Using polyethylene glycol as nonionic osmoticum to promote growth and lipid production of marine microalgae Nannochloropsis oculata

To promote the economic feasibility of Nannochloropsis oculata, efficacy of using polyethylene glycol (PEG) to increase microalgal growth and lipid accumulation was investigated. We first examined the effects of PEG concentrations on microalgal growth using 0–5 % (w/v) PEG-6000, and followed by exploring the effects of PEG molecular weights (400, 600, 2,000, 4,000, 6,000, and 20,000) on microalgal growth, size, as well as on yields of biomass, total lipids, and eicosapentaenoic acid. In addition, the capacity of PEG to reduce the effect of oxygen inhibition on microalgal growth was also investigated to evaluate its adaptability for use in large-scale and closed setting. Our results showed that PEG-induced osmotic stress (Π) in the range of 2.465–2.472 MPa can raise microalgal growth. The PEG with higher molecular weight exhibited greater efficacy of growth promotion but less lipid content under equal concentration. In this study, 0.5 % (w/v) PEG-20000 (Π = 2.466 MPa) remarkably enhanced microalgal growth without interference of intracellular lipid productivity and cellular size, yielding >50 % (w/w) increases in biomass, total lipid, and eicosapentaenoic acid amounts after 7 days that provided the optimal condition for microalgal cultivation. These positive effects possibly resulted from the moderate enhancement of osmotic stress in the medium and stronger chaotrope-like behavior from higher molecular weight PEG. With further verification that 0.5 % (w/v) PEG-20000 enabled to reduce the effect of oxygen inhibition on microalgal growth, the PEG-20000-mediated cultivation offers a feasible means for mass culture of N. oculata in closed setting.     

The Effect of Diel Temperature and Light Cycles on the Growth of Nannochloropsis oculata in a Photobioreactor Matrix

A matrix of photobioreactors integrated with metabolic sensors was used to examine the combined impact of light and temperature variations on the growth and physiology of the biofuel candidate microalgal species Nannochloropsis oculata. The experiments were performed with algal cultures maintained at a constant 20°C versus a 15°C to 25°C diel temperature cycle, where light intensity also followed a diel cycle with a maximum irradiance of 1920 µmol photons m⁻² s⁻¹. No differences in algal growth (Chlorophyll a) were found between the two environmental regimes; however, the metabolic processes responded differently throughout the day to the change in environmental conditions. The variable temperature treatment resulted in greater damage to photosystem II due to the combined effect of strong light and high temperature. Cellular functions responded differently to conditions before midday as opposed to the afternoon, leading to strong hysteresis in dissolved oxygen concentration, quantum yield of photosystem II and net photosynthesis. Overnight metabolism performed differently, probably as a result of the temperature impact on respiration. Our photobioreactor matrix has produced novel insights into the physiological response of Nannochloropsis oculata to simulated environmental conditions. This information can be used to predict the effectiveness of deploying Nannochloropsis oculata in similar field conditions for commercial biofuel production.     

The effect of ultrasound on the growth and viability of microalgae cells

Ultrasound has shown potential for both increasing microalgal lipid extraction yields and for the control of microalgal blooms through cell disruption. The effect of ultrasound on the viability of microalgae was investigated on the following species: Dunaliella salina, Chlamydomonas concordia and Nannochloropsis oculata. Sonication with a 20 kHz probe (0.086 W cm^?3) caused complete cell disruption of D. salina after 4 min. This microalgae species does not have a true cell wall. In the case of C. concordia which has a thin cell wall complete cell disruption under the same conditions took 16 min. Under the same conditions, there was no visible disruption of N. oculata, a species which has a thick cell wall. However spectro-fluorophotometer analysis of the sonicated suspension of N. oculata showed that although the cells were intact, the level of intracellular chlorophyll was reduced by ~10 %. This clearly indicated damage to the microalgal cell wall. After 16 min, treatment cultures of all three species remained viable. Programmed cell death (PCD) has been induced in some microalgal species to terminate algal blooms; ultrasonic application did not induce PCD in any species tested. The supernatant of sonicated D. salina and C. concordia has also been shown to be able to boost the growth of established cultures. These results provide important information concerning the uses of ultrasound in both the microalgal biofuels industry and for the control of microalgal blooms.     

