using lake water from Lake Lunz in Austria, under high (300 ppm) and low (150 ppm) concentrations of either surfactant over a four-week period. Our results show that both surfactants significantly affected microbial diversity. Up to 36% of microbial operational taxonomic units were lost after surfactant exposure. Rhamnolipid exposure also increased the production of the extracellular enzymes, leucine aminopeptidase, and glucosidase, while SDS exposure reduced leucine aminopeptidase and glucosidase. This study demonstrates that exposure of freshwater biofilms to chemical and biological surfactants caused a reduction of microbial diversity and changes in biofilm metabolism, exemplified by shifts in extracellular enzyme activities.SG is funded by an Ulster University Vice Chancellors Doctoral Research Fellowship, and received additional support through an Ulster University Broadening Horizons Travel Bursary. Analytical costs were partly supported by the HYDRO-DIVERSITY project funded by the Environmental Systems Sciences Program of the Austrian Academy of Sciences (ÖAW) to JS, and core funding of the AFBI Aquatic Chemistry Laboratory (WH)

Climate-Induced Changes in Spring Snowmelt Impact Ecosystem Metabolism and Carbon Fluxes in an Alpine Stream Network

Although stream ecosystems are recognized as an important component of the global carbon cycle, the impacts of climate-induced hydrological extremes on carbon fluxes in stream networks remain unclear. Using continuous measurements of ecosystem metabolism, we report on the effects of changes in snowmelt hydrology during the anomalously warm winter 2013/2014 on gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP) in an Alpine stream network. We estimated ecosystem metabolism across 12 study reaches of the 254 km2 subalpine Ybbs River Network (YRN), Austria, for 18 months. During spring snowmelt, GPP peaked in 10 of our 12 study reaches, which appeared to be driven by PAR and catchment area. In contrast, the winter precipitation shift from snow to rain following the low-snow winter in 2013/2014 increased spring ER in upper elevation catchments, causing spring NEP to shift from autotrophy to heterotrophy. Our findings suggest that the YRN transitioned from a transient sink to a source of carbon dioxide (CO2) in spring as snowmelt hydrology differed following the high-snow versus low-snow winter. This shift toward increased heterotrophy during spring snowmelt following a warm winter has potential consequences for annual ecosystem metabolism, as spring GPP contributed on average 33% to annual GPP fluxes compared to spring ER, which averaged 21% of annual ER fluxes. We propose that Alpine headwaters will emit more within-stream respiratory CO2 to the atmosphere while providing less autochthonous organic energy to downstream ecosystems as the climate gets warmer

Forestry impact on water quality: a landscape perspective on dissolved organic carbon

Dissolved organic carbon (DOC) is a fundamental variable defining boreal stream ecosystems. In this thesis the impact of forestry practices that are commonly performed in the boreal regions of Scandinavia for stream water quality were evaluated. The thesis is based on combining the use of primary data from the Balsjö paired catchment experiment in northern Sweden with various modeling approaches. Final-felling strongly increased DOC concentrations in boreal first-order streams during the first four years after harvest. Median concentrations increased by 3.0 mg/L after clear-cutting and 6.2 mg/L after site preparation with concentrations being 5-24 mg/L higher in the clear-cut than in the reference catchment during summer storms. Clear-cutting also increased the riverine carbon (C) export significantly from 95 kg C ha⁻¹ yr⁻¹ to 183 kg C ha⁻¹ yr⁻¹ and to 280 kg C ha⁻¹ yr⁻¹ during pre-treatment, clear-cut and site-preparation periods, respectively. This export represents an important part of the C-balance of a forest in the region. Hydrological effects of clear-cutting included increased snow accumulation by 29 mm (27%) and a modified spring snowmelt. However, the largest effect on the water balance (~189 mm = 31%) was found during summer, when stream runoff was increased due to reduction in evapotranspiration. The drivers of the increased DOC concentrations were identified as changing flow-pathways in riparian soils activating more surficial, DOC rich soil layers, as well as increased soil temperatures that enhanced the DOC availability in riparian soils and therefore increased DOC mobilization from clear-cuts during the summer. In a final step, the impact of these increased, clear-cut induced DOC inputs into a larger scale boreal stream network were investigated by using a mixing model approach. DOC inputs were transferred to downstream sites, which resulted in increases in DOC concentrations at these locations. Further, the modeling approach showed that increases in DOC concentrations can be statistically detected, if the total area harvested within the stream network exceeds threshold values of 11% (p>0.05) and 23-25% (p<0.001) of the catchment area. Thus, this thesis suggests that threshold values for the maximum percentage of harvested area within a river basin should be implemented into forest planning for boreal catchments that are sensitive to changes in DOC concentrations

