Colonization dynamics and grazing activity of ciliates in stream biofilms

Cross-barriers such as small dams cause local flow velocity heterogeneities, which might affect the formation, structure, and function of biofilms. The main objective of this thesis was to investigate the impact of flow velocity on colonization dynamics, grazing activity, and behavioral changes of biofilm associated ciliates. In addition, the effect of protists grazing activity on spatial biofilm morphology was examined at slow flow velocities. Surfaces were rapidly colonized by ciliates at a slow flowing reservoir of the third order Ilm stream (Thuringia, Germany) and in slow flowing flow channels. Initial stream biofilms inhabited all functional groups of ciliates irrespective of flow velocity implementing all trophic links within the microbial loop. The low abundance of sessile filter feeders at faster flow velocities suggested that the attachment of these cells was inhibited. Grazing activity of a previously attached filter feeder seemed to be inhibited at faster flow velocities since cells remained about 45% of the observed time in a contracted state. Vagile flattened gulper feeder tolerated faster flow velocities and responded with a positive rheotactic movement. Grazing activity and motility of ciliates altered spatial biofilm morphology. Mushroom shaped microcolonies, a higher porosity, and a higher biofilm surface area to volume ratio indicated that nutrient exchange between biofilm and the surrounding fluid was improved and bacterial growth might be accelerated. With respect to restoration ecology the data indicated that flow velocity differences at small low-head dams increased the diversity of biofilm associated ciliates, which might contribute to enhanced ecosystem resilience

Streambed migration frequency drives ecology and biogeochemistry across spatial scales

The bed of fluvial ecosystems plays a major role in global biogeochemical cycles. All fluvial sediments migrate and although responses of aquatic organisms to such movements have been recorded there is no theoretical framework on how the frequency of sediment movement affects streambed ecology and biogeochemistry. We here developed a theoretical framework describing how the moving-resting frequencies of fine-grained sediments constrain streambed communities across spatial scales. Specifically, we suggest that the most drastic impact on benthic and hyporheic communities will exist when ecological and biogeochemical processes are at the same temporal scale as the sediment moving-resting frequency. Moreover, we propose that the simultaneous occurrence of streambed patches differing in morphodynamics should be considered as an important driver of metacommunity dynamics. We surmise that the frequency of patch transition will add new dimensions to the understanding of biogeochemical cycling and metacommunities from micro-habitat to segment scales. This theoretical framework is important for fluvial ecosystems with frequent sediment movement, yet it could be applied to any other dynamic habitat.German Research Foundation joint funding grant RI 2093/2-1 and MU 1464/7-1Carl Zeiss Foundation, P2021-00-004Israel Science Foundation, grant 682/17),NSF-BSF joint funding grant EAR-1734300UK-Israel Science Fellowship Scheme 2018–2019Israeli Science Foundation, grant 944\2

Diatoms Reduce Decomposition of and Fungal Abundance on Less Recalcitrant Leaf Litter via Negative Priming

Heterotrophic microbial decomposers colonize submerged leaf litter in close spatial proximity to periphytic algae that exude labile organic carbon during photosynthesis. These exudates are conjectured to affect microbial decomposers' abundance, resulting in a stimulated (positive priming) or reduced (negative priming) leaf litter decomposition. Yet, the occurrence, direction, and intensity of priming associated with leaf material of differing recalcitrance remains poorly tested. To assess priming, we submerged leaf litter of differing recalcitrance (Alnus glutinosa [alder; less recalcitrant] and Fagus sylvatica [beech; more recalcitrant]) in microcosms and quantified bacterial, fungal, and diatom abundance as well as leaf litter decomposition over 30 days in absence and presence of light. Diatoms did not affect beech decomposition but reduced alder decomposition by 20% and alder-associated fungal abundance by 40% in the treatments including all microbial groups and light, thus showing negative priming. These results suggest that alder-associated heterotrophs acquired energy from diatom exudates rather than from leaf litter. Moreover, it is suggested that these heterotrophs have channeled energy to alternative (reproductive) pathways that may modify energy and nutrient availability for the remaining food web and result in carbon pools protected from decomposition in light-exposed stream sections

Hydrodynamics Alter the Tolerance of Autotrophic Biofilm Communities Toward Herbicides

Multiple stressors pose potential risk to aquatic ecosystems and are the main reasons for failing ecological quality standards. However, mechanisms how multiple stressors act on aquatic community structure and functioning are poorly understood. This is especially true for two important stressors types, hydrodynamic alterations and toxicants. Here we perform a mesocosm experiment in hydraulic flumes connected as a bypass to a natural stream to test the interactive effects of both factors on natural (inoculated from streams water) biofilms. Biofilms, i.e., the community of autotrophic and heterotrophic microorganisms and their extracellular polymeric substances (EPS) in association with substratum, are key players in stream functioning. We hypothesized (i) that the tolerance of biofilms toward toxicants (the herbicide Prometryn) decreases with increasing hydraulic stress. As EPS is known as an absorber of chemicals, we hypothesize (ii) that the EPS to cell ratio correlates with both hydraulic stress and herbicide tolerance. Tolerance values were derived from concentration-response assays. Both, the herbicide tolerance and the biovolume of the EPS significantly correlated with the turbulent kinetic energy (TKE), while the diversity of diatoms (the dominant group within the stream biofilms) increased with flow velocity. This indicates that the positive effect of TKE on community tolerance was mediated by turbulence-induced changes in the EPS biovolume. This conclusion was supported by a second experiment, showing decreasing effects of the herbicide to a diatom biofilm (Nitzschia palea) with increasing content of artificial EPS. We conclude that increasing hydrodynamic forces in streams result in an increasing tolerance of microbial communities toward chemical pollution by changes in EPS-mediated bioavailability of toxicants