Bicarbonate-based carbon capture and algal production system on ocean with floating inflatable-membrane photobioreactor

This study aims to develop a low-cost microalgae culture system which uses a simple closed vessel as photobioreactor to save manufacturing cost, waves for mixing to save energy cost, and high concentration of bicarbonate for carbon supply to avoid the high cost of CO₂-bubbling pipeline construction on the ocean as well as to control pH by buffering the effect of bicarbonate/carbonate. To test this idea, the alkalihalophilic cyanobacterium Euhalothece sp. was cultured with 1.0 M NaHCO₃ in small-scale floating photobioreactors (PBRs) on 10-cm-high artificial waves at first. The final biomass concentration was up to 0.91 and 1.47 g L^?1 for indoor and outdoor cultures, respectively. However, the recorded dissolved oxygen (DO) was occasionally over-saturated (> 500% of air saturation), indicating mass transfer problem. k _L a in these PBRs with different culture depth was measured then, and the results showed great variation, from 0.13 to 4.87 h^?1. At the scale of 1.0 m², this floating PBR was made with low-cost membrane and inflatable design. It was placed on the ocean surface and mixed with natural waves. Biomass concentration of 1.63 g L^?1 and productivity of 8.27 g m^?2 day^?1 were obtained in this culture. With these results, the feasibility of a low-cost microalgae culture system was proven, which could systematically reduce the cost of photobioreactor manufacturing, operating, and maintenance.     

Outdoor production of <Emphasis Type="Italic">Tisochrysis lutea</Emphasis> in pilot-scale tubular photobioreactors

In this paper we study the outdoor production of Tisochrysis lutea in pilot-scale tubular photobioreactors (3.0 m³). Experiments were performed modifying the dilution rate and evaluating biomass productivity and quality, in addition to the overall performance of the system. Results confirm that T. lutea can be produced outdoors on a commercial scale in continuous mode, obtaining productivities of up to 20 g m^?2 day^?1 of biomass, which are rich in proteins (45 % d.wt.) and lipids (25 % d.wt.). The utilization of this type of photobioreactor allows one to control the levels of contamination and pH within the cultures, but daily variations in solar radiation impose elevated dissolved oxygen concentrations and insufficient temperature conditions on the cells inside the reactor. Excessive dissolved oxygen reduces biomass productivity to 68 % of that which is maximal, whereas inadequate temperature reduces it to 63 % of maximum. Thus, by optimally controlling these parameters, biomass productivity can be almost doubled. These results confirm the potential for producing this valuable strain on a commercial scale in optimally designed/operated tubular photobioreactors as a viable biotechnological industry.     

Study of hydrodynamic characteristics in tubular photobioreactors

In this work, the hydrodynamic characteristics in tubular photobioreactors with a series of helical static mixers built-in were numerically investigated using computational fluid dynamics (CFD). The influences of height and screw pitch of the helical static mixer and fluid inlet velocity on the cell trajectories, swirl numbers and energy consumption were examined. In order to verify the actual results for cultivation of microalgae, cultivation experiments of freshwater Chlorella sp. were carried out in photobioreactor with and without helical static mixer built-in at the same time. It was shown that with built-in helical static mixer, the mixing of fluid could be intensified, and the light/dark cycle could also be achieved which is of benefit for the growth of microalgae. The biomass productivity of Chlorella sp. in tubular photobioreactor with helical static mixer built-in was 37.26 % higher than that in the photobioreactor without helical static mixer.     

