Abstract
Microalgae are important green energy resources. With high efficiency in fixing carbon dioxide, microalgae are broadly applied for biofuel production. Integrating various cultivation parameters, we applied ultraviolet (UV) mutagenesis, one of the most common approaches, to induce genomic mutation in microalgae and thus enhance the production of lipid content, but the screening process is convoluted and labor-intensive. In this study, we aimed to develop an accessible microfluidic platform to optimize the biofuel production of microalgae. Instead of traditional lithography, we designed hanging-drop microfluidic chips that were fabricated using a cheap computer numerical control (CNC) micro-milling technique. On each chip, we cultured in parallel Botryococcus braunii, one of the most common freshwater microalgae for biofuel production, in sets of ten separated hanging drops (~30 µL each); we monitored their growth in each individual drop for more than 14 days. To optimize the culturing conditions, using drops of varied diameter, we first identified the influence of cell density on algae growth and lipid production. After introducing UV-induced random mutations, we quantified the lipid content of the microalgae in situ; the optimized UV-C dosage was determined accordingly. In comparison with wild-type B. braunii, the results showed increased biomass growth (137%) and lipid content (149%) of the microalgae mutated with the desired UV process. Moreover, we showed a capacity to modulate the illumination on an addressed chip area. In summary, without using an external pump system, we developed a hanging-drop microfluidic system for long-term microalgae culturing, which can be easily operated using laboratory pipettes. This microfluidic system is expected to facilitate microalgae mutation breeding, and to be applied for algae cultivation optimization.
Highlights
Global warming, caused by CO2 emissions and the depletion of fossil fuels, has necessitated the search for sustainable and renewable energy sources [1]
On day 14, we found that the total chlorophyll signal in the well with a diameter of 1.8 mm was 4.86 times that in the well with a diameter 1.1 mm and 5.65 times that in the well with a diameter 1.5 mm; once normalized with the initial chlorophyll signal on day 0, we noticed that the growth rate of microalgae in a small hanging drop was greater than that in larger drops (Figure 2C)
It has been reported that the fluorescent intensity of cells stained with Nile red and the lipid content in B. braunii as determined with a conventional solvent extraction system show a linear relation (R2 = 0.998) [27]
Summary
Global warming, caused by CO2 emissions and the depletion of fossil fuels, has necessitated the search for sustainable and renewable energy sources [1]. The use of farm land for biofuel production can cause increased emissions of greenhouse gases, which are emitted during the land use changes [6]. Under these circumstances, microalgae can be considered as a promising feedstock for biodiesel production, as they have a greater growth rate and lipid content than agricultural crops [3]. B. braunii has attracted substantial interest as a potential species for the production of biofuels [1]; when the extracted hydrocarbon-rich oil from B. braunii is cracked, which is a process of catalytic and thermal degradation of long-chain hydrocarbons to form usable shorter-chain hydrocarbons, it yields petrol 67%, aviation fuel 15% and diesel distillate 15%, which proportions are similar to the gasoil fraction of crude oil; it is considered to be an effective candidate for third-generation biofuels
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