Anthropogenic stressors affect fungal more than bacterial communities in decaying leaf litter: A stream mesocosm experiment

Multiple stressors affecting microbial decomposer and litter decomposition in restored urban streams: Assessing effects of salinization, increased temperature, and reduced flow velocity in a field mesocosm experiment

A multitude of anthropogenic stressors impact biological communities and ecosystem processes in urban streams. Prominent among them are salinization, increased temperature, and altered flow regimes, all of which can affect microbial decomposer communities and litter decomposition, a fundamental ecosystem process in streams. Impairments caused by these stressors individually or in combination and recovery of communities and ecosystem processes after release from these stressors are not well understood. To improve our understanding of multiple stressors impacts we performed an outdoor stream mesocosm experiment with 64 experimental units to assess the response of microbial litter decomposers and decomposition. The three stressors we applied in a full-factorial design were increased salinity (NaCl addition, 0.53 mS cm-1 above ambient), elevated temperature (3.5 °C above ambient), and reduced flow velocity (3.5 vs 14.2 cm s-1). After two weeks of stressor exposure (first sampling) and two subsequent weeks of recovery (second sampling), we determined leaf-associated microbial respiration, fungal biomass, and the sporulation activity and community composition of aquatic hyphomycetes in addition to decomposition rates of black alder (Alnus glutinosa) leaves confined in fine-mesh litter bags. Microbial colonization of the litter was accompanied by significant mass loss in all mesocosms. However, there was little indication that mass loss, microbial respiration, fungal biomass, sporulation rate or community composition of aquatic hyphomycetes was strongly affected by either single stressors or their interactions. Two exceptions were temperature effects on sporulation and decomposition rate. Similarly, no notable differences among mesocosms were observed after the recovery phase. These results suggest that microbial decomposers and leaf litter decomposition are either barely impaired by exposure to the tested stressors at the levels applied in our experiment, or that communities in restored urban streams are well adapted to cope with these stressor levels.

Effects of flow reduction and artificial light at night (ALAN) on litter decomposition and invertebrate communities in streams: A flume experiment

River ecosystems are heavily impacted by multiple stressors, where effects can cascade downstream of point sources. However, a spatial approach to assess the effects of multiple stressors is missing. We assessed the local and downstream effects on litter decomposition, and associated invertebrate communities of two stressors: flow reduction and artificial light at night (ALAN). We used an 18-flow-through mesocosm system consisting of two tributaries, where we applied the stressors, merging in a downstream section. We assessed the changes in decomposition rate and invertebrate community structure in leaf bags. We found no effect of ALAN or its interaction with flow reduction on the litter decomposition or the invertebrate community in the tributaries. Flow reduction alone led to a 14.8 % reduction in decomposition rate in the tributaries. We recorded no effect of flow reduction on the macroinvertebrates community composition in the litter bags. We also observed no effects of the spatial arrangement of the stressors on the litter decomposition and macroinvertebrate community structure downstream. Overall, our results suggest the impact of stressors on litter decomposition and macroinvertebrate communities remained local in our experiment. Our work thus calls for further studies to identify the mechanisms and the conditions under which spatial effects dominate over local processes.

Hidden Decomposers: the Role of Bacteria and Fungi in Recently Intermittent Alpine Streams Heterotrophic Pathways

The frequency of flow intermittency and drying events in Alpine rivers is expected to increase due to climate change. These events can have significant consequences for stream ecological communities, though the effects of reduced flow conditions on microbial communities of decomposing allochthonous leaf material require additional research. In this study, we investigated the bacterial and fungal communities associated with the decomposition of two common species of leaf litter, chestnut (Castanea sativa), and oak (Quercus robur). A sampling of experimentally placed leaf bags occurred over six collection dates (up to 126 days after placement) at seven stream sites in the Western Italian Alps with historically different flow conditions. Leaf-associated bacterial and fungal communities were identified using amplicon-based, high-throughput sequencing. Chestnut and oak leaf material harbored distinct bacterial and fungal communities, with a number of taxonomic groups differing in abundance, though bacterial community structure converged later in decomposition. Historical flow conditions (intermittent vs perennial rivers) and observed conditions (normal flow, low flow, ongoing drying event) had weaker effects on bacterial and fungal communities compared to leaf type and collection date (i.e., length of decomposition). Our findings highlight the importance of leaf characteristics (e.g., C:N ratios, recalcitrance) to the in-stream conditioning of leaf litter and a need for additional investigations of drying events in Alpine streams. This study provides new information on the microbial role in leaf litter decomposition with expected flow changes associated with a global change scenario.