Comparison of growth of Tetraselmis in a tubular photobioreactor (Biocoil) and a raceway pond

Microalgae cultivation systems can be divided broadly into open ponds and closed photobioreactors. This study investigated the growth and biomass productivity of the halophilic green alga Tetraselmis sp. MUR-233, grown outdoors in paddle wheel-driven open raceway ponds and in a tubular closed photobioreactor (Biocoil) at a salinity of 7 % NaCl (w/v) between mid-March and June 2010 (austral autumn/winter). Volumetric productivity in the Biocoil averaged 67 mg ash-free dry weight (AFDW) L^?1 day^?1 when the culture was grown without CO₂ addition. This productivity was 86 % greater, although less stable, than that achieved in the open raceway pond (36 mg L^?1 day^?1) grown at the same time in the autumn period. The Tetraselmis culture in the open raceway pond could be maintained in semi-continuous culture for the whole experimental period of 3 months without an additional CO₂ supply, whereas in the Biocoil, under the same conditions, reliable semi-continuous culture was only achievable for a period of 38 days. However, stable semi-continuous culture was achieved in the Biocoil by the addition of CO₂ at a controlled pH of ~7.5. With CO₂ addition, the volumetric biomass productivity in the Biocoil was 85 mg AFDW L^?1 day^?1 which was 5.5 times higher than the productivity achieved in the open raceway pond (15 mg AFDW L^?1 day^?1) with CO₂ addition and 8 times higher compared to the productivity in the open raceway pond without CO₂ addition (11 mg AFDW L^?1 day^?1), when cultures were grown in winter. The illuminated area productivities highlight an alternative story and showed that the open raceway pond had a three times higher productivity (3,000 mg AFDW m^?2 day^?1) compared to the Biocoil (850 mg AFDW m^?2 day^?1). Although significant differences were found between treatments and cultivation systems, the overall average lipid content for Tetraselmis sp. MUR-233 was 50 % in exponential phase during semi-continuous cultivation.     

Effects of nitrogen supplementation of the culture medium on the growth, total lipid content and fatty acid profiles of three microalgae (Tetraselmis subcordiformis, Nannochloropsis oculata and Pavlova viridis)

To improve the properties of microalgae as sources for biodiesel production, three microalgae (Tetraselmis subcordiformis SHOU-S05, Nannochloropsis oculata SHOU-S14 and Pavlova viridis SHOU-S16) were cultured in media supplemented with different amounts of nitrogen (0, 0.22, 0.44, 0.88 and 1.76 mmol N·L^?1). The growth, total lipid contents, lipid classes and fatty acid profiles of the three microalgae were assayed after 10 days of cultivation. The results indicated that the specific growth rates of T. subcordiformis, N. oculata and P. viridis were lowest (0.014, 0.033 and 0.018, respectively) in the treatments without nitrogen supplementation and increased significantly with increasing nitrogen supplementation. The microalgae treated with 0.22 mmol N·L^?1 had the highest total lipid contents, which were 29.77, 35.85 and 32.10 % in T. subcordiformis, N. oculata and P. viridis, respectively. The total lipid contents as well as the proportions of neutral lipid in the three microalgae decreased significantly with increasing nitrogen supplementation between 0.22 and 1.76 mmol N·L^?1. The fatty acid profiles of the three microalgae were significantly different. The obvious characteristic of the fatty acid profile of T. subcordiformis was a high amount (17.68–22.22 %) of 18:3n3. However, EPA (C20:5n3) and C16 fatty acids were significantly high in N. oculata and P. viridis, respectively. N. oculata and P. viridis accumulated more 16-carbon fatty acids and fewer polyunsaturated fatty acids in nitrogen-limited media. It is therefore suggested that a limited nitrogen treatment is effective for improving the properties of P. viridis and N. oculata as sources for biodiesel.     

A theoretical consideration on oxygen production rate in microalgal cultures

Because algal cells are so efficient at absorbing incoming light energy, providing more light energy to photobioreactors would simply decrease energy conversion efficiency. Furthermore, the algal biomass productivity in photobioreactor is always proportional to the total photosynthetic rate. In order to optimize the productivity of algal photobioreactors (PBRs), the oxygen production rate should be estimated. Based on a simple model of light penetration depth and algal photosynthesis, the oxygen production rate in high-density microalgal cultures could be calculated. The estimated values and profiles of oxygen production rate by this model were found to be in accordance with the experimental data. Optimal parameters for PBR operations were also calculated using the model.     

