  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.
