Active sinking particles: sessile suspension feeders significantly alter the flow and transport to sinking aggregates

Abstract

Sinking or sedimentation of biological aggregates plays a critical role in carbon sequestration in the ocean and in vertical material fluxes in wastewater treatment plants. In both these contexts, the sinking aggregates are ‘active’, since they are biological hot-spots and are densely colonized by microorganisms including bacteria and sessile protists, some of which generate feeding currents. However, the effect of these feeding currents on the sinking rates, trajectories and mass transfer to these ‘active sinking particles’ has not previously been studied. Here, we use a novel scale-free vertical tracking microscope (a.k.a. gravity machine; Krishnamurthy et al. 2020 Nat. Methods 17 , 1040–1051 ( doi:10.1038/s41592-020-0924-7 )) to follow model sinking aggregates (agar spheres) with attached protists ( Vorticella convallaria ), sinking over long distances while simultaneously measuring local flows. We find that activity due to attached V. convallaria causes significant changes to the flow around aggregates in a dynamic manner and reshapes mass transport boundary layers. Further, we find that activity-mediated local flows along with sinking modify the encounter and plume cross-sections of the aggregate and induce sustained aggregate rotations. Overall, our work shows the important role of biological activity in shaping the near-field flows around aggregates with potentially important effects on aggregate fate and material fluxes.

Repulsion from slow-diffusing nutrients improves chemotaxis towards moving sources

“…Meanwhile, a microbe starting further away and approaching from the side experiences a strain-induced torque towards vertically upwards or downwards 36 that makes it crucial to be pointing sufficiently downwards upon entering the relevant region – which the earlier downturn of the differential strategy would provide. That sinking particles are likely irregular non-spherical shapes with both solid and mucus components should also be considered 43,44 . Detailed calculations of differential and purely attractive strategies with fluid flows and their effects on microbe trajectories should be performed by future works, but it appears likely the benefits of differential chemotaxis will be robust to these corrections.…”

“…That sinking particles are likely irregular non-spherical shapes with both solid and mucus components should also be considered 43,44 . Detailed calculations of differential and purely attractive strategies with fluid flows and their effects on microbe trajectories should be performed by future works, but it appears likely the benefits of differential chemotaxis will be robust to these corrections.…”

Orthogonal-view microscope for the biomechanics investigations of aquatic organisms

“…Among the existing DIY microscopes, the scale-free tracking system [19] , [20] is an elegant approach for providing a free path for the movement of organisms in one direction. However, conventional chambers, micro, and millifluidics can still serve as valuable tools for many investigations.…”

“…Moreover, researchers have successfully produced 3D-printed components enabling imaging and control of model organisms (e.g., temperature) [14] , as well as microscopes tailored for controlling optofluidic applications [7] , DIY incubator-contained microscope (i.e., Matchboxscope [15] ), and a low-cost automated scanning microscope using the mechanical components of a 3D printer (for example, EnderScope [16] ). In the study of marine organisms, PlanktoScope [17] , [18] and the scale-free tracking microscope [19] , [20] are of particular interest. Additionally, the use of mobile phones in conjunction with DIY hardware has been successful [21] , [22] , [23] , [24] .…”

Feeding Rates in Sessile versus Motile Ciliates are Hydrodynamically Equivalent

“…The "swim" or "stay" strategies shape material transport through this essential link in aquatic trophic systems, thus affecting not only the fitness of these microorganisms, [8][9][10][11] but also impacting global biogeochemical cycles and the food web chain [12][13][14][15]. Therefore, understanding the flow physics underlying the exchange of nutrients and wastes at this scale is important across disparate fields of the life sciences, from evolutionary biology [16,17] to ecosystem ecology [18,19].…”