Effects of carbon source and light intensity on the growth and total lipid production of three microalgae under different culture conditions

We attempted to enhance the growth and total lipid production of three microalgal species, Isochrysis galbana LB987, Nannochloropsis oculata CCAP849/1, and Dunaliella salina, which are capable of accumulating high content of lipid in cells. Low nitrogen concentration under photoautotrophic conditions stimulated total lipid production, but a decreasing total lipid content and an increasing biomass were observed with increasing nitrogen concentration. Among the different carbon sources tested for heterotrophic cultivation, glucose improved the growth of all three strains. The optimal glucose concentration for growth of I. galbana LB987 and N. oculata CCAP849/1 was 0.02 M, and that of D. salina was 0.05 M. Enhanced growth occurred when they were cultivated under heterotrophic or mixotrophic conditions compared with photoautotrophic conditions. Meanwhile, high total lipid accumulation in cells occurred when they were cultivated under photoautotrophic or mixotrophic conditions. During mixotrophic cultivation, biomass production was not affected significantly by light intensity; however, both chlorophyll concentration and total lipid content increased dramatically with increasing light intensity up to 150 µmol/m²/s. The amount and composition ratio of saturated and unsaturated fatty acids in cells were different from each other depending on both species and light intensity. The highest accumulation of total fatty acid (C16–C18) among the three strains was found from cells of N. oculata CCAP849/1, which indicates that this species can be used as a source for production of biodiesel.     

Comparison of four outdoor pilot-scale photobioreactors

Background

Microalgae are a potential source of sustainable commodities of fuels, chemicals and food and feed additives. The current high production costs, as a result of the low areal productivities, limit the application of microalgae in industry. A first step is determining how the different production system designs relate to each other under identical climate conditions. The productivity and photosynthetic efficiency of Nannochloropsis sp. CCAP 211/78 cultivated in four different outdoor continuously operated pilot-scale photobioreactors under the same climatological conditions were compared. The optimal dilution rate was determined for each photobioreactor by operation of the different photobioreactors at different dilution rates.

Results

In vertical photobioreactors, higher areal productivities and photosynthetic efficiencies, 19–24 g m^?2 day^?1 and 2.4–4.2 %, respectively, were found in comparison to the horizontal systems; 12–15 g m^?2 day^?1 and 1.5–1.8 %. The higher ground areal productivity in the vertical systems could be explained by light dilution in combination with a higher light capture. In the raceway pond low productivities were obtained, due to the long optical path in this system. Areal productivities in all systems increased with increasing photon flux densities up to a photon flux density of 30 mol m^?2 day^?1. Photosynthetic efficiencies remained constant in all systems with increasing photon flux densities. The highest photosynthetic efficiencies obtained were; 4.2 % for the vertical tubular photobioreactor, 3.8 % for the flat panel reactor, 1.8 % for the horizontal tubular reactor, and 1.5 % for the open raceway pond.

Conclusions

Vertical photobioreactors resulted in higher areal productivities than horizontal photobioreactors because of the lower incident photon flux densities on the reactor surface. The flat panel photobioreactor resulted, among the vertical photobioreactors studied, in the highest average photosynthetic efficiency, areal and volumetric productivities due to the short optical path. Photobioreactor light interception should be further optimized to maximize ground areal productivity and photosynthetic efficiency.

     

Effect of liquid circulation velocity and cell density on the growth ofParietochloris incisa in flat plate photobioreactors

For more accurately describing the durations of the light and the dark phases of microalgal cells over the whole light-dark cycle, and probing into the relationship between the liquid circulation time or velocity, the aeration rate and cell density, a series of experiments was carried out in 10 cm light-path flat plate photobioreactors. The results indicated that the liquid flow in the flat plate photobioreactor could be described by liquid dynamic equations, and a high biomass output, higher content and productivity of arachidonic acid, 70.10 gm⁻²d⁻¹, 9.62% and 510.3 mg/L, respectively, were obtained under the optimal culture conditions.     

