Maximum photosynthetic yield of green microalgae in photobioreactors

Growth parameter estimation and model simulation for three industrially relevant microalgae: Picochlorum, Nannochloropsis, and Neochloris

Multiple models have been developed in the field to simulate growth and product accumulation of microalgal cultures. These models heavily depend on the accurate estimation of growth parameters. In this paper growth parameters are presented for three industrially relevant microalgae species: Nannochloropsis sp., Neochloris oleoabundans, and Picochlorum sp. (BPE23). Dedicated growth experiments were done in photobioreactors to determine the maximal biomass yield on light and maintenance rate, while oxygen evolution experiments were performed to estimate the maximal specific growth rate. Picochlorum sp. exhibited the highest specific growth rate of 4.98 ± 0.24 day⁻¹ and the lowest specific maintenance rate of 0.079 day⁻¹, whereas N. oleoabundans showed the highest biomass yield on light of 1.78 g_x·mol_ph⁻¹. The measured growth parameters were used in a simple kinetic growth model for verification. When simulating growth under light conditions as found at Bonaire (12 °N, 68° W), Picochlorum sp. displayed the highest areal biomass productivity of 32.2 g.m⁻²·day⁻¹ and photosynthetic efficiency of 2.8%. The presented growth parameters show to be accurate compared to experimental data and can be used for model calibration by scientists and industrial communities in the field.

Starch Rich Chlorella vulgaris: High-Throughput Screening and Up-Scale for Tailored Biomass Production

The use of microalgal starch has been studied in biorefinery frameworks to produce bioethanol or bioplastics, however, these products are currently not economically viable. Using starch-rich biomass as an ingredient in food applications is a novel way to create more value while expanding the product portfolio of the microalgal industry. Optimization of starch production in the food-approved species Chlorella vulgaris was the main objective of this study. High-throughput screening of biomass composition in response to multiple stressors was performed with FTIR spectroscopy. Nitrogen starvation was identified as an important factor for starch accumulation. Moreover, further studies were performed to assess the role of light distribution, investigating the role of photon supply rates in flat panel photobioreactors. Starch-rich biomass with up to 30% starch was achieved in cultures with low inoculation density (0.1 g L−1) and high irradiation (1800 µmol m−2 s−1). A final large-scale experiment was performed in 25 L tubular reactors, achieving a maximum of 44% starch in the biomass after 12 h in nitrogen starved conditions.

Experimental and Model-Based Analysis to Optimize Microalgal Biomass Productivity in a Pilot-Scale Tubular Photobioreactor

A dynamic coarse-grained model of microalgal growth considering light availability and temperature under discontinuous bioprocess operation was parameterized using experimental data from 15 batch cultivations of Nannochloropsis granulata in a pilot-scale tubular photobioreactor. The methodology applied consists of a consecutive two-step model parameter estimation using pooled, clustered and reorganized data to obtain initial estimates and multi-experiment fitting to obtain the final estimates, which are: maximum specific growth rate μ_max = 1.56 d⁻¹, specific photon half-saturation constant K_S,ph = 1.89 molphgX-1d-1, specific photon maintenance coefficient m_ph = 0.346 molphgX-1d-1 and the cardinal temperatures T_min = 2.3°C, T_opt = 27.93°C and T_max = 32.59°C. Biomass productivity prediction proved highly accurate, expressed by the mean absolute percent error MAPE = 7.2%. Model-based numerical optimization of biomass productivity for repeated discontinuous operation with respect to the process parameters cultivation cycle time, inoculation biomass concentration and temperature yielded productivity gains of up to 35%. This optimization points to best performance under continuous operation. The approach successfully applied here to small pilot-scale confirms an earlier one to lab-scale, indicating its transferability to larger scale tubular photobioreactors.