Hydrology controls dissolved organic matter export and composition in an Alpine stream and its hyporheic zone

Streams and rivers transport dissolved organic matter (DOM) from the terrestrial environment to downstream ecosystems. In light of climate and global change it is crucial to understand the temporal dynamics of DOM concentration and composition, and its export fluxes from headwaters to larger downstream ecosystems. We monitored DOM concentration and composition based on a diurnal sampling design for 3 years in an Alpine headwater stream. We found hydrologic variability to control DOM composition and the coupling of DOM dynamics in the streamwater and the hyporheic zone. High-flow events increased DOM inputs from terrestrial sources (as indicated by the contributions of humic- and fulvic-like fluorescence), while summer baseflow enhanced the autochthonous imprint of DOM. Diurnal and seasonal patterns of DOM composition were likely induced by biological processes linked to temperature and photosynthetic active radiation (PAR). Floods frequently interrupted diurnal and seasonal patterns of DOM, which led to a decoupling of streamwater and hyporheic water DOM composition and delivery of aromatic and humic-like DOM to the streamwater. Accordingly, DOM export fluxes were largely of terrigenous origin as indicated by optical properties. Our study highlights the relevance of hydrologic and seasonal dynamics for the origin, composition and fluxes of DOM in an Alpine headwater stream

Gravel bars are sites of increased CO2 outgassing in stream corridors

Streams are significant sources of CO2 to the atmosphere. Estimates of CO2 evasion fluxes (fCO2) from streams typically relate to the free flowing water but exclude geomorphological structures within the stream corridor. We found that gravel bars (GBs) are important sources of CO2 to the atmosphere, with on average more than twice as high fCO2 as those from the streamwater, affecting fCO2 at the level of entire headwater networks. Vertical temperature gradients resulting from the interplay between advective heat transfer and mixing with groundwater within GBs explained the observed variation in fCO2 from the GBs reasonably well. We propose that increased temperatures and their gradients within GBs exposed to solar radiation stimulate heterotrophic metabolism therein and facilitate the venting of CO2 from external sources (e.g. downwelling streamwater, groundwater) within GBs. Our study shows that GB fCO2 increased f CO2 from stream corridors by [median, (95% confidence interval)] 16.69%, (15.85–18.49%); 30.44%, (30.40–34.68%) and 2.92%, (2.90–3.0%), for 3rd, 4th and 5th order streams, respectively. These findings shed new light on regional estimates of fCO2 from streams, and are relevant given that streamwater thermal regimes change owing to global warming and human alteration of stream corridors

Sources and variability of CO 2 in a pre‐alpine stream gravel bar

Gravel bars (GBs) contribute to carbon dioxide (CO2) emissions from stream corridors, with CO2 concentrations and emissions dependent on prevailing hydraulic, biochemical, and physicochemical onditions. We investigated CO2 concentrations and fluxes across a GB in a prealpine stream over three different ischarge‐temperature conditions. By combining field data with a reactive transport groundwater model, we were able to differentiate the most relevant ydrological and biogeochemical processes contributing to CO2 dynamics. GB CO2 concentrations showed significant spatial and temporal variability and were highest under the lowest flow and highest temperature conditions. Further, observed GB surface CO2 evasion fluxes, measured CO2 concentrations, and modelled aerobic respiration were highest at the tail of the GB over all conditions. Modelled CO2 transport via streamwater downwelling contributed the largest fraction of the measured GB CO2 concentrations (31% to 48%). This contribution increased its relative share at higher discharges as a result of a decrease in other sources. Also, it decreased from the GB head to tail across all dischargetemperature conditions. Aerobic respiration accounted for 17% to 36% of measured surface CO2 concentrations. Zoobenthic respiration was estimated to contribute between 4% and 8%, and direct groundwater CO2 inputs 1% to 23%. Unexplained residuals accounted for 6% to 37% of the observed CO2 concentrations at the GB surface. Overall, we highlight the dynamic role of subsurface aerobic respiration as a driver of spatial and temporal variability of CO2 concentrations and evasion fluxes from a GB. As hydrological regimes in prealpine streams are predicted to change following climatic change, we propose that warming temperatures combined with extended periods of low flow will lead to increased CO2 release via enhanced aerobic respiration in newly exposed GBs in prealpine stream corridors