Reactive
oxygen species (ROS) are ubiquitous on earth and drive
numerous redox-centered biogeochemical processes. The rhizosphere
of wetland plants is a highly dynamic interface for the exchange of
oxygen and electrons, presenting the basis of the precedent for ROS
production, yet whether extracellular ROS are produced in the rhizosphere
remains unknown. Here, we designed a microfluidic chip setup to detect in-situ ROS productions in the rhizosphere of rice with
spatial and temporal resolutions. Fluorescence imaging clearly displayed
the hot spots of ROS generation in the rhizosphere. The formation
concentration of the hydroxyl radical (•OH, a representative
ROS, 10–6 M) was comparable to those by the classical
photochemical route (10–6–10–7 M) in aquatic systems, therefore highlighting the rhizosphere as
an unrecognized hotspot for ROS production. Moreover, the rhizosphere
ROS production exhibits diel fluctuation, which simultaneously fluctuated
with dissolved oxygen, redox potential, and pH, all driven by radial
oxygen loss near the root in the daytime. The production and diel
fluctuation of ROS were confirmed in the rhizosphere of rice root
incubated in natural soils. We demonstrated that the extracellular
ROS production was triggered by the interplay between root-released
oxygen and microbial respiration released extracellular electrons,
while iron mineral and organic matter might play key roles in storing
and shuttling electrons. Our results highlight the rhizosphere as
a widespread but previously unappreciated hotspot for ROS production,
which may affect pollutant redox dynamics and biogeochemical processes
in soils.
Soil science is an inherently diverse and multidisciplinary subject that cannot develop further without the continuous introduction and promotion of emerging technologies. One such technology that is widely used in biomedicine and similar research fields, microfluidics, poses significant benefits for soil research; however, this technology is still underutilized in the field. Microfluidics offers unparalleled opportunities in soil bacterial cultivation, observation, and manipulation when compared to conventional approaches to these tasks. This review focuses on the use of microfluidics for bacteria research and, where possible, pulls from examples in the literature where the technologies were used for soil related research. The review also provides commentary on the use of microfluidics for soil bacteria research and discusses the key challenges researchers face when implementing this technology. We believe that microfluidic chips and their associated auxiliary technologies provide a prime inroad into the future of soil science research.
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