Outdoor Large-Scale Cultivation of the Acidophilic Microalga Coccomyxa onubensis in a Vertical Close Photobioreactor for Lutein Production

The large-scale biomass production is an essential step in the biotechnological applications of microalgae. Coccomyxa onubensis is an acidophilic microalga isolated from the highly acidic waters of Río Tinto (province of Huelva, Spain) and has been shown to accumulate a high concentration of lutein (9.7 mg g−1dw), a valuable antioxidant, when grown at laboratory-scale. A productivity of 0.14 g L−1 d−1 was obtained by growing the microalga under outdoor conditions in an 800 L tubular photobioreactor. The results show a stable biomass production for at least one month and with a lutein content of 10 mg g−1dw, at pH values in the range 2.5–3.0 and temperature in the range 10–25 °C. Culture density, temperature, and CO2 availability in highly acidic medium are rate-limiting conditions for the microalgal growth. These aspects are discussed in this paper in order to improve the outdoor culture conditions for competitive applications of C. onubensis.

Optimal proteome allocation strategies for phototrophic growth in a light-limited chemostat

BACKGROUND

Cyanobacteria and other phototrophic microorganisms allow to couple the light-driven assimilation of atmospheric [Formula: see text] directly to the synthesis of carbon-based products, and are therefore attractive platforms for microbial cell factories. While most current engineering efforts are performed using small-scale laboratory cultivation, the economic viability of phototrophic cultivation also crucially depends on photobioreactor design and culture parameters, such as the maximal areal and volumetric productivities. Based on recent insights into the cyanobacterial cell physiology and the resulting computational models of cyanobacterial growth, the aim of this study is to investigate the limits of cyanobacterial productivity in continuous culture with light as the limiting nutrient.

RESULTS

We integrate a coarse-grained model of cyanobacterial growth into a light-limited chemostat and its heterogeneous light gradient induced by self-shading of cells. We show that phototrophic growth in the light-limited chemostat can be described using the concept of an average light intensity. Different from previous models based on phenomenological growth equations, our model provides a mechanistic link between intracellular protein allocation, population growth and the resulting reactor productivity. Our computational framework thereby provides a novel approach to investigate and predict the maximal productivity of phototrophic cultivation, and identifies optimal proteome allocation strategies for developing maximally productive strains.

CONCLUSIONS

Our results have implications for efficient phototrophic cultivation and the design of maximally productive phototrophic cell factories. The model predicts that the use of dense cultures in well-mixed photobioreactors with short light-paths acts as an effective light dilution mechanism and alleviates the detrimental effects of photoinhibition even under very high light intensities. We recover the well-known trade-offs between a reduced light-harvesting apparatus and increased population density. Our results are discussed in the context of recent experimental efforts to increase the yield of phototrophic cultivation.

Marimo machines: oscillators, biosensors and actuators

BACKGROUND

The green algae balls (Aegagropila linnaei), known as Marimo, are large spherical colonies of live photosynthetic filaments, formed by rolling water currents in freshwater lakes. Photosynthesis therein produces gas bubbles that can attach to the Marimo, consequently changing its buoyancy. This property allows them to float in the presence of light and sink in its absence.

RESULTS

We demonstrate that this ability can be harnessed to make actuators, biosensors and bioprocessors (oscillator, logic gates). Factors affecting Marimo movement have been studied to enable the design, construction and testing of working prototypes.

CONCLUSIONS

A novel actuator design is reported, incorporating an enhanced bubble retention system and the design and optimisation of a bio-oscillator is demonstrated. A range of logic gates (or, and, nor, nand, xor) implementable with Marimo have been proposed.