Effects of dominant microalgae species and bacterial quantity on shrimp production in the final culture season

The dominant microalgal species, quantity of heterotrophic bacteria and Vibrio in the intestines and gills of Litopenaeus vannamei (Pacific white shrimp), positive detection rate of white spot syndrome virus (WSSV), and water quality indices were investigated at the final culture stage (88th day in culture season). Correlation of microalgal community, bacteria quantity, and shrimp production were analyzed by statistical analysis methods. Every 10 days, probiotics were used in groups A, B, and C, consisting of Bacillus, photosynthetic bacteria (PSB), and equal parts Bacillus and PSB, respectively. The results showed that production (25597.33?±?928.46 kg ha^?1) and survival rate (77.06?±?9.00 %) was the highest in group C, but positive detection rate of WSSV was the lowest. The microalgal community of group C was significantly dominated by Chlorella pyrenoidosa, with an average density and dominance of (289.52?±?142.10)?×?10⁷ cells L^?1 and 0.878?±?0.161, respectively. The correlation analysis indicated a significant negative correlation between Cyanophyta dominance and shrimp production (P?<?0.05), while the relationship between production and Vibrio quantity was not significantly correlated (P?>?0.05). Accordingly, microalgal dominant species should be controlled as a key factor in the shrimp culture season; in particular, the dominance of Cyanophyta should be restricted to a low level. Meanwhile, the combined use of Bacillus and PSB probiotics was considered an effective solution to optimize microalgal communities and controlling the cell density of Cyanophyta.     

The oxygen evolution methodology affects photosynthetic rate measurements of microalgae in well‐defined light regimes

Designing photobioreactors correctly is a must for the success of microalgal mass production. Optimal photobioreactor design requires a precise knowledge of photosynthesis dynamics in fluctuating light conditions and hence a method for the measurement of photosynthetic rates in specific light regimes. However, it is not uncommon in literature that experimental protocols used to obtain oxygen generation rates are described ambiguously and the reported rates of photosynthesis vary widely depending on the methodology. Additionally, quite a number of methods overlook certain aspects that can affect the estimated rates significantly, and can therefore affect photobioreactor design. We have developed a method based on oxygen evolution measurements that accurately determines photosynthetic rates under well‐defined light regimes. Our experimental protocol takes into account most of the issues that can affect the rates of oxygen generation, such as depletion of nutrients during the measurements and precision of the measurements. We have focused on the basic applications in photobioreactor design and used a dynamic model of photosynthesis to analyze our results and compare them with available published data. The results suggest that our oxygen evolution method is consistent. Biotechnol. Bioeng. 2010;106: 228–237. © 2010 Wiley Periodicals, Inc.     

Irradiance optimization of outdoor microalgal cultures using solar tracked photobioreactors

Photosynthetic activity and temperature regulation of microalgal cultures (Chlorella vulgaris and Scenedesmus obliquus) under different irradiances controlled by a solar tracker and different cell densities were studied in outdoor flat panel photobioreactors. An automated process control unit regulated light and temperature as well as pH value and nutrient concentration in the culture medium. CO₂ was supplied using flue gas from an attached combined block heat and power station. Photosynthetic activity was determined by pulse amplitude modulation fluorometry. Compared to the horizontal irradiance of 55 mol photons m^?2 d^?1 on a clear day, the solar tracked photobioreactors enabled a decrease and increase in the overall light absorption from 19 mol photons m^?2 d^?1 (by rotation out of direct irradiance) to 79 mol photons m^?2 d^?1 (following the position of the sun). At biomass concentrations below 1.1 g cell dry weight (CDW) L^?1, photoinhibition of about 35 % occurred at irradiances of ≥1,000 μmol photons m^?2 s^?1 photosynthetic active radiation (PAR). Using solar tracked photobioreactors, photoinhibition can be reduced and at optimum biomass concentration (≥2.3 g CDW L^?1), the culture was irradiated up to 2,000 μmol photons m^?2 s^?1 to overcome light limitation with biomass yields of 0.7 g CDW mol photons^?1 and high photosynthetic activities indicated by an effective quantum yield of 0.68 and a maximum quantum yield of 0.80 (F _v/F _m). Overheating due to high irradiance was avoided by turning the PBR out of the sun or using a cooling system, which maintained the temperature close to the species-specific temperature optima.     

