# Evaporation-driven convective flows in suspensions of non-motile bacteria

## TL;DR

This study uncovers a novel evaporation-driven convection in bacterial suspensions caused by salt accumulation at the air-water interface, independent of bacterial motility, supported by experiments and mathematical modeling.

## Contribution

It demonstrates that salt accumulation from evaporation can induce convection in non-motile bacteria, a mechanism previously unrecognized in biological systems.

## Key findings

- Salt accumulation drives convection patterns.
- Motility is not necessary for observed flows.
- Mathematical model supports the evaporation-driven mechanism.

## Abstract

We report a novel form of convection in suspensions of the bioluminiscent marine bacterium $Photobacterium~phosphoreum$. Suspensions of these bacteria placed in a chamber open to the air create persistent luminiscent plumes most easily visible when observed in the dark. These flows are strikingly similar to the classical bioconvection pattern of aerotactic swimming bacteria, which create an unstable stratification by swimming upwards to an air-water interface, but they are a puzzle since the strain of $P.~phosphoreum$ used does not express flagella and therefore cannot swim. Systematic experimentation with suspensions of microspheres reveals that these flow patterns are driven not by the bacteria but by the accumulation of salt at the air-water interface due to evaporation of the culture medium; even at room temperature and humidity, and physiologically relevant salt concentrations, the rate of water evaporation is sufficient to drive convection patterns. A mathematical model is developed to understand the mechanism of plume formation, and linear stability analysis as well as numerical simulations were carried out to support the conclusions. While evaporation-driven convection has not been discussed extensively in the context of biological systems, these results suggest that the phenomenon may be relevant in other systems, particularly those using microorganisms of limited motility.