Exploring the Impact of Environmental Conditions and Bioreactors on Microalgae Growth and Applications

1. Introduction

Microalgae are unicellular, photosynthetic organisms with a variety of potential applications. For instance, they can serve as a renewable energy source and are efficient producers of compounds commonly utilized in a variety of fields, such as pharmaceuticals and nutraceuticals, as depicted in Figure 1 [1,2,3,4]. Which has made microalgae highly sought after in research and industrial capacities, and to improve sustainability.

One such function of microalgae is in environmental betterment. Microalgae can be utilized to remediate wastewater while recovering useful products such as pigments and biogas [5]. This methodology, which utilizes wastewater in place of a traditional growth medium for the recovery process, encourages a circular bioeconomy by taking urban and industrial food wastewater and cultivating microalgae with the ability to reclaim useful products [5]. Furthermore, due to the organisms’ photosynthetic status, their consumption of carbon dioxide can be utilized for CO₂ sequestering and simultaneous production of biomass [6]. The previously mentioned biomass productivity can include the synthesis of many appealing compounds.

Microalgae productivity encompasses the creation of lipids like polyunsaturated fatty acids (PUFAs) that can be used in the synthesis of biofuel or nutraceuticals [2,7]. For instance, Chlorella vulgaris can be the source of monounsaturated and saturated fatty acids that contribute to its potential in nutraceutical production [8]. Microalgae can also produce pigments like carotenoids, including compounds such as β-carotene, zeaxanthin, and astaxanthin, with high antioxidant capabilities [9,10,11]. As such, microalgae-based nutraceuticals have been synthesized to make use of microalgae extract’s healthy properties. For instance, compounds extracted from C. vulgaris exhibit a variety of health benefits, namely anti-obesity activity [2]. Furthermore, these bioproducts can be included in animal feed to improve the health of livestock. This has revealed it is possible to enhance the growth of domesticated chicken flocks under heat stress through the introduction of antioxidant-rich microalgae in their diet [12]. The cultivation of these photosynthetic organisms, thus, becomes a point of focus in obtaining these desirable microalgae bioproducts.

There are a variety of factors that impact microalgae growth and, occasionally, productivity. These include conditions such as light availability, temperature, pH, the rates of CO₂ in air flow, nitrogen supply, and carbon source [13,14,15,16]. Under a 12-hour dark and 12-hour light period, the biomass and growth rate of Nannochloropsis salina is less compared to cultures grown under constant light [13]. However, under the former conditions, it was noted that cells were more efficient during the 12-hour light period [13]. When grown under constant light, a larger productivity rate was also observed in Nannochloropsis QU130, a strain that has adapted to the harsh climate of the Qatar desert, compared to cultures grown under light cycles [15]. Additionally, in the same strain, fluctuating temperatures increased cell size compared to constant temperatures [15]. Wherein observing cells cultivated under varied temperatures and continuous light resulted in the highest productivity [15].

pH is also a consideration when cultivating microalgae. As more CO₂ is taken up by an increasing microalgae population, pH fluctuations occur, thus rendering it necessary to balance pH to keep the microalgae within a tolerable range [13]. The preferred pH for growth can vary depending on strain [17,18]. However, it has been observed that lipid productivity and accumulation are not significantly influenced by unideal pH environments in different microalgae strains [17,18].

In terms of airflow rates, 1L/h of 5% CO₂ supplemented air facilitated faster growth for N. salina, as cultures reached a plateau or the stationary phase at about 10 days [13]. Whereas at the lower rate of 0.25 L/h, the stationary phase occurred at approximately 17 days [13]. By increasing both the nitrogen and CO₂ sources, an approximately four-fold increase in biomass was observed [13]. Furthermore, in the nitrogen-deficient medium, 63% of N. salina dry weight was comprised of lipids [13]. Beyond air supply, the type of carbon source provided for microalgae cultures can also impact growth rates.

