Metabolic activity controls the emergence of coherent flows in microbial suspensions

TL;DR

This study shows how metabolic activity, controlled by light, influences collective microbial movement and flow patterns, revealing reversible transitions between stable and convective states in microbial suspensions.

Contribution

It uncovers the link between photosynthetic activity and collective flow emergence, demonstrating reversible control of microbial motility and flow structures via light intensity.

Findings

High light induces reverse sedimentation of microbes.

Decreased photosynthetic activity leads to bioconvective flows.

Flow patterns are fully reversible with light adjustments.

Abstract

Photosynthetic microbes have evolved and successfully adapted to the ever-changing environmental conditions in complex microhabitats throughout almost all ecosystems on Earth. In the absence of light, they can sustain their biological functionalities through aerobic respiration, and even in anoxic conditions through anaerobic metabolic activity. For a suspension of photosynthetic microbes in an anaerobic environment, individual cellular motility is directly controlled by its photosynthetic activity, i.e. the intensity of the incident light absorbed by chlorophyll. The effects of the metabolic activity on the collective motility on the population level, however, remain elusive so far. Here, we demonstrate that at high light intensities, a suspension of photosynthetically active microbes exhibits a stable reverse sedimentation profile of the cell density due to the microbes' natural bias to move against gravity. With decreasing photosynthetic activity, and therefore suppressed individual motility, the living suspension becomes unstable giving rise to coherent bioconvective flows. The collective motility is fully reversible and manifests as regular, three-dimensional plume structures, in which flow rates and cell distributions are directly controlled via the light intensity. The coherent flows emerge in the highly unfavourable condition of lacking both light and oxygen and, thus, might help the microbial collective to expand the exploration of their natural habitat in search for better survival conditions.