Study of Optical Configurations for Multiple Enhancement of Microalgal Biomass Production

Microalga is a promising biomass feedstock to restore the global carbon balance and produce sustainable bioenergy. However, the present biomass productivity of microalgae is not high enough to be marketable mainly because of the inefficient utilization of solar energy. Here, we study optical engineering strategies to lead to a breakthrough in the biomass productivity and photosynthesis efficiency of a microalgae cultivation system. Our innovative optical system modelling reveals the theoretical potential (>100 g m⁻² day⁻¹) of the biomass productivity and it is used to compare the optical aspects of various photobioreactor designs previously proposed. Based on the optical analysis, the optimized V-shaped configuration experimentally demonstrates an enhancement of biomass productivity from 20.7 m⁻² day⁻¹ to 52.0 g m⁻² day⁻¹, under the solar-simulating illumination of 7.2 kWh m⁻² day⁻¹, through the dilution and trapping of incident energy. The importance of quantitative optical study for microalgal photosynthesis is clearly exhibited with practical demonstration of the doubled light utilization efficiencies.

Targeted knockout of phospholipase A2 to increase lipid productivity in Chlamydomonas reinhardtii for biodiesel production

Biofuel derived from microalgae have several advantages over other oleaginous crops, however, still needs to be improved with its cost aspect and can be achieved by developing of a strain with improved lipid productivity. In this study, the CRISPR-Cas9 system was incorporated to carry out a target-specific knockout of the phospholipase A₂ gene in Chlamydomonas reinhardtii. The targeted gene encodes a key enzyme in the Lands cycle. As a result, the mutants showed a characteristic of increased diacylglycerol pool, followed by a higher accumulation of triacylglycerol without being significantly compensated with the cell growth. As a result, the overall lipid productivities of phospholipase A₂ knockout mutants have increased by up to 64.25% (to 80.92 g L^-1 d^-1). This study can provide crucial information for the biodiesel industry.

Photosynthetic oxygenation for urine nitrification

Human urine accounts for only a fraction of the sewage volume, but it contains the majority of valuable nutrient load in wastewater. In this study, synthetic urine was nitrified in a closed photo-bioreactor through photosynthetic oxygenation by means of a consortium of microalgae and nitrifying bacteria. In situ production of oxygen by photosynthetic organisms has the potential to reduce the energy costs linked to conventional aeration. This energy-efficient strategy results in stable urine for further nutrient recovery, while part of the nutrients are biologically recovered in the form of valuable biomass. In this study, urine was nitrified for the first time without conventional aeration at a maximum photosynthetic oxygenation rate of 160 mg O₂ gVSS^-1 d^-1 (VSS: volatile suspended solids). A maximum volumetric nitrification rate of 67 mg N L^-1 d^-1 was achieved on 12% diluted synthetic urine. Chemical oxygen demand (COD) removal efficiencies were situated between 44% and 83% at a removal rate of 24 mg COD gVSS^-1 d^-1. After 180 days, microscopic observations revealed that Scenedesmus sp. was the dominant microalga. Overall, photosynthetic oxygenation for urine nitrification is promising as a highly electricity efficient approach for further nutrient recovery.

Glocal assessment of integrated wastewater treatment and recovery concepts using partial nitritation/Anammox and microalgae for environmental impacts

This study explored the feasibility and estimated the environmental impacts of two novel wastewater treatment configurations. Both include combined bioflocculation and anaerobic digestion but apply different nutrient removal technologies, i.e. partial nitritation/Anammox or microalgae treatment. The feasibility of such configurations was investigated for 16 locations worldwide with respect to environmental impacts, such as net energy yield, nutrient recovery and effluent quality, CO₂ emission, and area requirements. The results quantitatively support the applicability of partial nitritation/Anammox in tropical regions and some locations in temperate regions, whereas microalgae treatment is only applicable the whole year round in tropical regions that are close to the equator line. Microalgae treatment has an advantage over the configuration with partial nitritation/Anammox with respect to aeration energy and nutrient recovery, but not with area requirements. Differential sensitivity analysis points out the dominant influence of microalgal biomass yield and wastewater nutrient concentrations on area requirements and effluent quality. This study provides initial selection criteria for worldwide feasibility and corresponding environmental impacts of these novel municipal wastewater treatment plant configurations.