Molecular and Morphological Snapshot Characterisation of the Protist Communities in Contrasting Alpine Glacier Forefields

Phagotrophic protist diversity in oligotrophic soils such as alpine glacier forefields is still poorly studied. Combining morphologic observations with molecular-based analyses, we assessed the diversity of major phagotrophic protist groups in two contrasting glacier forefields in the Swiss Alps (Tiefen glacier forefield, siliceous bedrock, and Wildstrubel glacier forefield, calcareous bedrock), at sites differing in soil development. Ciliates and heterotrophic flagellates could be detected with both approaches, while amoebae could be observed only microscopically. Soils from Tiefen and Wildstrubel glacier forefields harboured distinctly different ciliate, flagellate and amoebae communities. The ciliate clone libraries from the Tiefen glacier forefield were dominated by Oligohymenophorea-related sequences while those from the Wildstrubel glacier forefield were dominated by Spirotrichea-related sequences. Testate amoebae morphospecies of the genera Corythion, Cryptodifflugia, Euglypha and Tracheleuglypha were restricted to the Tiefen glacier forefield, while Centropyxis and Trinema to the Wildstrubel one. No ciliate sequences and only a few ciliate and testate amoebae morphospecies could be retrieved from unvegetated soils of both glacier forefields. The ciliate and testate amoebae community detected at unvegetated sites were a subset of the community developed at vegetated sites. Overall, our results suggest that alpine glacier forefields are colonised by a diverse community of phagotrophic protists which seems to be shaped by bedrock geology and vegetation cover

Overlooked Implication of Sediment Transport at Low Flow: Migrating Ripples Modulate Streambed Phototrophic and Heterotrophic Microbial Activity

Sandy streambeds are mobile even at low flow velocities at which sediments can be transported as bedload, more specifically as migrating ripples. Small variations in discharge can result in transitions between sediment transport and no‐transport. Despite being inherent processes of streams and rivers, the effect of sediment transport and transport regime transition on the phototrophic and heterotrophic activity of streambed microbial communities remains unclear. We performed a microcosm experiment mimicking sediment transport as migrating ripples (i.e., migrating) and no sediment transport (i.e., stationary), and their transition to observe the response of the phototrophic and heterotrophic microbial community. Both net community production and community respiration were respectively 77% and 40% suppressed in migrating sediments compared to stationary sediments. In migrating sediments, a combination of mechanical stress, light limitation, and limited habitable area likely hampered microbial metabolism. Stationary conditions facilitated an active community of phototrophs, mainly diatoms, as indicated by high net community production, high rates of dissolved organic carbon release and silicon retention. After transitioning migrating to stationary and vice versa, differences were maintained regardless of the change in mechanical stress and associated stressors, most likely as a result of the interaction between their antecedent transport conditions and developmental stage that shaped the microbial community. Our results indicate that sediment transported as migrating ripples at low flow velocity can strongly modulate streambed metabolism, and discharge oscillations resulting in sediment transport transitions will result in a mosaic of microbial activity and biomass that will emerge at larger scales determining reach‐scale metabolism.Key Points: A microcosm approach was used to identify the effect of ripple migration on the metabolism and composition of microbial communities. Migrating ripples hampered the phototrophic and heterotrophic community respiration, net production, and composition. The influence of transition from (to) migrating ripple to (from) stationary sediment was modulated by community developmental stage.Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659Basque Country GovernmentDeutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/50110000165

Metabolism, nutrient dynamics and community composition in sandy sediments - A microcosms experiment

Sandy streambeds can be mobilized at base flow and sediments are transported as bedload, more specifically as migrating ripples. Within migrating ripples, microbial communities experience an erosion-resting cycle of sediment grains. Besides, small changes in discharge can results in frequent transitions between migrating ripples and no-transport. Despite the ubiquity of both migrating ripples and sediment transport transition, their effect on streambed functioning and microbial community composition remain unclear. We performed a microcosm experiment mimicking two sediment transport conditions, namely ripple and no transport (i.e., stable), and their transition to observe the response of sediment community function and composition. Both net community production (NCP) and community respiration (CR) were suppressed in ripple sediments compared to stable sediments. In ripples, a combination of mechanic stress, advective supply and light limitation likely hampered microbial metabolism. Sediment stability likely facilitated an active community of autotrophs, mainly diatoms, as indicated by high NCP, high rates of DOC release and Si-SiO2 retention. Retention of nitrate and the high DIN : SRP ratio indicated efficient resource utilization in stable sediments. After the transition, microbial communities from each treatment responded differently to sediment transport, most likely as a result of the interaction between their previous environmental conditions and functional status in response to the new conditions. Our data indicate that sediment transport in the form of migrating ripples at low flow can strongly modulate streambed metabolism, and discharge oscillations (transitions) will result in a mosaic of metabolism and communities that will emerge at larger scales determining reach scale metabolism