Dynamic in situ detection in iRhizo-Chip reveals diurnal fluctuations of <i>Bacillus subtilis</i> in the rhizosphere

Effective colonization by microbe in the rhizosphere is critical for establishing a beneficial symbiotic relationship with the host plant. <i>Bacillus subtilis</i>, a soil-dwelling bacterium that is commonly found in association with plants and their rhizosphere, has garnered interest for its potential to enhance plant growth, suppress pathogens, and contribute to sustainable agricultural practices. However, research on the dynamic distribution of <i>B. subtilis</i> within the rhizosphere and its interaction mechanisms with plant roots remains insufficient due to limitations in existing in situ detection methodologies. To achieve dynamic in situ detection of the rhizosphere environment, we established iRhizo-Chip, a microfluidics-based platform. Using this device to investigate microbial behavior within the rhizosphere, we found obvious diurnal fluctuations in the growth of <i>B. subtilis</i> in the rhizosphere. Temporal dynamic analysis of rhizosphere dissolved oxygen (DO), pH, dissolved organic carbon, and reactive oxygen species showed that diurnal fluctuations in the growth of <i>B. subtilis</i> are potentially related to a variety of environmental factors. Spatial dynamic analysis also showed that the spatial distribution changes of <i>B. subtilis</i> and DO and pH were similar. Subsequently, through in vitro control experiments, we proved that rhizosphere DO and pH are the main driving forces for diurnal fluctuations in the growth of <i>B. subtilis</i>. Our results show that the growth of <i>B. subtilis</i> is driven by rhizosphere DO and pH, resulting in diurnal fluctuations, and iRhizo-Chip is a valuable tool for studying plant rhizosphere dynamics.