Algal biomass. Photosynthetic algae have the potential to achieve very high biomass productivities, namely 108 gDW/m2/day (grams of dry weight per m2 per day). Indeed, photosynthesis requires 8 photons to fix a C atom from CO2, biomass is roughly 50% C, we assume a solar irradiance of 6 kWh/ m2/day as well as a high quantum yield of 0.83 seen in dark-adapted algal cells. PIARCS’s proposed photobioreactor design (proprietary) aims to reach 70% of this maximum theoretical productivity, namely 76 gDW/m2/day (or 110 tons/acre/year, which compares to 6 tons/acre/year for corn).
Algal biofuel. As has been known for decades, certain eukaryotic algal strains accumulate lipids (a biodiesel precursor) when subjected to limiting levels of nitrogen. Assuming that the chosen algal strain produces continuously biomass containing 20% recoverable lipids at a productivity of 60 gDW/m2/day (the lower biomass productivity would result from nitrogen limitation), the resulting biofuel production would be on the order of 5400 gal biofuel/acre/yr.
- Nevertheless, current outdoor algal biomass productivities remain low, on the order of 18 gDW/m2/day, while the 2030 DOE BioEnergy Technology Office (BETO) target is only 25 gDW/m2/day.
Continuous lipid production has not been documented to date. Current strategies rely on intermittent nitrogen exhaustion from the growth medium, which further impairs overall areal productivity.
Solutions under development
- At the interface between Biochemical Engineering, Algal Physiology, PAM Fluorimetry and Photosynthesis, Alexandra Holland’s publications (see below) lay the foundation to address the two main shortcomings in photobioreactor designs, namely the need for:
Chemostat operation (open source)
Bioreactor parameterization is necessary to operate the reactor as a chemostat, which is easily done under low light. The underlying mass balances allow for complete nutrient utilization, which is key to reduce input waste. In addition, this allows to induce lipid accumulation through mild or severe nitrogen limitation. Maximum lipid productivity under low light can therefore be estimated for various algal strains of interest.
In order to validate the proposed bioreactor parameterization, Arduino-based chemostats (~800$) are currently being developed with a Public University partner. Results will be published, and additional resources will be made available on the PIARCS website.
Achieving low photon flux per cell under high light (proprietary)
Under high irradiance, current algal photobioreactor designs fail to achieve a low photon flux per cell due to poor mixing. As a result, photosynthetic algal cells are unable to sustain high quantum yields, namely quantum yields on the order of 70%-100% of the quantum yields measured in dark-adapted nutrient-replete cells. Light-adapted quantum yields often fall below 20% of dark-adapted quantum yields under irradiances above 1500 micromoles/ m2/s Photosynthetic Photon Flux Density (PPFD), light intensities which are commonly measured in mid-day temperate climates. Since the quantum yield is defined as the fraction of photons processed into biochemical energy, these lower quantum yields solely account for a 3-5 fold reduction in biomass productivity.
PIARCS is proposing to engineer a novel mixing strategy to allow for light-dark cycling frequencies conducive to high-yield photosynthesis. The mixing qualitative pattern and energy requirements were validated experimentally in February 2021, but the question of whether it would enhance algal growth under high light remains to be answered. In order to validate the proprietary prototype design, PIARCS is currently seeking funding to work in collaboration with a National Laboratory.
High lipid productivity under high light?
Once the ‘low photon flux per cell’ condition has been achieved under high light (proprietary prototype), the resulting bioreactor becomes amenable to chemostat parameterization and operation. This means that the limiting nitrogen quotient (or biomass nitrogen concentration) achieving the maximum lipid productivity under low light can now be used under outdoor high light conditions, therefore yielding high lipid productivity outdoor.
Why low photon flux per cell?
Even though documented in the early 50’s by Bessel Kok, the fact that quantum yield plummets with increasing irradiance has been broadly overlooked. Yet, it makes sense: in the light zone, photons get absorbed and ‘close’ photosystems, such that subsequent photons hitting the closed photosystems will most likely see their energy wasted as heat or fluorescence (Non Photochemical Quenching). The well-documented ‘flashing light effect’ illustrates that, following a short period under high light, algae can spend time in the dark to process that excitation energy (Photochemical Quenching) in order to re-open (relax) their photosystems.
This phenomenon is analogous to a dense fishtank with fish pellets being fed from the surface. We assume that a fish with a full mouth wastes the pellets – which dissolves before it can be taken-up by another fish. Now a strategy to maximize yield (namely the amount of fish body weight per pellet) is to allow time for the fish to ‘digest’ its pellet in the depths, in order to free-up its mouth as it comes back to the surface. As a result, the fish trajectory/speed could be designed to allow for enough time at the surface to pick-up a pellet, and enough time in the depths to allow for its digestion. Except that, fish do not respond to mind control all that well : )
Luckily for us, microalgae are well behaved in that their trajectory simply follows fluid flow.
Importance of growth rate
The use of growth rate as a productivity indicator is an erroneous approach. Alexandra Holland has validated that growth rate and biomass yield do not correlate. Growth rate, however, is widely used as a strain selection criterion in the field.
Interestingly, heterotrophs display a trade-off between growth rate and yield , since bacterial metabolism optimizes for both adaptation in the event of sudden stress and biomass formation. In addition, Wong et al. theoretically derived that the yield provides an upper bound for growth rate, but does not correlate with growth rate.
The lack of significance of growth rate makes sense, however, since it describes how fast cells can divide under nutrient excess (or ‘wasted food’), whereas the yield describes how much biomass is produced per input feed (or output/input metrics). In the case of photosynthetic algae, the way to waste no food is to feed the photons in small packets (‘low photon flux per cell’), namely under continuous low light or under high light subjected to vigorous mixing.
Reporting a volumetric productivity in the absence of all key parameters (reactor geometry, culture volume, incident light level, area of light incidence) is utterly useless. Indeed, biomass yield (in grams biomass per mole of photons absorbed) cannot be inferred from volumetric productivities.
This is analogous to saying “my car is fast, it goes 120 miles” [and no, you have not suffered a stroke mid-sentence, you just assumed the ‘per hour’ was insignificant].
Holland A., Dragavon J. (2014) Algal Reactor Design Based on Comprehensive Modeling of Light and Mixing. In: Bajpai R., Prokop A., Zappi M. (eds) Algal Biorefineries. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-7494-0_2
Holland AD, Dragavon J and DC Sigee. Methods for Estimating Intrinsic Autotrophic Biomass Yield and Productivity in Algae: Emphasis on Experimental Methods for Strain Selection. Biotechnology Journal. 2011 6:572-583. https://doi.org/10.1002/biot.201000260
Holland AD and D Wheeler. Methods for Estimating Intrinsic Autotrophic Biomass Yield and Productivity in Algae: Modeling Spectrum and Mixing-Rate Dependence. Biotechnology Journal. 2011 6:584-599. https://doi.org/10.1002/biot.201000261
Licensing revenue target
Targeted licensing revenues would be on the order of 0.2% of the biomass sales revenue.
For example a 100-acre farm producing 60 gDW/m2/day would yield 8900 tons of biomass per year, or a gross revenue of $ 1.3M at a price of $150/ton, with a PIARCS licensing fee on the order of $2,600/year. The USDA reported that 51% of US farmers declared incomes under $10,000 in 2019, with an average farm size of 81 acres. Provided adequate access to freshwater and capital investment, PIARCS’ technology could enable a 100-fold increase in small farm gross income. If the technology becomes broadly adopted, a 10% market share of the US petroleum consumption would generate a licensing revenue of $4,3M/year.