Some microalgae are mixotrophic and thus can utilize inorganic and organic forms of carbon and light for growth. An example is C. vulgaris [14]. In comparison to photoautotrophic growth, mixotrophy can increase productivity, which is dependent on microalgae species and other growth factors [14,19]. To elucidate the impact of mixotrophy in microalgae, a mathematical model was designed to observe the photoautotrophic, heterotrophic, and mixotrophic growth of C. vulgaris [14]. This revealed, the organism prefers photoautotrophic methods in a mixotrophic environment [14]. Therefore, to cater to specific growth and productivity rates, these growth factors must be adjusted accordingly.

Productivity can also be altered by culturing microalgae under stress conditions or extreme growth conditions. For instance, adjusting factors, like temperature, can increase the production of PUFAs [20]. Wherein, Nannochloropsis oculata and Isochrysis galbana were identified as microalgae species with high eicosapentaenoic acid (EPA) productivity at 20 °C and docosahexaenoic acid (DHA) productivity at 14 °C, respectively [20].

Light intensity can also be adjusted to increase growth rates. For instance, high light (360 µmol photons/(m²s)) increased growth approximately three-fold compared to low light conditions (6 µmol photons/(m²s)) for Nannochloropsis gaditana [21]. A similar result was observed in other microalgae species, where increases in light intensity invoked an approximately three-fold increase in fatty acid content in both Desmodesmus sp. and Scenedesmus obliquus after 15 days of cultivation [22]. Furthermore, under higher light intensity conditions, or 300 µE/m²s, the percentage of lipids within their biomass increased compared to the lower light intensity environment of 50 µE/m²s [22].

Nutritional deficiencies and unideal salt concentrations are also stress factors related to growth in microalgae [23]. Wherein under salinity stress, three strains of freshwater microalgae, Ankistrodesmus braunii, Ankistrodesmus falcatus, and Scenedesmus incrassatulus, have heightened lipid content present in their dry biomass [23]. With the addition of varying temperatures and nitrogen supply, the accumulation of lipids increased in Xanthonema hormidioides [24]. Where with a nitrogen concentration at 3 mM and temperature at 25 °C, lipid content within microalgal dry weight increased to its highest at 57.49% dry weight [24]. However, achieving stress conditions is not a requirement to ensure high biomass and productivity.

Microalgae growth can also be assisted by providing optimal growth conditions, which can result in an increase in growth and biomass productivity. This was described by Josephine et al. for C. vulgaris. Which revealed that specific temperature, pH, salinity, and light provide the ideal conditions for microalgae growth [25]. Thus, to provide optimal growth conditions, cultivation systems or devices were introduced to provide these growth factors for both research and industrial purposes alike.

The most common microalgae cultivation systems can be divided into two categories: open systems, such as ponds, and closed systems, like bioreactors [26,27]. Open systems are generally susceptible to contamination, whether it is bacterial or cross-contamination [28]. There are systems that prevent the latter, but still detect concentrations of the former, such as the twin-layer solid-state photobioreactor [28]. Photobioreactors (PBRs) are an example of a closed cultivation system with an additional light module, and thus tend to be preferable for microalgae cultivation. They do not require as much space as open raceway ponds, and have a low possibility of contamination [26]. However, for an open system, like a raceway pond, space requirements and the possibility of contamination are higher [26]. However, it is important to note that, generally, other factors, such as maintenance and setup costs, can be more reasonable [26]. One of the limiting factors for microalgae growth in open raceway ponds is light, as natural light tends to be the only source of this key cultivation factor [16]. Thus, it can only be distributed from the sun, which requires harvesting to occur more often to inhibit negative responses to light attenuation [16]. Furthermore, closed systems, like a bubble column photobioreactor, can cultivate higher biomass yield and nitrogen intake in microalgae compared to an open high-rate pond (HRP) system [29]. However, in an HRP, the net energy ratio or energy efficiency is higher, rendering it more favorable in that aspect [29]. Therefore, dependent on the purpose of the microalgae culture, there are many growth systems that can be tailored for a desirable result.