Productivity and biochemical composition of <Emphasis Type="Italic">Tetradesmus obliquus</Emphasis> and <Emphasis Type="Italic">Phaeodactylum tricornutum</Emphasis>: effects of different cultivation approaches

The present work evaluated biomass productivity, carbon dioxide fixation rate, and biochemical composition of two microalgal species, Phaeodactylum tricornutum (Bacillariophyta) and Tetradesmus obliquus (Chlorophyta), cultivated indoors in high-technology photobioreactors (HT-PBR) and outdoors both in pilot ponds and low-technology photobioreactors in a greenhouse in southern Italy. Microalgae were grown in standard media, under nitrogen starvation, and in two liquid digestates obtained from anaerobic digestion of agro-zootechnical and vegetable biomass. P. tricornutum, cultivated in semi-continuous mode in indoor HT-PBRs with standard medium, showed a biomass productivity of 21.0?±?2.3 g m^?2 d^?1. Applying nitrogen starvation, the lipid productivity increased from 2.3 up to 4.5?±?0.5 g m^?2 d^?1, with a 24 % decrease of biomass productivity. For T. obliquus, a biomass productivity of 9.1?±?0.9 g m^?2 d^?1 in indoor HT-PBR was obtained using standard medium. Applying liquid digestates as fertilizers in open ponds, T. obliquus gave a biomass productivity (10.8?±?2.0 g m^?2 d^?1) not statistically different from complete medium such as P. tricornutum (6.5?±?2.2 g m^?2 d^?1). The biochemical data showed that the fatty acid composition of the microalgal biomass was affected by the different cultivation conditions for both microalgae. In conclusion, it was found that the microalgal productivity in standard medium was about doubled in HT-PBR compared to open ponds for P. tricornutum and was about 20 % higher for T. obliquus.     

pH-induced flocculation,indirect electrocoagulation,and hollow fiber filtration techniques for harvesting the saltwater microalga Dunaliella

This research assessed the efficacy of three harvesting methods on a strain of Dunaliella viridis. While there is strong potential to use lipids from microalgae as a feedstock for biofuels to replace petroleum-based fuel, at present microalgal harvesting for biofuel production is not yet economically feasible or energy efficient. pH-induced flocculation (by adjusting the pH of exponentially growing cells), indirect electrocoagulation (applying aluminum hydroxide coagulant to culture), and hollow fiber filtration (separating biomass from medium using tangential flow) were compared as potential harvesting mechanisms for small-scale (3–10 L) and large-scale (30–150 L) volumes of D. viridis. Both pH-induced flocculation and electrocoagulation yielded significant biomass recovery (>95 %), but both methods required removal of added chemicals and/or coagulant before the medium could be reused. In contrast, hollow fiber filtration did not require added chemicals or coagulant, and as another advantage, the filtrate was successfully reused as culture medium without apparent detrimental effects on cell size, cell shape, or cell production. When high salinity stress was imposed on the concentrate produced from the filtration method, total fatty acids (FAs) did not increase. However, total FAs did significantly increase after hollow fiber filtration (49 %) in comparison to FA content before filtration (36 %). This research indicates that hollow fiber filtration as a commercial harvesting mechanism offers attractive advantages as a harvesting mechanism for microalgae such as Dunaliella, relative to pH-induced flocculation and indirect electrocoagulation.     

