A Method for Adapting Euglena gracilis to Cassava-Based Substrates for Heterotrophic Cultivation

Our research focuses on adapting new Euglena gracilis leaves to cassava waste and studying growth, cyanide reduction, and protein-rich biomass production. This protocol can be applied in sustainable cassava processing, agricultural waste valorization, feed production, and low-cost biotechnology systems. To begin, label sterile 500-milliliter vented cap flasks with the substrate type as garri.

Mark the appropriate target concentration on the flask. Label the flask with the replicate identification and planned sterilization condition as autoclaved or non-autoclaved. Place a weighboat on the analytical balance.

After pressing tare, weigh the garri to match the final working volume. Aseptically add the weighed garri into a sterile 500-milliliter flask containing 300 milliliters of deionized or distilled water. Reapply the vented cap to each flask.

Mix each suspension for three minutes until the mixture is homogenized. If magnetic stirring is used, place the flask on a stir plate and set the speed to 200G to form a shallow vortex without splashing. Allow the suspension to hydrate undisturbed for 10 minutes at 25 degrees Celsius.

Tighten the vent cap immediately prior to fermentation and swirl the flask by hand five times to gently homogenize the mixture and re-suspend the settled material. To prepare additional cassava product media, label the flasks and weigh the substrate as previously demonstrated to achieve 30 grams per liter based on the working volume. Aseptically add the weighed substrate into a sterile flask containing 300 milliliters of deionized or distilled water.

Reapply the vented cap to each flask. Hydrate and mix the suspension as previously demonstrated. Confirm that flasks are fitted with a sterile gas-permeable vented cap to permit gas exchange while minimizing environmental contamination.

Place the flasks upright at 25 degrees Celsius in a designated area away from direct sunlight and high traffic airflow and incubate the flasks without agitation for a minimum of 72 hours. Record the fermentation start date and time for each flask and condition. Confirm that each 500-milliliter flask contains no more than 300 milliliters of liquid following fermentation to prevent boilover during sterilization.

Once the vented cap is loosened to a finger tight position, apply autoclave indicator tape to the flask. After loading the flasks into an autoclave-safe tray, place the tray into the autoclave and select the P13 liquid cycle. Set the sterilization parameters to 121 degrees Celsius for 30 minutes for liquid sterilization.

Start the autoclave cycle. After the cycle completes, allow the autoclave chamber pressure to return to atmospheric pressure and open the autoclave door slowly. Using heat-resistant gloves, remove the flasks and allow them to cool to room temperature.

Store the cooled flasks overnight at room temperature prior to inoculation. Use the fermented cassava media directly without autoclaving to preserve native microbial communities. Inoculate the non-autoclaved media immediately after the 72-hour fermentation period to minimize unintended environmental contamination.

Maintain consistent handling across replicates by using the same type 500-milliliter vented cap flasks, the same working volume, and the same fermentation duration for all substrate conditions. Inoculate Euglena gracilis into fresh heterotrophic nutrient medium. Incubate the cultures in complete darkness at 28 degrees Celsius with continuous orbital shaking at 120G until the cultures reach mid-logarithmic growth.

Using a compound light microscope in brightfield mode, examine the cultures at 100x to 400x total magnification and confirm active motility, elongated cell morphology, and low levels of insistment. Exclude cultures exhibiting excessive cellular debris, abnormal morphology, or elevated insistment prior to adaptation experiments. After gently homogenizing the starter cultures by tightening the vent cap, swirl the flasks by hand for 15 seconds to ensure uniform cell distribution without inducing sheer stress.

Using a Neubauer hemocytometer, determine the cell density. Once the culture is mixed by swirling the flask for 15 seconds, pipette one milliliter of culture into a sterile 1.5 milliliter microcentrifuge tube. Clean the hemocytometer and cover slip with 70%ethanol and allow the hemocytometer and cover slip to air dry.

Pipette 10 microliters of culture onto the two grids on the hemocytometer and place the cover slip onto the wet sample. Using brightfield microscopy with a 10x objective, focus on the grid and count the cells within a defined set of squares on the large grid by applying the standard boundary rules. Repeat the cell counts on the second chamber and calculate the mean cells per milliliter and cell density.

Adjust the inoculation volumes to achieve a final inoculum of 5%volume by volume across all experimental media. Prepare separate inocula for each treatment condition to maintain consistent initial cell densities between experimental groups. Add the prepared E.gracilis inoculum to autoclaved and non-autoclaved cassava media prepared at final garri concentrations of 10, 20, and 30 grams per liter.

Use equal inoculation volumes across all treatment conditions to standardize the initial cell density. After tightening the vented cap, gently swirl the flask in a circular motion for 15 seconds to distribute the cells. Incubate the cultures in complete darkness at 25 degrees Celsius to initiate heterotrophic growth.

Minimize agitation of the non-autoclaved media to preserve the native microbial community structure and substrate stratification. Mix all cultures once daily using gentle manual swirling for 15 seconds to maintain contact between E.gracilis cells and suspended cassava material. Using a sterile pipette tip, every 48 hours, aseptically collect one milliliter samples from each culture.

Tighten the vented cap and gently swirl the culture flask for 15 seconds prior to sampling to ensure even distribution of cells and suspended material. After transferring an aliquot of culture into a sterile 1.5 milliliter microcentrifuge tube, dilute the sample with deionized water as needed to improve visualization of individual cells. Prepare a wet mount microscopy slide by pipetting 10 microliters of the diluted sample onto a clean microscope slide and covering the sample with a cover slip.

Using a compound light microscope in brightfield mode, examine the samples at 100x to 400x total magnification and assess cell motility, morphology, and the frequency of insistment as indicators of adaptation status. Every seven days, aseptically transfer 20%volume by volume of each culture into freshly prepared cassava media with the same garri concentration and sterilization conditions. Maintain consistent transfer volumes between passages and repeat serial passaging until the cultures exhibit consistent growth, sustained motility, and low insistment frequency in 30 grams per liter cassava media.

Following high concentration cassava adaptation and comparative growth analysis, use colorimetric strips to assess cyanide levels and observe the developed color. After centrifugation, harvest biomass for downstream analysis. Under fully dark cultivation conditions, cultures grown at 30 grams per liter of garri exhibited higher integrated growth than lower concentrations.

Lower garri concentrations of 10 to 20 grams per liter showed detectable but reduced and more variable growth. Higher concentrations of 40 to 50 grams per liter reached elevated peak cell densities but showed delayed stabilization and greater variability. Trypan blue exclusion assays indicated that cultures maintained at 30 grams per liter garri retained high viability across sampled time points.

Higher garri concentrations showed transient reductions in viability followed by partial recovery with increased variability over time. Positive outcomes included rounded or semi-insisted cells with reduced motility that remained intact across repeated subculturing. Adapted E.gracilis cultures displayed reproducible growth across multiple substrate types and showed earlier stabilization with reduced late-stage decline relative to wild-type cultures.

Successful implementation of the protocol yielded E.gracilis biomass with measurable crude protein content as quantified by external laboratory analysis using the combustion Dumas method. Protein percentages observed in adapted cultures were comparable to those measured in wild-type cultures with higher total protein output corresponding to increased biomass accumulation. This protocol allows researchers to study Euglena growth, cyanide reduction, biomass production, and nutritional potential in different cassava-based environments.

The critical challenge in this protocol is maintaining a stable temperature, consistent culture conditions, and a balanced microbial community for Euglena growth. Future studies can optimize and scale up this protocol, improve protein yield, and test different industrial applications using diverse cassava waste streams on a large scale.