However, for research purposes, it may be preferable to utilize PBRs, as they can cover a variety of roles. The first role largely involves microalgae cultivation. PBRs allow the controlled delivery of a variety of growth factors, including mass transfer and agitation of the culture, which allows for the ability to adjust these factors for or to observe a certain outcome [30]. For instance, by taking advantage of the device’s ability to control growth conditions, PBRs have been used to observe the impact of various cultivation conditions on productivity in various microalgal species, such as C. vulgaris and Nannochloropsis oceanica [31,32,33,34]. They can also play an indirect role by cultivating microalgae for the design of a model to optimize the organism for biofuel production [32]. This paper will, therefore, delve into the importance of PBRs and their design in microalgae research and industry.

2. Types and Uses of Bioreactors

PBRs come in various designs and capacities (Table 1). For instance, a Fibonacci-type PBR was designed and scaled up to cultivate Dunaliella salina in an extreme solar environment, where growth factors like temperature and pH were controlled within optimal values for the microalgae [35]. This design scaled the culture to 1250 L and increased biomass concentrations three-fold compared to the culture grown in the same conditions in a raceway pond [35]. There are also tubular and panel PBRs, which can increase the efficiency of microalgae biomass productivity in the aerobic phase and biohydrogen production phase, respectively, for Chlamydomonas reinhardtii [36].

In an outdoor photobioreactor with a capacity of 50 L, C. vulgaris FSP-E was grown to optimize the production of protein in the microbial species to lower protein production costs for a fishmeal alternative [31]. A biomass productivity level of 268.1 mg/L/d and the protein productivity level of 155.4 mg/L/d were achieved with this cultivation system [31]. Analysis of the synthesized proteins validated the potential of utilizing the species as a feedstock for the production of a fishmeal alternative [31]. Beyond research applications, PBRs can be used in larger-scale capacities.

PBRs also have diverse uses for industrial purposes. For large-scale wastewater treatment and biofuel production, a floating offshore PBR was designed to address a couple of identifiable issues with biofuel feedstock cultivation [37]. This included scaling up the culture and its longevity [37]. One key element of this PBR’s wastewater remediation function is the cultivation of polycultures, which has aided in the stability, efficiency, and consistency of the culture and its products [37]. Beyond microalgae, wastewater can also be utilized for the cultivation of other microbes, including Zoogloea [38]. These microbes can produce polyhydroxyalkanoates (PHA), a biopolymer studied as an alternative for common plastics, but have not yet been widely adopted due to high production costs [38,49]. As such, a model was designed to predict microbe growth and PHA production in rice winery wastewater with varying organic loading rates (OLRs), which is related to microbe metabolism and productivity [38]. As an average OLR improved PHA productivity [38]. Beyond wastewater treatment, PBRs can also be involved in the mitigation of CO₂.

When scaling up Tetraselmis sp. CTP4, tubular PBRs were tested for industrial-scale cultivation [39]. The productivity of photosynthetic microalgae is tied to CO₂. As such, CO₂ mitigation is a factor observed when considering the success of microalgae. Within a tubular PBR, CO₂ mitigation efficiency reached 65%, contributing to the potential of the species for industrial-sized production [39]. Additionally, to improve biomass, this system can be used during the spring and summer months or modified to provide mixotrophic growth conditions [39]. In addition to industrial-scale PBR usage, microalgae applications in industrial capacities have been explored.

Prospective industrial applications of microalgae have been identified with the use of PBRs. This has been achieved through the production of PBR models to simulate microalgae growth and productivity in various climates [40]. Through studying Microchloropsis salina, such a model was created for open thin-layer cascade (TLC) PBRs by observing the species’ lipid productivity while utilizing two scalable TLC PBRs [40]. Through microalgae cultivation in the TLC PBRs and an accompanying model, the potential of M. salina for full-scale productivity of lipids on an industrial scale was revealed and confirmed [40]. However, these examples describe a fraction of bioreactors’ contribution to the research of microbes.

