Microalgae Cultivation: History
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Microalgae is a renewable resource with potential value as a commodity and for combating climate change. Commercial cultivation can be done through open ponds or photobioreactors (PBRs), with open ponds being less expensive. PBRs offer potential better controls but are hindered by cost-related issues. Outdoor PBRs are a promising solution, but challenges related to relying on natural resources remain. Further research is needed to make PBRs commercially viable.

  • microalgae
  • outdoor cultivation
  • photobioreactor (PBR)
  • cultivation strategy

1. Introduction

Global warming has changed our perception of how we should view our lifestyles. It has driven us to re-evaluate our individual, social, national, and regional spheres, as well as our commercial and industrial practices. Policies, regulations, and practices to reduce greenhouse gas (GHG) build up in the atmosphere are becoming increasingly common around the world. Various strategies have been adopted to reduce GHG emissions and decrease their concentration in the atmosphere. While increasing forest cover has a crucial role in reducing CO2 in the atmosphere and halting deforestation is important, other options should also be considered and explored. More than two-thirds of the Earth’s surface is covered by oceans, which house numerous micro-organisms that have the potential to contribute to utilizing CO2 if their biomass can be utilized before decomposition. Commercial cultivation of microalgae has the potential to utilize atmospheric and flue gas CO2 as the produced algal biomass can be used for various applications [1,2]. Microalgae can play a crucial role in combating global warming and reducing the consumption of chemicals used in traditional gas upgrading plants [3,4]. However, microalgae are not currently being considered to play a significant role in addressing global warming.
Among all ocean organisms, microalgae are an essential component. They are unicellular plants that grow using carbon, water, and other nutrients through the process of photosynthesis. Microalgae cells are free of roots, stems, or leaves, and they uptake nutrients more efficiently and grow faster than terrestrial plants. Microscopic organisms, phytoplanktons, including microalgae, are essential for maintaining biodiversity in oceans, and with their high multiplying capability, they contribute to around 50% of the planet’s primary production [5]. Algal biomass is a source of protein, lipids, and carbohydrates, and depending on the species, they can be used to produce food, feed, fuel, bioplastics, cosmetics, and other value-added products [6,7].
Microalgae can be produced in properly designed cultivation units built on non-fertile or semi-fertile substrates where growing traditional plants and crops is challenging. Microalgae can grow in an environment where only a few nutrients are available. Although optimized algal growth cannot be achieved, microalgae accumulate different components in a nutrient-starved medium. For example, Rhodomonas sp. is shown to produce enhanced lipids in a nitrogen-starved medium [8]. By manipulating the nutrient ratios, the biochemical composition of microalgae can be altered. They can be grown commercially in indoor environments, as well as outdoors, such as on a roof. EnerGaia, a start-up from Bangkok, is an example of this. They grow Spirulina in 15 L to 20 L cylindrical photobioreactors (PBRs) on rooftops [9,10]. Another example is Algalif, an algal production facility in Iceland that produces natural astaxanthin products from microalgae and cultivates microalgae inside indoor tubular PBR systems [11].
Microalgae can be cultivated commercially using open ponds or closed PBRs. Open ponds are commonly used due to their lower cost and ease of maintenance. However, they lack the ability to facilitate complex operations, which are necessary for providing additional benefits. PBRs have the potential to fill this void, but their higher construction, operation, and maintenance costs pose a challenge for commercial use. Most laboratory-scale experiments on PBRs have been conducted indoors, where optimum control can be achieved but at a higher cost. Algal productivity in outdoor environments is typically lower than laboratory yields, as outdoor temperatures and light intensities are more variable and extreme. These environmental conditions are often not replicated when selecting strains indoors [12]. According to Dahlin et al. (2018), microalgae that are grown in outdoor environments have to cope with natural fluctuations in temperature and light conditions, making it challenging to identify strains that can be cultivated successfully in a controlled laboratory setting [13]. It is worth mentioning that most large-scale commercial microalgae production farms, including both open ponds and closed photobioreactors, are operated outdoors (personal communication). Despite this, few studies have been conducted on outdoor PBRs.