Meta-analysis of three-stressor combinations on population-level fitness reveal substantial higher-order interactions

Although natural populations are typically subjected to multiple stressors, most past research has focused on single-stressor and two-stressor interactions, with little attention paid to higher-order interactions among three or more stressors. However, higher-order interactions increasingly appear to be widespread. Consequently, we used a recently introduced and improved framework to re-analyze higher-order ecological interactions. We conducted a literature review of the last 100 years (1920-2020) and reanalyzed 142 ecological three-stressor interactions on species' populations from 38 published papers; the vast majority of these studies were from the past 10 years. We found that 95.8 % (n = 136) of the three-stressor combinations had either not been categorized before or resulted in different interactions than previously reported. We also found substantial levels of emergent properties-interactions that are not due to strong pairwise interactions within the combination but rather uniquely due to all three stressors being combined. Calculating net interactions-the overall accounting for all possible interactions within a combination including the emergent and all pairwise interactions-we found that the most prevalent interaction type is antagonism, corresponding to a smaller than expected effect based on single stressor effects. In contrast, for emergent interactions, the most prevalent interaction type is synergistic, resulting in a larger than expected effect based on single stressor effects. Additionally, we found that hidden suppressive interactions-where a pairwise interaction is suppressed by a third stressor-are found in the majority of combinations (74 %). Collectively, understanding multiple stressor interactions through applying an appropriate framework is crucial for answering fundamental questions in ecology and has implications for conservation biology and population management. Crucially, identifying emergent properties can reveal hidden suppressive interactions that could be particularly important for the ecological management of at-risk populations.

Integrated Ecohydrological Models in Aquatic Ecosystems

As a critical component of the global environment, aquatic ecosystems support a wide range of organisms, including bacteria, fungi, algae, invertebrates, plants, and fish [...]

Rated-M for mesocosm: allowing the multimodal analysis of mature root systems in 3D

A plants' water and nutrients are primarily absorbed through roots, which in a natural setting is highly dependent on the 3-dimensional configuration of the root system, collectively known as root system architecture (RSA). RSA is difficult to study due to a variety of factors, accordingly, an arsenal of methods have been developed to address the challenges of both growing root systems for imaging, and the imaging methods themselves, although there is no 'best' method as each has its own spectrum of trade-offs. Here, we describe several methods for plant growth or imaging. Then, we introduce the adaptation and integration of three complementary methods, root mesocosms, photogrammetry, and electrical resistance tomography (ERT). Mesocosms can allow for unconstrained root growth, excavation and preservation of 3-dimensional RSA, and modularity that facilitates the use of a variety of sensors. The recovered root system can be digitally reconstructed through photogrammetry, which is an inexpensive method requiring only an appropriate studio space and a digital camera. Lastly, we demonstrate how 3-dimensional water availability can be measured using ERT inside of root mesocosms.

Long-Term Fertilization History Alters Effects of Microplastics on Soil Properties, Microbial Communities, and Functions in Diverse Farmland Ecosystem

Microplastics (MPs) pollution has caused a threat to soil ecosystem diversity and functioning globally. Recently, an increasing number of studies have reported effects of MPs on soil ecosystems. However, these studies mainly focused on soil bacterial communities and a few limited functional genes, which is why MPs effects on soil ecosystems are still not fully understood. Fertilization treatment often coinsides with MPs exposure in practice. Here, we studied effects of an environmentally relevant concentration of polyethylene on soil properties, microbial communities, and functions under different soil types and fertilization history. Our results showed that 0.2% PE MPs exposure could affect soil pH, but this effect varied according to soil type and fertilization history. Long-term fertilization history could alter effects of MPs on soil bacterial and fungal communities in diverse farmland ecosystems (P < 0.05). Soil fungal communities are more sensitive to MPs than bacterial communities under 0.2% PE MPs exposure. MPs exposure has a greater impact on the soil ecosystem with a lower microbial diversity and functional genes abundance and increases the abundance of pathogenic microorganisms. These findings provided an integrated picture to aid our understanding of the impact of MPs on diverse farmland ecosystems with different fertilization histories.

Distinct fungal plastisphere across different river functional zones: A watershed scale study

Fungi not only play important roles in biogeochemical processes but also can form biofilm on plastic debris. However, knowledge of structure composition and spatiotemporal pattern of fungal plastisphere on different kinds of plastic debris in river with specific usages, known as river functional zones, is still missing. In this study, we investigated the spatial distribution of the fungal plastisphere across a complete urban river with different functional zones (drinking, farm irrigation, aquaculture, and tail lake). Our research was performed based on both field residual plastic debris collection and a 30-day field in situ incubation experiments. Our study revealed that plastic debris enriched distinct fungal communities (including pathogenic fungi) significantly different from the surrounding water. Tracking the source of the fungi colonized on plastic debris suggested that the fungal taxa colonized on the different kinds of plastic debris were not from the surrounding water. Human activities had considerable effects on the fungal community structure on plastic debris, and the plastisphere fungal community structure strikingly varied across different river functional zones. Plastisphere may be used as an indicator for fungi biogeography and pathogenic fungi pollution in river with different functional zones. These findings are essential for ecological risk assessment and management decisions for pollution control of plastic debris and maintaining ecological health.