Bioreactors have diverse functions in microbial research, as they can also play an additional role in research by cultivating transgenic organisms. A genetically engineered strain of Streptococcus equi subsp. zooepidemicus was cultivated in a 3 L bioreactor to observe the strain’s ability to produce chondroitin, a precursor of chondroitin sulfate, and hyaluronic acid [44]. The latter two compounds are glycosaminoglycans, which are used in the cosmetic and pharmaceutical industries [44]. The inclusion of genes from Escherichia coli known to be associated with the synthesis of a chondroitin-like sugar, kfoC and kfoA, gave the recombinant S. equi subsp. zooepidemicus, the ability to produce both chondroitin and hyaluronic acid [44]. The 3 L bioreactors were utilized in the cultivation of the recombinant bacteria and, therefore, the production of desired compounds [44]. Bioreactors have also been involved in the larger-scale production of transgenic organisms. Saccharomyces cerevisiae was genetically engineered to produce high amounts of tyrosol and salidroside, used in the production of cosmetics, nutraceuticals, and pharmaceuticals [41]. In terms of bioreactor usage, a 5 L bioreactor was used to scale up the productivity of S. cerevisiae from shake flasks [41]. With this method, the yeast species was able to produce a high amount of the desired products [41].

A bioreactor was also utilized to increase lipid productivity, squalene, and DHA in Aurantiochytrium sp. T66 [46]. The latter of which has nutritional benefits for humans when incorporated into the diet [46,50,51]. Moreover, squalene is the precursor to human steroids, the production of which makes Aurantiochytrium sp. a good candidate for the industrial production of nutraceuticals, cosmetics, and pharmaceuticals [46]. After cultivation in a bioreactor, total lipid concentration was 5.90 g/L, where DHA made up 35.76%, and squalene yield increased by approximately 0.28 g/L in comparison to flask-cultivated microbes [46].

Bioreactors can also be used to cultivate plant cell lineswith the ability to produce compounds with various applications in cosmetics, nutraceuticals, and food coloring [45]. This plant biomass, from the red carrot R4G cell line, was observed to contain high concentrations of anthocyanins, which may point to its use as a food colorant [45]. Furthermore, the carrot cells’ biomass had a variety of health benefits in mouse cells, including anti-aging, anti-inflammatory, and antioxidant activity [45]. These results further identify the impact of bioreactors on research for the production of a variety of useful products, the diverse applications of which are noted in Figure 2.

When scaling up the production of microalgae with photobioreactors, several issues may arise (Table 2). Notably, keeping the process low-cost, culture collapse, and contamination [33,43]. However, to address the latter two, a two-stage process was created with C. vulgaris culture [33]. The first stage of this methodology requires the cultivation of the microalgae species in fermenters [33]. Stage two is inoculating the culture in a larger capacity flat panel PBR with the ability to culture 1000 L of microalgae [33]. These steps describe a heterotrophic method of scaling up [33]. By utilizing the autotrophic growth method, growing 1000 L of C. vulgaris would take approximately 35 days, whereas the previously mentioned method requires approximately five days [33]. Furthermore, the heterotrophic method essentially decreases the time and space required for scaled-up microalgae production in PBRs [33]. Therefore, this emphasizes the importance of identifying the ideal growth system for an intended purpose, whether that be for research or industry.

3. Photobioreactors for Energy

Currently, there is interest in increasing sustainability through the production of biofuels sourced from microbes like microalgae, illustrated in Figure 3. However, there are several challenges that have inhibited the usage of the product worldwide [42]. For instance, identifying the microbe species to be used as feedstock and determining its success in scaled-up systems can be difficult [42].

Biodiesel quality is correlated with the concentration of fatty acid type, which plays a role in the biofuel’s characteristics, such as cold-flow properties [48]. This makes it necessary to design an efficient method for microalgae cultivation to maximize ideal lipid concentrations for biodiesel production [48]. Which was performed through validation of Coelastrum sp. SM cultivated in an air-lift PBR as a feedstock for biofuel and fixation of CO₂ [48]. Wherein analysis recorded that the highest lipid content reached 37.91% of dry weight and the highest carbohydrate content was 58.45% of dry weight. Furthermore, the culture had a CO₂ fixation rate of 0.302 g/L*h, confirming the utility of the species in biodiesel production and CO₂ fixation [48].