2. Drivers of Algae Research

The algae industry has demonstrated significant commercial potential in producing food, feed, biofuel, and biomaterials, which presents a great opportunity for achieving sustainability and decarbonization [14]. These advantages have garnered attention from policymakers worldwide. For instance, a study conducted on several experts and stakeholders stated that European Union policymakers are adapting regulations in favour of the algae market in Europe [14]. Additionally, consumers are opting for sustainable and environmentally friendly products more frequently, which has led to a greater demand for algae [14]. Consequently, the Blue Economy Forum is investigating gaps in financial support, lack of support due to proper policies and regulations, and market challenges [14]. In Europe, there are hardly any socio-cultural barriers to algae products and applications, except for genetically modified microalgae [15]. A new study revealed the potential of algae-based protein food as a viable alternative [16]. The study identified several important drivers for consumer acceptance, including health and nutritional benefits, taste, natural and fresh characteristics, organic or sustainable certification, and less environmental impact [16]. These drivers are encouraging the algae industry to expand further. For instance, the demand for algae products, such as Spirulina and Chlorella, is on the rise in Europe [15]. A study analysed data from multiple studies conducted in different regions and found that consumer perceptions varied across regions [16]. For instance, Australian respondents did not exhibit significant concerns about food safety and responsibility [17]. Food neophobia, lack of trust in the promoters, and perceived high cost were identified as major consumer concerns [16]. In order to address the concerns and meet the requirements of specific consumer groups, as well as attract new groups, further research is necessary.
Microalgae are gaining practical considerations in national and regional policies of North America, the Asia Pacific, and Australia, as well as being used as a tool to combat climate change and promote renewable resources of food, feed, and fuel [18,19,20]. Microalgae are already present in the North American and Australian markets, and attempts are being made to diversify their uses. In the Asia Pacific region, markets are being assessed, feasibility studies are being conducted, and even pilot-scale experiments collaborating with the private sector have been carried out [19,20]. However, algal research faces barriers in all regions, particularly in developing countries. The predictability of the yield of microalgae, as well as the overall cost and energy efficiency of production, are still questionable. The uncertainties, followed by the immature status of the current algal market, have played a significant role in deterring investors from funding new microalgal projects. This is one of the dominant factors hindering the growth of the algal market in developing countries [19].
The majority of research activities related to the microalgae market applications have been dedicated to six major market opportunities: pharmaceuticals, biofertilizers, biofuels, nutraceuticals, bioplastics, and cosmetics [21]. European experts believe that these four sectors will play a major role in the commercialization of microalgae in the near future. These sectors include food (such as protein and polysaccharide production), bioremediation and biofertilizers, the invention and commercialization of antibiotics and antimicrobial compounds, and the production of vaccines and recombinant proteins [15]. A recent study on the European Atlantic algal market identified key obstacles to industrializing microalgae, including the need to strengthen existing growth and production systems for effective technology transfer from R&D platforms to the industrial sector, the absence of information on reproducible levels of bulk production, and the challenge of ensuring sustainable long-term production. Addressing these obstacles was deemed crucial to promoting the industrialization of microalgae [15]. Other major barriers include the lack of a financially efficient commercial metabolite extraction and purification system, energy and cost-efficient biomass harvesting techniques, and innovative and energy-efficient production processes [15]. Other areas, such as biomass production and harvesting, extraction and purification, PBR design, regulation, and certification, and scale-up of production systems, were identified as medium-speed evolving areas [15].

3. Trends and Concerns of Outdoor Algal Cultivation Methods

3.1. Ideal vs. Non-Ideal Conditions

Ideal PBRs are characterized by optimal algal productivity, which cannot be surpassed by any practical technology. The concept of an ideal PBR postulates that its productivity is impervious to any modification or adaptation concerning radiation, as ideal PBRs do not consider productivity to be a function of geographical location but rather depend on the highest solar radiation available on Earth [22]. However, solar intensity fluctuations and deviations from ideal conditions result in lower performance for a given location. The location of interest is also impacted by varying meteorological conditions, induced shading by other PBRs, differing lengths of daytime, and light intensity fluctuations at different periods of the day [22]. Furthermore, the efficiency of radiation utilization by microalgal cells, which is influenced by biological kinetics, is directly affected by temperature, pH, harvesting strategies, and other experimental factors [22]. A non-ideal or real PBR has to contend with these factors daily.