A Synthetic Biology Approach to Engineering Living Photovoltaics

The ability to electronically interface living cells with electron accepting scaffolds is crucial for the development of next-generation biophotovoltaic technologies. Although recent studies have focused on engineering synthetic interfaces that can maximize electronic communication between the cell and scaffold, the efficiency of such devices is limited by the low conductivity of the cell membrane. This review provides a materials science perspective on applying a complementary, synthetic biology approach to engineering membrane-electrode interfaces. It focuses on the technical challenges behind the introduction of foreign extracellular electron transfer pathways in bacterial host cells and the past and future efforts to engineer photosynthetic organisms with artificial electron-export capabilities for biophotovoltaic applications. The article highlights advances in engineering protein-based, electron-exporting conduits in a model host organism, E. coli, before reviewing state-of-the-art biophotovoltaic technologies that use both unmodified and bioengineered photosynthetic bacteria with improved electron transport capabilities. A thermodynamic analysis is used to propose an energetically feasible pathway for extracellular electron transport in engineered cyanobacteria and identify metabolic bottlenecks amenable to protein engineering techniques. Based on this analysis, an engineered photosynthetic organism expressing a foreign, protein-based electron conduit yields a maximum theoretical solar conversion efficiency of 6-10% without accounting for additional bioengineering optimizations for light-harvesting.

Photon management for augmented photosynthesis

Microalgae and cyanobacteria are some of nature's finest examples of solar energy conversion systems, effortlessly transforming inorganic carbon into complex molecules through photosynthesis. The efficiency of energy-dense hydrocarbon production by photosynthetic organisms is determined in part by the light collected by the microorganisms. Therefore, optical engineering has the potential to increase the productivity of algae cultivation systems used for industrial-scale biofuel synthesis. Herein, we explore and report emerging and promising material science and engineering innovations for augmenting microalgal photosynthesis.

Photosynthetic microalgae could provide an ecologically sustainable route to produce solar biofuels and high-value chemicals. Here, the authors review various optical management strategies used to manipulate the incident light in order to increase the efficiency of microalgae biofuel production.

Modelling of oxygen-evolving-complex ionization dynamics for energy-efficient production of microalgal biomass, pigment and lipid with carbon capture: an engineering vision for a biorefinery

Development of an algal growth kinetics model, incorporating oxygen-evolving-complex ionization dynamics, for sustainable production of algal biomass, lipid, and chlorophyll (with associated carbon dioxide capture) in an algal biorefinery.

Microalgal TAG production strategies: why batch beats repeated-batch

BACKGROUND

For a commercially feasible microalgal triglyceride (TAG) production, high TAG productivities are required. The operational strategy affects TAG productivity but a systematic comparison between different strategies is lacking. For this, physiological responses of Nannochloropsis sp. to nitrogen (N) starvation and N-rich medium replenishment were studied in lab-scale batch and repeated-batch (part of the culture is periodically harvested and N-rich medium is re-supplied) cultivations under continuous light, and condensed into a mechanistic model.

RESULTS

The model, which successfully described both strategies, was used to identify potential improvements for both batch and repeated-batch and compare the two strategies on optimized TAG yields on light (amount of TAGs produced per mol of supplied PAR photons). TAG yields on light, for batch, from 0.12 (base case at high light) to 0.49 g molph (-1) (at low light and with improved strain) and, for repeated-batch, from 0.07 (base case at high light) to 0.39 g molph (-1) (at low light with improved strain and optimized repeated-batch settings). The base case yields are in line with the yields observed in current state-of-the-art outdoor TAG production.

CONCLUSIONS

For continuous light, an optimized batch process will always result in higher TAG yield on light compared to an optimized repeated-batch process. This is mainly because repeated-batch cycles start with N-starved cells. Their reduced photosynthetic capacity leads to inefficient light use during the regrowth phase which results in lower overall TAG yields compared to a batch process.

