Encounter rates between bacteria and small sinking particles

Abstract

The ecological interaction between bacteria and sinking particles, such as bacterial degradation of marine snow particles, is regulated by their encounters. Current encounter models focus on the diffusive regime, valid for particles larger than the bacterial run length, yet the majority of marine snow particles are small, and the encounter process is then ballistic. Here, we analytically and numerically quantify the encounter rate between sinking particles and non-motile or motile microorganisms in the ballistic regime, explicitly accounting for the hydrodynamic shear created by the particle and its coupling with micro-organism shape. We complement results with selected experiments on non-motile diatoms. The shape-shear coupling has a considerable effect on the encounter rate and encounter location through the mechanisms of hydrodynamic focusing and screening, whereby elongated micro-organisms preferentially orient normally to the particle surface downstream of the particle (focusing) and tangentially to the surface upstream of the particle (screening). Non-motile elongated micro-organisms are screened from sinking particles because shear aligns them tangentially to the particle surface, which reduces the encounter rate by a factor proportional to the square of the micro-organism aspect ratio. For motile elongated micro-organisms, hydrodynamic focusing increases the encounter rate when particle sinking speed is similar to microorganism swimming speed, whereas for very quickly sinking particles hydrodynamic screening can reduce the encounter rate below that of non-motile micro-organisms. For natural ocean conditions, we connect the ballistic and diffusive limits and compute the encounter rate as a function of shape, motility and particle characteristics. Our results indicate that shear should be taken into account to predict the interactions between bacteria and sinking particles responsible for the large carbon flux in the ocean's biological pump.

Sea-ice retreat may decrease carbon export and vertical microbial connectivity in the Eurasian Arctic basins

“…Alternatively, the relatively long sinking time of days to weeks could have led to PA microbial communities at depth containing an imprint of surface water's microbial communities that were no longer present or horizontally offset during our sampling. Overall, we conclude from our data that the observed vertical changes in the PA communities are substantially affected by sinking speed, causing different encounter rates and colonization in the surface ocean 46 , and differences in time for ecological succession within the particles 47 . The succession can result from transformations in the aggregate composition during aging and turnover 48 , grazing by protozoans 49 , viral infection 50 , or changing environmental conditions, such as increasing hydrostatic pressure 51,52 .…”

The influence of motility on bacterial accumulation in a microporous channel

“…They identified the shear induced reorientation as the mechanism allowing the bacteria to accumulate in these specific regions. Using a similar approach, Słomka et al 52 came to the conclusion Fig. 9 Transition rates for the from-obstacle-to-obstacle K o,o and for the from-wall-to-obstacle rate K w,o as a function of the (average) running duration T r .…”

Environmental fluctuations and their effects on microbial communities, populations and individuals

“…This heterogeneous distribution of nutrient among a population arises from the spatial heterogeneity in how bacteria and nutrient are distributed and can be further accentuated by heterogeneities in cell shape and fluid flow (Słomka et al . 2020 ). Thus, the heterogeneous distributions of environmental factors produce striking differences in the temporal environment experienced between individuals within a microbial population.…”