CO₂ fixation has also been recorded in C. vulgaris cultivated in tubular PBRs [34]. The system was tested for its ability to scale up the microalgae culture to 100 L while utilizing sodium bicarbonate as a CO₂ source [34]. As a result, lipid concentrations increased to about 26%, and CO₂ fixation was approximately 0.925 g/L*d [34]. As the maximum concentration of sodium bicarbonate to completely increase lipid productivity has not yet been elucidated, we can only conclude that lipid productivity and sodium bicarbonate concentration are directly related, wherein an increase in the latter results in the increase of the former [34].

To produce biodiesel or biofuel, microalgae’s lipid productivity is a point of interest. In an open system, a raceway pond, Nannochloropsis sp. KMMCC 290 was cultivated to produce biodiesel [1]. In a flat-plate photobioreactor (FPP) under control conditions, or with a light intensity of 5,800 lux and continuous air supply with no carbon dioxide, lipid content was higher than in the raceway pond [1]. Whereas, under increased light intensity, gas exchange rates, and CO₂ supply, lipid productivity was about 16.6-fold higher than in the raceway pond [1]. At the conclusion of this research, the productivity in different systems, such as a bubble column PBR and air-lift PBR, provided varying productivity rates, thus further pointing to the importance of identifying the optimal growth system for a desired result [1]. Another species, S. obliquus, was cultivated in a plastic-type flat panel PBR with a volume of 5 L [47]. When the nitrogen source for the culture was urea and light intensity reached 3000 lux with white-colored, fluorescent bulbs, the resulting culture’s dry biomass contained 40% lipids [47]. Furthermore, 66.6% of lipids were unsaturated fatty acids, which makes the species ideal for biofuel production [47]. The potential of microalgae species for alternative energy sources has also been realized through the production of models.

A computational fluid dynamic model was utilized to experimentally scale up a mutant cell line of C. reinhardtii by simulating varying sparger designs in a 120 L flat-plate photobioreactor [42]. The model predicted a biomass productivity increase of 18% in C. reinhardtii grown within the optimized closed PBR [42]. These studies have revealed the importance of PBRs in the synthesis of renewable energy sources utilizing the productivity of microalgae.

4. Incubation Factors for Photobioreactors

An effective photobioreactor design must take into consideration a variety of growth factors, such as hydrodynamics, light, growth kinetics, agitation, nutrient supply, and gas exchange (Table 3, Figure 4) [53].

Light is an important factor for microalgae growth. For example, a light intensity of 350 μmol/m²s, results in the highest productivity of carbohydrates at 48.11 gC/m³d for I. galbana [57].

Light intensity, gas exchange, and CO₂ supply impact cell count in Nannochloropsis sp. KMMCC 290 cultures [1]. Increasing the growth factors previously listed can improve lipid content in cells [1]. Furthermore, light cycles can inhibit cell replication and stress cells, which induces an increase in lipid production [1]. Light as a growth factor, can be controlled with cultivation systems.

The thickness of a flat plate photobioreactor (FPP) impacts the light intensity received by the algae culture in the device [55]. The thinner the FPP, or the smaller the light path, the higher the lipid productivity and content in cells [55]. However, for a light path of 5 cm compared with the 10 cm light path, values such as growth rate, cell dry weight, and biomass productivity, were lower [55]. Thus, it is important to consider light intensity for the purpose of providing ideal growth conditions.

In addition to light intensity, cell density, growth medium, and light cycles also influence microalgae growth [56]. For Chlorococcum sp., saline water, 2500-3500 lux, and growth under 24 hours of light, resulted in optimal growth of the species on the fifth day [56]. However, this growth rate additionally relies on cell density, wherein low initial cell densities have a longer death phase after the 11^th day of growth [56].