Impacts of multiple anthropogenic stressors on stream macroinvertebrate community composition and functional diversity

Ensuring the provision of essential ecosystem services in systems affected by multiple stressors is a key challenge for theoretical and applied ecology. Trait-based approaches have increasingly been used in multiple-stressor research in freshwaters because they potentially provide a powerful method to explore the mechanisms underlying changes in populations and communities. Individual benthic macroinvertebrate traits associated with mobility, life history, morphology, and feeding habits are often used to determine how environmental drivers structure stream communities. However, to date multiple-stressor research on stream invertebrates has focused more on taxonomic than on functional metrics. We conducted a fully crossed, 4-factor experiment in 64 stream mesocosms fed by a pristine montane stream (21 days of colonization, 21 days of manipulations) and investigated the effects of nutrient enrichment, flow velocity reduction and sedimentation on invertebrate community, taxon, functional diversity and trait variables after 2 and 3 weeks of stressor exposure. 89% of the community structure metrics, 59% of the common taxa, 50% of functional diversity metrics, and 79% of functional traits responded to at least one stressor each. Deposited fine sediment and flow velocity reduction had the strongest impacts, affecting invertebrate abundances and diversity, and their effects translated into a reduction of functional redundancy. Stressor effects often varied between sampling occasions, further complicating the prediction of multiple-stressor effects on communities. Overall, our study suggests that future research combining community, trait, and functional diversity assessments can improve our understanding of multiple-stressor effects and their interactions in running waters.

Fine sediment and flow velocity impact bacterial community and functional profile more than nutrient enrichment

Freshwater ecosystems face many simultaneous pressures due to human activities. Consequently, there has been a rapid loss of freshwater biodiversity and an increase in biomonitoring programs. Our study assessed the potential of benthic stream bacterial communities as indicators of multiple-stressor impacts associated with urbanization and agricultural intensification. We conducted a fully crossed four-factor experiment in 64 flow-through mesocosms fed by a pristine montane stream (21 d of colonization, 21 d of manipulations) and investigated the effects of nutrient enrichment, flow-velocity reduction and added fine sediment after 2 and 3 weeks of stressor exposure. We used high-throughput sequencing and metabarcoding techniques (16S rRNA genes), as well as curated biological databases (METAGENassit, MetaCyc), to identify changes in bacterial relative abundances and predicted metabolic functional profile. Sediment addition and flow-velocity reduction were the most pervasive stressors. They both increased α-diversity and had strong taxon-specific effects on community composition and predicted functions. Sediment and flow velocity also interacted frequently, with 88% of all bacterial response variables showing two-way interactions and 33% showing three-way interactions including nutrient enrichment. Changes in relative abundances of common taxa were associated with shifts in dominant predicted functions, which can be extrapolated to underlaying stream-wide mechanisms such as carbon use and bacterial energy production pathways. Observed changes were largely stable over time and occurred after just 2 weeks of exposure, demonstrating that bacterial communities can be well-suited for early detection of multiple stressors. Overall, added sediment and reduced flow velocity impacted both bacterial community structure and predicted function more than nutrient enrichment. In future research and stream management, a holistic approach to studying multiple-stressor impacts should include multiple trophic levels with their functional responses, to enhance our mechanistic understanding of complex stressor effects and promote establishment of more efficient biomonitoring programs.

Evaluating ecosystem functioning following river restoration: the role of hydromorphology, bacteria, and macroinvertebrates

Ecological restoration of freshwater ecosystems is now being implemented to mitigate anthropogenic disruption. Most emphasis is placed on assessing physico-chemical and hydromorphological properties to monitor restoration progress. However, less is known about the structural integrity and ecosystem health of aquatic ecosystems. In particular, little is known about how ecosystem function changes following river habitat restoration, especially in China. Leaf litter decomposition can be used as an indicator of stream ecosystem integrity. Therefore, the leaf breakdown rate was measured to assess the ecosystem function of restored rivers. By comparing leaf breakdown rates in urban rivers undergoing habitat restoration with that in degraded urban rivers and rivers in forested areas (i.e., reference conditions), we aimed to determine: (i) how habitat restoration affected leaf litter decomposition? (ii) the relationship between leaf litter decomposition to both environmental (habitat and physico-chemical variables) and biological factors (benthic communities), and (iii) identify the factors that contribute most to the variance in leaf litter breakdown rates. The results demonstrated a significant increase in leaf breakdown rate (120% in summer and 28% in winter) in the restored rivers compared to the degraded rivers. All environmental and biotic factors evaluated contributed synergistically to the differences in leaf litter decomposition among the three river types. The role of macroinvertebrates, mainly shredders, appeared to be particularly important, contributing 52% (summer) and 33% (winter) to the variance in decomposition, followed by habitat characteristics (e.g. substrate diversity, water velocity; 17% in summer, 29% in winter), physico-chemical variables (e.g. nutrient and organic pollutants; 11% in summer, 1% in winter) and biofilm bacteria (0% in summer, 15% in winter). Habitat restoration positively affected the structure and function of the previously degraded streams. Knowledge on controlling variables and their attribution to changes of ecosystem functioning provides guidance to assist the future planning of ecological restoration strategies.