Closing Domestic Nutrient Cycles Using Microalgae

This study demonstrates that microalgae can effectively recover all P and N from anaerobically treated black water (toilet wastewater). Thus, enabling the removal of nutrients from the black water and the generation of a valuable algae product in one step. Screening experiments with green microalgae and cyanobacteria showed that all tested green microalgae species successfully grew on anaerobically treated black water. In a subsequent controlled experiment in flat-panel photobioreactors, Chlorella sorokiniana was able to remove 100% of the phosphorus and nitrogen from the medium. Phosphorus was depleted within 4 days while nitrogen took 12 days to reach depletion. The phosphorus and nitrogen removal rates during the initial linear growth phase were 17 and 122 mg·L(-1)·d(-1), respectively. After this initial phase, the phosphorus was depleted. The nitrogen removal rate continued to decrease in the second phase, resulting in an overall removal rate of 80 mg·L(-1)·d(-1). The biomass concentration at the end of the experiment was 11.5 g·L(-1), with a P content of approximately 1% and a N content of 7.6%. This high algal biomass concentration, together with a relatively short P recovery time, is a promising finding for future post-treatment of black water while gaining valuable algal biomass for further application.

A bioenergetic assessment of photosynthetic growth of Synechocystis sp. PCC 6803 in continuous cultures

BACKGROUND

Synechocystis sp. PCC 6803, a model organism used for bioenergy and bioplastic production, was grown in continuous culture to assess its most important bioenergetic parameters.

RESULTS

Biomass yield on light energy of 1.237 g mol photons(-1) and maintenance energy requirement of 0.00312 mol photons g(-1) h(-1) were calculated. This corresponded to a light conversion efficiency of 12.5 %, based on the model of Pirt which was about 35 % lower than the theoretical one based on the stoichiometric equation for the formation of biomass on carbon dioxide, water, and nitrate. The maximum F v/F m ratio recorded in the Synechocystis cultures was 0.57; it progressively declined to 0.45 as the dilution rate increased. An over-reduction of reaction centers at a high dilution rate was also recorded, together with an increased VJ phase for the chlorophyll fluorescence transient. In contrast, the chlorophyll optical cross section increased by about 40 % at the fastest dilution rate, and compensated for the decline in F v/F m, thus resulting in a constant total photosynthesis rate (photosynthesis plus respiration). Chlorophyll content was maximum at the lowest dilution rate and was 48 % lower at the highest one, while phycocyanin, and total carotenoids decreased by about 42 % and 37 %, respectively. Carotenoid analysis revealed increased echinenone, zeaxanthin, and myxoxanthophyll contents as the dilution rate increased (40.6, 63.8 and 35.5 %, respectively, at the fastest dilution rate). A biochemical analysis of the biomass harvested at each different dilution rates showed no changes in the lipid content (averaging 11.2 ± 0.6 % of the dry weight), while the protein content decreased as the dilution rate increased, ranging between 60.7 ± 1.1 and 72.6 ± 0.6 %. Amino acids pattern did not vary with the dilution rate. Carbohydrate content ranged from 9.4 to 16.2 % with a mean value of 11.2 ± 1.4 %.

CONCLUSIONS

The present work provides useful information on the threshold of light conversion efficiency in Synechocystis, as well as basic bioenergetic parameters that will be helpful for future studies related to its genetic transformation and metabolic network reconstruction.

Characterization of nutrient removal and microalgal biomass production on an industrial waste-stream by application of the deceleration-stat technique

Industrial wastewaters can serve as a nutrient and water source for microalgal production. In this study the effluent of an internal circulation (IC) reactor anaerobically treating the wastes of a biotechnology production facility were chosen as the cultivation medium for Chlorella sorokiniana in batch and continuous cultures. The aim was to evaluate the rates of nutrient removal and biomass production possible at various dilution rates. The results demonstrate that the industrial wastewater served as a highly effective microalgae culture medium and that dilution rate strongly influenced algae productivity in a short light-path photobioreactor. Batch culture on undiluted wastewater showed biomass productivity of 1.33 g L(-1)day(-1), while removing over 99% of the ammonia and phosphate from the wastewater. Deceleration-stat (D-stat) experiments performed at high and low intensities of 2100 and 200 (μmol photon m(2)s(-1)) established the optimal dilution rates to reach volumetric productivity of 5.87 and 1.67 g L(-1)day(-1) respectively. The corresponding removal rates of nitrogen were 238 and 93 mg L(-1)day(-1) and 40 and 19 mg L(-1)day(-1) for phosphorous. The yield on photons at low light intensity was as high as had been observed in any previous report indicating that the waste stream allowed the algae to grow at its full potential.