Temperature conditions are also an important consideration for microalgae cultivation. Under high-temperature stress, at 30 °C, lipid productivity improved in a mixed microalgae culture collected from the Nacharam Cheruvu in India [62]. Furthermore, analysis of lipid content noted a high concentration of saturated fatty acids due to stress phase growth under the mentioned temperature conditions [62]. At 25 °C, the growth rate was higher than hotter temperatures, for Chaetoceros sp. FIKU035 and Nannochloropsis sp. FIKU036 [54]. Whereas Tetraselmis suecica FIKU032 had a slightly higher growth rate at 30 °C, and could not be cultivated in higher temperatures [54]. Variability was also observed in biomass productivity and concentration [54]. Where these values were the highest for Nannochloropsis sp. FIKU036 cultivated at 25 °C, and the highest for Chaetoceros sp. FIKU035 and T. suecica FIKU032 at 30 °C [54].

Nutrient supply is highly involved with productivity in microalgae [60]. In freshwater with fertilizer, Tetradesmus almeriensis cultivated in a pilot-scale thin-layer cascade PBR had the most biomass productivity of 30.3 g/m²*day [60]. Whereas when the microalgae were grown in wastewater, protein and lipid productivity increased [60]. When Chlorella sp. GN1 was grown in nitrogen deprivation conditions, the lipid content increased in dry cell weight compared to phosphorous limitation or nitrogen and phosphorous sufficient conditions [55]. Under nitrogen-deprived conditions, lipid productivity reached 63.5 mg/L*day [55]. However, lipid concentration under nutrient-limited conditions was lower than Chlorella sp. GN1 was grown in nitrogen and phosphorous-sufficient conditions after eight days of growth [55]. Nutrient availability also impacts the productivity of other beneficial compounds in microalgae.

Fucoxanthin is a pigment with a variety of health benefits, including as an antioxidant, which has made the compound highly sought after to produce nutraceuticals and pharmaceuticals [3,4]. As such, methods to optimize the productivity of fucoxanthin have been studied [4]. This has revealed that providing high nitrogen levels in the initial f/2 growth medium for Phaeodactylum tricornutum can result in high productivity thus, increase biomass and concentrations of fucoxanthin [4]. Furthermore, high nitrogen media and low light conditions can increase fucoxanthin production [4]. This points to both growth factors attributing to the fucoxanthin production process in P. tricornutum [4].

Higher air flow correlated with higher cell density in Tisochrysis lutea cultivated in a bench-scale air-lift PBR [61]. Generally, for column-style photobioreactors, air supply has a positive correlation with gas exchange, as a result of each microalgae cell within the culture receiving more light exposure [61]. Furthermore, reducing hydrodynamic stress, caused by factors like shear stress, can further increase cell growth [61].

Different microalgae species may react to hydrodynamic stress differently [63]. Both shear rate and shear stress are values associated with calculating shear forces [64]. High shear stress and shear rate can damage cells, and microalgae’s reaction and resilience to such stress varies by strain [64]. However, shear stress near the walls of closed growth systems is necessary to inhibit the growth of a biofilm [65]. Biofilm formation can prevent the microalgae culture from receiving light [65]. Thus, C. vulgaris biofilm prevention and removal through wall shear forces was pursued through cultivation in a flat-panel PBR [65]. It was revealed that 0.2 Pa of wall shear stress prevents biofilm formation, whereas 6 Pa can disrupt an established biofilm, and 53 Pa is necessary to remove it [65].

Shear force can also be utilized to increase mass transfer, thus improving microalgae’s CO₂ fixation [59]. This was achieved through applying shear force through water centrifugation to decrease the overall size of bubbles, or increase the bubbles’ surface area, within the microalgae culture [59]. This method increased biomass productivity by 50.7% [59]. However, high aeration creates high levels of shear stress, preventing biomass productivity of Arthrospira platensis in 2 L photobioreactors [66]. Thus, it is important to develop optimal growth settings to introduce reasonable levels of shear stress for the growing microalgae cultures.

Simulations of an air-lift photobioreactor predict that higher gas flow rates cause increases in photosynthetic efficiency [58]. This is due to the cells’ light exposure increasing as a result of introducing agitation [58]. However, this comes with a disclaimer, as high gas flow rates also cause high levels of shear stress, which decreases biomass [58]. Thus, for Porphyridium sp. the increased fluid mixing, a result of higher gas flow rates, also increases shear stress, causing cell damage,thus leading to decreased biomass productivity [58]. Additionally, challenges in microalgae cultivation come in the form of contamination.