3.2. Challenges and Limitations of Outdoor Experimentations

Using naturally available resources to cultivate microalgae can reduce costs. However, unpredictable outdoor weather makes algal growth uncertain, which is a concern for commercial production. Outdoor cultivation requires the selected algal strain to not only be a competitive feedstock for value-added chemicals but also to adapt to diurnal changes. The majority of current studies have been conducted indoors because exposure to complete outdoor conditions increases the risk of contamination with viruses, bacteria, fungi, insect pupae, rotifers, protozoa, or other unwanted algae [23,24]. The simple body structure and fast reproduction capability of rotifers and protozoa allow them to rapidly colonize an ecosystem [24]. Especially when investigating new species, close-to-ideal experimental conditions are necessary to determine their optimal cultivation parameters. Properly controlled indoor experiments better serve this purpose. Indoor cultivations provide information about the optimized culture conditions of a strain, but outdoor cultivation is necessary to overcome the challenge of commercial applications.

3.3. Batch Culture

The structural simplicity, flexibility in uses, and low construction and operation costs make batch microalgae culture systems a popular choice among researchers worldwide [25]. Batch production techniques are widely used for algal cultivation due to their ability to accommodate different types of experiments, such as nutrient limitation experiments, more conveniently than continuous mode. Each batch cycle requires starting from a pre-stock culture and typically has a shorter cycle period that allows for lower sanitary demands [25]. The shorter cycle time does not allow predatory or competing organisms to achieve high concentrations [26]. Although scaling-up benchtop PBRs to an industrial scale remains a challenge, research institutes prefer batch culture. As a relatively new field of research, many research institutes and universities have recently initiated algal research. Like all other new research initiatives, they face the challenge of limited resources to conduct research and train new researchers. Batch experiments play an important role in exploring new knowledge of microalgae within limited resources.

3.4. Online Monitoring Tool

Improving the commercial cultivation of microalgae requires significant effort, including better process control and the acquisition and study of real-time data. One way to achieve this is by developing online monitoring tools. In the laboratory, all analyses and observations are based on physical procedures, where samples are collected from the production units and later analysed. Some of these analyses are laborious and time-consuming. Offline analysis systems do not provide many facilities, such as knowing when the highest biomass or any particular metabolite yield has been achieved. Among the spectrophotometric technologies, fluorescence spectroscopy has been recommended as an effective real-time monitoring tool for simultaneously and noninvasively monitoring several compounds in microalgae production [27,28,29,30,31,32].

3.5. Virtual Laboratory

A study introduced the concept of a virtual laboratory designed for outdoor algal cultivation, which was researched by the Department of Chemical Engineering at the University of Almeria, Spain [33]. The virtual laboratory can serve both teaching and research purposes regarding PBR systems. It is intended to aid in comprehending the interrelationship between various parameters of a PBR. Given that hydrodynamic and geometric parameters are critical decision makers for the algal productivity of a PBR, this interactive tool can analyse the performance of a particular PBR design [33]. Moreover, the virtual laboratory can help to identify and address complications during microalgal production and optimize PBR design. Multiple controls over the growth environment of the algal cultivation unit can be manipulated, resulting in a compelling way of achieving the highest productivity [33].
According to the study, the interactive virtual laboratory is capable of (a) “accurately reproducing the structure of a real plant”, (b) “simulating a generic tubular PBR by changing the PBR geometry”, (c) “simulating the effects of different operating parameters, such as culture conditions (pH, biomass concentration, dissolved O2, injected CO2, etc.)”, (d) “simulating the PBR in its environmental context, by changing the geographic location of the system or the solar irradiation profile”, (e) “applying different control strategies to adjust various variables such as CO2 injection, culture circulation rate, or culture temperature to maximize biomass production”, and (f) “simulating the harvesting process”. The simulator was developed using “Easy Java Simulations”, a tool specifically designed for interactive dynamic simulations [33].

This entry is adapted from the peer-reviewed paper 10.3390/en16073105

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