Quantifying the effects of light intensity on bioproduction and maintenance energy during photosynthetic growth of Rhodobacter sphaeroides

Obtaining a better understanding of the physiology and bioenergetics of photosynthetic microbes is an important step toward optimizing these systems for light energy capture or production of valuable commodities. In this work, we analyzed the effect of light intensity on bioproduction, biomass formation, and maintenance energy during photoheterotrophic growth of Rhodobacter sphaeroides. Using data obtained from steady-state bioreactors operated at varying dilution rates and light intensities, we found that irradiance had a significant impact on biomass yield and composition, with significant changes in photopigment, phospholipid, and biopolymer storage contents. We also observed a linear relationship between incident light intensity and H2 production rate between 3 and 10 W m(-2), with saturation observed at 100 W m(-2). The light conversion efficiency to H2 was also higher at lower light intensities. Photosynthetic maintenance energy requirements were also significantly affected by light intensity, with links to differences in biomass composition and the need to maintain redox homeostasis. Inclusion of the measured condition-dependent biomass and maintenance energy parameters and the measured photon uptake rate into a genome-scale metabolic model for R. sphaeroides (iRsp1140) significantly improved its predictive performance. We discuss how our analyses provide new insights into the light-dependent changes in bioenergetic requirements and physiology during photosynthetic growth of R. sphaeroides and potentially other photosynthetic organisms.

Microplate-based method for high-throughput screening of microalgae growth potential

Microalgae cultivation conditions in microplates will differ from large-scale photobioreactors in crucial parameters such as light profile, mixing and gas transfer. Hence volumetric productivity (P(v)) measurements made in microplates cannot be directly scaled up. Here we demonstrate that it is possible to use microplates to measure characteristic exponential growth rates and determine the specific growth rate light intensity dependency (μ-I curve), which is useful as the key input for several models that predict P(v). Nannochloropsis salina and Chlorella sorokiniana specific growth rates were measured by repeated batch culture in microplates supplied with continuous light at different intensities. Exponential growth unlimited by gas transfer or self-shading was observable for a period of several days using fluorescence, which is an order of magnitude more sensitive than optical density. The microplate datasets were comparable to similar datasets obtained in photobioreactors and were used an input for the Huesemann model to accurately predict P(v).

Improving the sunlight-to-biomass conversion efficiency in microalgal biofactories

Microalgae represent promising organisms for the sustainable production of commodities, chemicals or fuels. Future use of such systems, however, requires increased productivity of microalgal mass cultures in order to reach an economic viability for microalgae-based production schemes. The efficiency of sunlight-to-biomass conversion that can be observed in bulk cultures is generally far lower (35-80%) than the theoretical maximum, because energy losses occur at multiple steps during the light-driven conversion of carbon dioxide to organic carbon. The light-harvesting system is a major source of energy losses and thus a prime target for strain engineering. Truncation of the light-harvesting antenna in the algal model organism Chlamydomonas reinhardtii was shown to be an effective way of increasing culture productivity at least under saturating light conditions. Furthermore engineering of the Calvin-Benson cycle or the creation of photorespiratory bypasses in A. thaliana proved to be successful in terms of achieving higher biomass productivities. An efficient generation of novel microalgal strains with improved sunlight conversion efficiencies by targeted engineering in the future will require an expanded molecular toolkit. In the meantime random mutagenesis coupled to high-throughput screening for desired phenotypes can be used to provide engineered microalgae.