Bacteria‐induced mixing in natural waters

Abstract

Swimming organisms can enhance mixing in their natural environments by creating eddies in their wake and by dragging water along. However, these mixing mechanisms are inefficient for microorganisms, because swimming‐induced variations in velocity, temperature, and dissolved substances are evened out before they can be advected. In bioconvection, however, microorganisms induce water movement not by propulsion directly but by locally changing the fluid density, which drives convection. Observations of bioconvection have so far mainly been limited to laboratory settings. We report the first observation and quantification of bioconvection within a stratified natural water body. Using in situ measurements, laboratory experiments, and numerical simulations, we demonstrate that the bacterium Chromatium okenii is capable of mixing 0.3 to 1.2 m thick water layers at around 12 m water depth in the Alpine Lake Cadagno (Switzerland). As many species are capable of driving bioconvection, this phenomenon potentially plays a role in species distributions and influences large‐scale phenomena like algal blooms.

Dark aerobic sulfide oxidation by anoxygenic phototrophs in anoxic waters

“…Conservative properties such as temperature and conductivity were constant throughout the chemocline in all years sampled (Supporting Information Fig. S1 and 2) indicating mixing of this zone (Sommer et al, ). Flat conductivity profiles revealed stronger mixing of the chemocline in 2013 and 2014 (Supporting Information Fig.…”

“…No transient response was observed right at the start of the imaging, and the swimming velocity remained steady throughout the duration of the measurements. This is in contrast to swimming behaviour at higher light intensities where the swimming cells exhibited a positive phototactic response (Sommer et al ., ). We could therefore rule out a light‐induced effect on motility at the minimum illumination level used for our measurements.…”

Convection‐Diffusion Competition Within Mixed Layers of Stratified Natural Waters

“…Bioconvection is a process in which dense and motile organisms change the density of their fluid environment triggering hydrodynamic instabilities. Initially observed in laboratory experiments, this phenomenon was recently detected in natural waters by Sommer et al (2017) who describe a high concentration of dense and motile bacteria inducing formation of MLs in Lake Cadagno, Switzerland (Figure 1). The bioconvective MLs of Lake Cadagno are an exemplary case for studying convection-diffusion competition because wind-induced shearing is weak in the interior ( Figure S2).…”

“…Data from this field campaign was also presented in Sommer et al (). Although the analysis is similar, the results are independent and according to the methods presented herein.…”

“…These can induce formation of almost homogeneous mixed layers (hereafter referred to as MLs). Typical examples include double-diffusive convection (Huppert & Turner, 1972;Sommer, Carpenter, Schmid, Lueck, Schurter, & Wüest, 2013), radiatively driven under-ice convection (Bouffard et al, 2016;Kirillin et al, 2012;Yang et al, 2017), thermobaric convection (Crawford & Collier, 1997;Schmid et al, 2008), and bioconvection (Sommer et al, 2017). Yet, large-scale flows of surrounding stratified waters generate shear-induced turbulence (Bouffard et al, 2012;Saggio & Imberger, 2001) usually quantified by eddy diffusion, which can smooth out those MLs ( Figure S1 in the supporting information).…”