Maximizing the productivity of the microalgae <Emphasis Type="Italic">Scenedesmus</Emphasis> AMDD cultivated in a continuous photobioreactor using an online flow rate control

In this study, production of the microalga Scenedesmus AMDD in a 300 L continuous flow photobioreactor was maximized using an online flow (dilution rate) control algorithm. To enable online control, biomass concentration was estimated in real time by measuring chlorophyll-related culture fluorescence. A simple microalgae growth model was developed and used to solve the optimization problem aimed at maximizing the photobioreactor productivity. When optimally controlled, Scenedesmus AMDD culture demonstrated an average volumetric biomass productivity of 0.11 g L^?1 d^?1 over a 25 day cultivation period, equivalent to a 70 % performance improvement compared to the same photobioreactor operated as a turbidostat. The proposed approach for optimizing photobioreactor flow can be adapted to a broad range of microalgae cultivation systems.     

Comprehensive model of microalgae photosynthesis rate as a function of culture conditions in photobioreactors

In this paper, the influence of culture conditions (irradiance, temperature, pH, and dissolved oxygen) on the photosynthesis rate of Scenedesmus almeriensis cultures is analyzed. Short-run experiments were performed to study cell response to variations in culture conditions, which take place in changing environments such as outdoor photobioreactors. Experiments were performed by subjecting diluted samples of cells to different levels of irradiance, temperature, pH, and dissolved oxygen concentration. Results demonstrate the existence of photoinhibition phenomena at irradiances higher than 1,000 μE/m² s; in addition to reduced photosynthesis rates at inadequate temperatures or pH—the optimal values being 35 °C and 8, respectively. Moreover, photosynthesis rate reduction at dissolved oxygen concentrations above 20 mg/l is demonstrated. Data have been used to develop an integrated model based on considering the simultaneous influence of irradiance, temperature, pH, and dissolved oxygen. The model fits the experimental results in the range of culture conditions tested, and it was validated using data obtained by the simultaneous variation of two of the modified variables. Furthermore, the model fits experimental results obtained from an outdoor culture of S. almeriensis performed in an open raceway reactor. Results demonstrate that photosynthetic efficiency is modified as a function of culture conditions, and can be used to determine the proximity of culture conditions to optimal values. Optimal conditions found (T?=?35 °C, pH?=?8, dissolved oxygen concentration <20 mg/l) allows to maximize the use of light by the cells. The developed model is a powerful tool for the optimal design and management of microalgae-based processes, especially outdoors, where the cultures are subject to daily culture condition variations.     

Short-term evaluation of the photosynthetic activity of an alkaliphilic microalgae consortium in a novel tubular closed photobioreactor

A novel lab-scale tubular closed photobioreactor was developed and used for the assessment of the photosynthetic activity of an alkaliphilic microalgae mixed consortium under non-substrate limitation (i.e., bicarbonate excess), controlled irradiance, and mixing conditions. Two prominent haloalkaliphilic strains were identified as members of the consortium: Halospirulina sp. and Picochlorum sp. The photobioreactor (vol?=?0.5 L) consists of two interconnected U-shaped borosilicate glass tubes (internal diameter 2 cm) reaching a surface/volume ratio of 200 m² m^?3. This configuration specifically addressed the issue of the homogeneous light distribution among the microalgae suspended cells cultured by using fixed equidistant cool white light LEDs nearby the surface of the glass tubes. A soft homogeneous pneumatic mixing (i.e., airlift) was implemented in the culture fostering Reynolds numbers around 3000. The photosynthetic activity of the microalgae consortium was evaluated during different short-term kinetic assays by fitting the dynamics of the dissolved oxygen concentration to an oxygenic kinetic model. The photobioreactor operated in a closed loop allowed to control the produced oxygen by the extraction of the cumulated gas in the headspace. The use of this novel photobioreactor allowed the photosynthetic activity of microalgae suspended cells to be assessed, where the dissolved oxygen concentration and irradiance were the main parameters affecting the oxygenic rates under alkaline pH.