Carboxysomes are CO₂-fixing protein compartments present in all cyanobacteria and some proteobacteria. These structures are attractive candidates for carbon assimilation bioengineering because they concentrate carbon, allowing the fixation reaction to occur near its maximum rate, and because they self-assemble in diverse organisms with a set of standard biological parts. Recent discoveries have expanded our understanding of how the carboxysome assembles, distributes itself, and sustains its metabolism. These studies have already led to substantial advances in engineering the carboxysome and carbon concentrating mechanism into recombinant organisms, with an eye towards establishing the system in industrial microbes and plants. Future studies may also consider the potential of in vitro carboxysomes for both discovery and applied science.

Diffusion of biological nanoparticles in solution impedes our ability to continuously monitor individual particles and measure their physical and chemical properties. To overcome this, we previously developed the interferometric scattering anti-Brownian electrokinetic (ISABEL) trap, which uses scattering to localize a particle and applies electrokinetic forces that counteract Brownian motion, thus enabling extended observation. Here we present an improved ISABEL trap that incorporates a near-infrared scatter illumination beam and rapidly interleaves 405 and 488 nm fluorescence excitation reporter beams. With the ISABEL trap, we monitored the internal redox environment of individual carboxysomes labeled with the ratiometric redox reporter roGFP2. Carboxysomes widely vary in scattering contrast (reporting on size) and redox-dependent ratiometric fluorescence. Furthermore, we used redox sensing to explore the chemical kinetics within intact carboxysomes, where bulk measurements may contain unwanted contributions from aggregates or interfering fluorescent proteins. Overall, we demonstrate the ISABEL trap's ability to sensitively monitor nanoscale biological objects, enabling new experiments on these systems.