A high-throughput integrated biofilm-on-a-chip platform for the investigation of combinatory physicochemical responses to chemical and fluid shear stress

Influence of Hydrodynamic Forces on Electroactive Bacterial Adhesion in Microbial Fuel Cell Anodes

This investigation examined the role of shear stress on the dynamic development of microbial communities within anodic biofilms in single-chamber microbial fuel cells (MFCs). Bacterial attachment to surfaces, often regarded as a crucial step in biofilm formation, may significantly contribute to the selection of electroactive bacteria (EAB). It is well established that hydrodynamic forces, particularly shear forces, have a profound influence on bacterial adhesion. This study postulates that shear stress could select EAB on the anode during the adhesion phase by detaching non-EAB. To examine this hypothesis, MFC reactors equipped with a shear stress chamber were constructed, creating specific shear stress on the anode. The progression of adhesion under various shear stress conditions (1, 10, and 50 mPa) was compared with a control MFC lacking shear stress. The structure of the microbial community was assessed using 16S rRNA gene (rrs) sequencing, and the percentage of biofilm coverage was analyzed using fluorescence microscopy. The results indicate a significant impact of shear stress on the relative abundance of specific EAB, such as Geobacter, which was higher (up to 30%) under high shear stress than under low shear stress (1%). Furthermore, it was noted that shear stress decreased the percentage of biofilm coverage on the anodic surface, suggesting that the increase in the relative abundance of specific EAB occurs through the detachment of other bacteria. These results offer insights into bacterial competition during biofilm formation and propose that shear stress could be utilized to select specific EAB to enhance the electroactivity of anodic biofilms. However, additional investigations are warranted to further explore the effects of shear stress on mature biofilms.

A microscale system for in situ investigation of immobilized microalgal cell resistance against liquid flow in the early inoculation stage

In attached microalgae cultivation systems, cell detachment due to fluid hydrodynamic flow is not a subject matter that is commonly looked into. However, this phenomenon is of great relevance to optimizing the operating parameters of algae cultivation and feasible reactor design. Hence, this current work miniaturizes traditional benchtop assays into a microfluidic platform to study the cell detachment of green microalgae, Chlorella vulgaris, from porous substrates during its early cultivation stage under precisely controlled conditions. As revealed by time lapse microscopy, an increase in bulk flow velocity facilitated nutrient transport but also triggered cell detachment events. At a flow rate of 1000 μL min-1 of growth medium for 120 min, the algal cell coverage was up to 5% lower than those at 5 μL min-1 and 50 μL min-1. In static seeding, the evolution of attached cell resistance toward liquid flows was dependent on hydrodynamic zones. The center zone of the microchannel was shown to be a "comfortable zone" of the attached cells to sequester nutrients effectively at lower medium flow rates but there was a profile transition where outlet zones favored cell attachment the most at higher flow rates (1.13 times higher than the center zone for 1000 μL min-1). Besides, computational fluid dynamics (CFD) simulations illustrated that the focusing band varied between cross-sections and depths, while the streamline was the least concentrated along the side walls and bottom plane of the microfluidic devices. It was intriguing to learn that cell detachment was not primarily happening along the symmetry streamline. Insight gained from this study could be further applied in the optimization of operating conditions of attached cultivation systems whilst preserving laminar flow conditions.