Two-Dimensional Microfluidic System for the Simultaneous Quantitative Analysis of Phototactic/Chemotactic Responses of Microalgae

Abstract

Microalgae have been spotlighted as a renewable energy source to produce biofuels from CO2 by photosynthesis. However, their innate inefficiency of CO2 conversion using light energy has been a challenge to the commercialization of algae-based biofuel production. Photosynthetic organisms have evolved behavioral responses, including phototaxis and chemotaxis, to find optimal conditions for capturing light energy and inorganic carbon (Ci) sources for photosynthesis. In this context, investigation of phototaxis and chemotaxis to HCO3 –, the predominant form of Ci in neutral aqueous solutions, is necessary to understand the physiological role of tactic responses in photosynthesis. In this study, a two-dimensional microfluidic system enabled efficient analysis of phototactic and chemotactic responses by investigation of cell distribution in the outlet chambers. From statistical analysis (skewness and kurtosis) of tactic responses of different algal strains to external stimuli, the preferred concentrations of HCO3 – for Chlamydomonas reinhardtii CC125 (wild type), PTS42 (random insertional mutant of C. reinhardtii, high photosynthetic activity), and CC2702 (cia5 mutant of C. reinhardtii, unable to acclimate to low CO2 concentration) were determined to be 27.22, 43.23, and 36.95 mM, respectively. From the analysis of tactic responses of wild type and 14 mutant strains, it was found that the photosystem II (PSII) operating efficiency and CO2 fixation rate were strongly correlated with the phototactic (R 2 = 0.931) and chemotactic response (R 2 = 0.857), respectively. Finally, this system can be applied to high-throughput screening strategies for the rapid isolation of high photosynthetically productive microalgal strains based on their tactic responses.

Photo-bioconvection: towards light control of flows in active suspensions

“…In turn, this raises the question of whether the external control of stimuli could be used to govern large scale hydrodynamic instabilities in suspensions of swimming microorganisms, thereby harnessing the activity of the suspension to govern its macroscopic transport properties. Control of chemical gradients can be achieved within microfluidic devices [19][20][21][22], but advection of the chemical species by the underlying fluid [23][24][25] makes this a complex strategy for robust control of flows in macroscopic active matter suspensions. Conversely, arbitrary spatiotemporal patterns of light can be engineered easily, and are independent of ensuing flows within the suspension.…”

Cells collectively migrate during ammonium chemotaxis in Chlamydomonas reinhardtii

“…We believe these factors make an analysis of the mechanisms regulating ammonium chemotaxis difficult. To address this limitation, new methods using microfluidic devices to study Chlamydomonas chemotaxis have been developed 22,23 . Microfluidics possesses the superior capability to generate a well-defined, stable chemical gradient while allowing direct observation of cell migration with a microscope.…”

Microscope-based light gradient generation for quantitative growth studies of photosynthetic micro-organisms

“…Recently, researchers have developed methods to precisely control the intensity, spectrum, and spatial distribution of light at microscale, combined with microfluidics, to follow light-dependent cell growth in both space and time, screen microalgal oil production, or study the light-sensitive cell motility. [28][29][30][31][32][33][34][35][36] Graham et al developed an array of miniaturized photobioreactors for the screening of light properties such as intensity and spectral composition on photosynthetic growth. 29 In this system, the light intensity of each individual well was directly controlled by the programmable liquid crystal display (LCD) pattern overlaid above an LED light source.…”

“…36 In addition to photosynthetic growth, some motile microalgae have phototactic behavior that is also of interest. [30][31][32][33][34] Lam et al built a customized system that enabled the real-time manipulation of the swarming behavior of phototactic microalgae. 35 A digital light processing (DLP) projector and a 4× lens were used to generate light patterns with 20 μm resolution onto a microfluidic chip, and live cell motion was monitored by a camera.…”