Exploring flow-biofilm-sediment interactions: Opportunities and challenges
Authors: Gerbersdorf Sabine , Koca Kaan , deBeer Dirk , Chennu Arjun , Noss Christian , Risse-Buhl Ute , Weitere Markus , Eiff Olivier , Wagner Michael , Aberle Jochen , Schweikert Michael , Kristina Terheiden 
Published: July 2020

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&lt;p&gt;&lt;span lang=&quot;EN-GB&quot;&gt;The feedback mechanisms within the &amp;#8220;triangle&amp;#8221; flow-biofilm-sediment are crucial for a variety of important ecosystem functions within the aquatic habitat. With great progress made in measurement techniques in fluid mechanics as well as microbiology, but also in biomechanical understanding, it is now possible and timely to address &amp;#8220;old questions&amp;#8221; with new approaches within this triangle research. Nevertheless, setting new high standards in spatial resolution (e.g., measuring small scales relevant to biofilm) or addressing natural conditions (e.g., generating controlled, but fully turbulent flow conditions) brings new challenges.&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span lang=&quot;EN-GB&quot;&gt;To address these issues, a joint workshop was held in Stuttgart, Germany between June 2018 and February 2019, bringing experts from Germany in these key experimental areas together. The main goals of the workshop were i) to consolidate knowledge and identify knowledge gaps in understanding flow-biofilm-sediment interactions and ii) to perform a pilot experiment to demonstrate how the identified knowledge gaps can be addressed by co-application of cutting-edge methods in the fields of hydraulics, sedimentation engineering, microbial ecology and biochemistry. &lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span lang=&quot;EN-GB&quot;&gt;In the pilot experiment, we applied two different Particle Image Velocimetry (PIV) systems to measure flow field in the vicinity of the biofilm at contrasting flow conditions. Surface roughness of the biofilm was quantified through measurements by laser triangulation system and digital microscopy. This combination allowed showing that flow speed affects time of settlement, growth direction and subsequent topography of biofilm. In turn, the biofilm was found to impact flow pattern leading to significant increase in Reynolds stress. With varying flow regimes, we also observed significant shifts in species composition and diversity at both prokaryotic and eukaryotic level by using modern molecular techniques. The molecular microbiological analyses coupled with biochemical characterization of biofilm suggest that quantities of polymeric substances play a minor role in explaining the mechanisms of attachment and binding. Furthermore, microalgae appeared to play a dominant role in biostabilizing the sediment, with more or less impact according to the particular groups or species involved. Thus, the possibility to monitor by hyperspectral imaging the density, composition and distribution of phototrophic biofilms creates a strong predictor for sediment stability and metabolic activity. Microsensors provided measurements of metabolic activity (photosynthesis, respiration) and allowed for novel observations of potential prey-predator relation. Microsensor measurements could be used together with PIV, hyperspectral imaging and optical coherence tomography (OCT) measurements in analysing the external and internal mass transfer by combining substrate distribution, flow dynamics and biofilm structure. Bringing these tools together for improved understanding of flow-biofilm-sediment interactions and ecosystem functions would be a great research concept for hypothesis building. &lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span lang=&quot;EN-GB&quot;&gt;In reference to the workshop results above, we&lt;/span&gt;&lt;span lang=&quot;EN-US&quot;&gt; present a timely update on the current knowledge, research approaches and methodological progress that have been witnessed over the last years, especially relevant to biofilm, hydraulics and fine sediment dynamics. &lt;/span&gt;&lt;span lang=&quot;EN-US&quot;&gt;The presentation will be concluded by highlighting the important forefront research lines to be answered within the forthcoming years in the flow-biofilm-sediment triangle.&lt;/span&gt;&lt;/p&gt;