Rotifers are predators of microalgae [67,68,69]. These organisms can decimate microalgae cultures in both open ponds and closed photobioreactors when introduced [68]. However, rotifer contamination is restricted in thin-layer cascade reactors, as their population density is limited in the system [68]. This notes that the impacts o[67,68,69f rotifer contamination vary in different growth systems [68].

The role of growth kinetic models is to convey how microalgae grow in specific environments within a time period [16,70]. As such, there are currently a variety of growth kinetic models available for microalgae species [16]. These models can point to the downsides of different growth methods, for instance, in open raceway ponds, growth kinetics have pointed toward the main issue of light attenuation [16]. One such model for an open raceway pond notes that systems around 30-35 cm are advantageous for outdoor growth, whereas values above this range are not [16].

One method to analyze cell growth kinetics involves maximum growth rate, a value calculated with the biomass concentration of the desired microalgae strain at a specific time period [66]. Additionally, cell productivity must also be calculated through the varied cell density over the cultivation period [66]. This method was utilized to calculate the growth kinetics of A. platensis in PBRs [66]. Growth rate kinetics can also be calculated with the Contois Equation, the differential of substrate concentration with consideration to time, and the differential of carbohydrate concentration with consideration to time [71]. This method can reveal the growth rate of microalgae cultivated using thin-layer photobioreactors in different mediums [71].

Furthermore, different growth kinetic models fit with experimental data [70]. For instance, the Gompertz model is more accurate than the Logistic model in determining the cultivation of Scenedesmus parvus in a PBR [70]. Both of these models utilize the same variables to calculate growth kinetics, such as initial and largest biomass, growth rate, and time, however, are arranged differently to gain varying results [70]. As a result, the more accurate model can be utilized to scale up cultures for industrial purposes in the future [70].

Growth kinetics can also model growth limitations reliant on dissolved inorganic carbon (DIC) concentration in the microalgae culture [72]. Wherein low levels of DIC, or limited carbon, can result in a decrease in growth kinetics [72]. However, this model has not been developed to consider the impact of mixotrophic growth on CO₂ remediation by microalgae cultures [72]. Thus, growth kinetics can also assist with determining the ideal growth factors and system for microalgae cultivation.

5. Challenges and Perspectives

The cultivation of microalgae heavily relies upon growth conditions, wherein, the delivery of such factors can be dictated through the usage of cultivation systems [26,30]. These systems, categorized as either open or closed, differ greatly in their characteristics [26,27]. Generally, open systems have a higher risk of contamination from either other eukaryotic species or predators of microalgae, but can cater to larger volumes [26,28]. As such, for monoculture of smaller volumes, a PBR may be preferable, as the system provides the ability to deliver controlled growth conditions such as light and temperature. Of which these qualities in an open system, like a raceway pond, are more difficult to control [16,39]. Additionally, the usage of closed systems has a lower risk of contamination [26]. However, there are some recognizable challenges that PBRs face, which have been mainly tied to scaling up cultures [33,43]. Where production costs and limited cultivation volumes of PBRs pose issues [43]. Furthermore, large-volume outdoor PBRs are exposed to seasonal changes and natural light, which can vary drastically as time passes [35,39]. Thus, these challenges emphasize a need for more diverse PBR designs that can address some of the downsides of current models available. Presently, this has been pursued, as there are some PBR designs that are slightly more favorable for larger-scale microalgae production [33,35].

However, these models further confirm that there is a wide range of considerations when identifying or designing an ideal microalgae growth system. One must take into account the purpose of the final microalgae culture, whether that be for research or industrial purposes. The scale of the product and, if applicable, desired enhancements in productivity. This will assist in determining the type of microalgae growth system and the conditions required to induce desired biomass accumulation and productivity rates. For microalgae research, many of these considerations are addressed through the usage of PBRs that have allowed finer control of various growth factors.