Introduction and History of Commercial Algae Cultivation:

Algae’s usefulness in food, nutrition, and a variety of other industrial applications has long been recognized. With the rising demand for alternative energy sources, promising applications of algal biomass in biofuel generation have grown in significance. Apart from that, the issue of climate change and rising GHG emissions has driven the market for microalgae-based CO2 sequestration approaches in recent years. This has resulted in highly distinctive innovations in the algae business for improved mass culture. However, it is always interesting to examine the evolution and technological interventions that have occurred in the algae cultivation industry.  

People in several parts of the world have been consuming microalgae and seaweed as a major food source for many centuries. Around 2000 years ago, the Chinese population solved the hunger issue by eating Nostoc, as documented by Janssen et al. in 2002. In 1519, Spanish scientist Hernando Cortez and Conquistadors discovered the microalgae Arthrospira platensis (Spirulina). Initially, it was farmed in Mexico’s alkaline lakes and on the African continent, but now it is commercially grown and collected worldwide. Algae cultivation also occurred in other locations, such as Lake Texcoco in the Valley of Mexico, during the sixteenth century. In 1964-65, Jean Léonard reported the sale of greenish, edible cakes in the local markets of Fort-Lamy in Chad, Africa (Sow, S., & Ranjan, S. 2021). To promote economic recovery during World War II, Japan actively promoted the consumption of proteinaceous algae.

Market of Microalgae

Despite this extensive history, researchers have only domesticated and produced a few wild microalgae species for human food and/or usage. These include Arthrospira platensis (Spirulina), Chlorella vulgaris (Chlorella), and Aphanizomenon. Until a few decades ago, nature’s water bodies rich in naturally growing microalgae were the only sources of consumable microalgae. However, microalgae culture, or artificial farming, is now performed worldwide, and the development of biotechnology to cultivate microalgae began in the middle of the last century (Wang, Y., et al 2021, Spolaore et al. 2006).

According to the research conducted by Dogma Jr. et al. in 1990, Asian countries have always been well known for their significant algae production for food purposes. In the 1980s, Naylor (1976) determined the production of seaweed by various Asian countries. Their estimate revealed that Japan, China, and Korea ranked at the top of the producers list, collectively producing 1,733,500.0 mt of seaweed. Conversely, the Philippines, Indonesia, and India were identified as low producers, with a combined production of approximately 28,000.0 mt of seaweed. During the 1980s, these Asian countries emerged as the major seaweed producers.

Development of the Algae Industry:

In the decades from 1940 onward the development of the algae cultivation industry occurred in the following chronology:

• The year 1940-1950 – Beginning of commercial algae cultivation projects in European and Asian countries.
• The year 1950-1960 – The issues first encountered and resolved into the practical aspects of mass cultivation involved essential nutrients, vigorous mixing, and the supply of CO₂.
• The year 1960-1970 – Development in the design and operation of open ponds for mass cultivation and protein-rich microalgal biomass.
• The year 1970-1980 – R&D for algae for biofuels and Production of algal biomass on a large scale. Growth of the market for algae-based products in Western countries.
• The year 1980-1990 – Research on emerging products from algae biomass in pharmaceuticals, nutrients, cosmetics, biofuels, etc. Government agencies started supporting R&D programs for biofuels. 
• The year 1990-2000 – Development of a novel product market for algae-based products and commercial production of algal biomass for food, nutrition, and pharma applications.
• The year 2000-2010 – Researchers primarily focused on researching mass algae cultivation based on photobioreactors and hybrid cultivation systems. They successfully developed novel algae cultivation systems and demonstrated their potential.
• The year 2010-2020 – Development of newer industries in the algae sector. Importance algae in air and water treatment introduced along with biofuel production.
• The current decade (2020-2030) – Novel emerging technology development for algae-based climate mitigation solutions is taking place. Emerging innovators and investors started developing GHG mitigation solutions along with multiple biofuels and cleantech options. 

World Scenario of Algae Technology:

Human diets, functional components, cosmeceuticals, medicines, animal and aquaculture feeds, fatty acids, alginates, carotenoids, wastewater treatment, and biofuels are just a few of the biological and industrial applications for microalgal species. Companies sell cyanobacteria Arthrospira sp., and Chlorella vulgaris not only as protein-rich food ingredients and supplements but also as functional foods, and they generally regard them as safe (GRAS). Because of their high vitamin, mineral, and carotenoid concentrations. While annual global microalgae production is currently modest (5.0 104 tonnes dry matter) in comparison to macroalgae (seaweeds) production (7.5 106 tonnes dry matter), microalgae biomass and bio-actives extracted from it are of great nutritional and economic importance (values at USD 1.25 109 annually).

Importantly, researchers have yet to research, let alone commercialize, the vast majority of the tens of thousands of distinct species that make up the big, polyphyletic group of microalgae. As a result, there is enormous potential in researching and developing microalgae as sources of high-quality, sustainable protein for human meals and dietary supplements (www.bsb.murdoch.edu, McGill 2008, Javed and Farooq, 2013, Sankpa and Naikwade, 2013, Sarwer, A., et al. 2022).

Algae cultivation systems:

Algae can be cultivated in various types of systems, ranging from simple open ponds to complex photobioreactors. These systems have the potential to revolutionize the way we produce algae-based food, energy, and products while mitigating some of the environmental impacts associated with traditional production methods. In this article, we will explore various types of algae cultivation systems, and how they work on a large scale.

Algae cultivation systems are vessels that hold water as a medium and nourish algae by exposing them to plenty of light, which facilitates photosynthesis.

The basic algae cultivation system is equipped with:

• Growth Vessel: The cultivation vessel with a larger surface area to receive maximum light energy. (Pond Height ≤30cm, Tube ID≤20cm) (Yadala, S., & Cremaschi, S. 2016, Torzillo, G., & Chini Zittelli, G. 2015)
• Agitation: Mechanical or pneumatic way of vigorous agitation provided with baffles to avoid dead zones inside the system due to lack of proper agitation (de Souza Kirnev, P. C., et al. 2022)
• Light Source: Suitable Natural or Artificial light source supplying Photosynthetic Photon Flux Density in the range of 26−700 µmol photons m⁻² s⁻¹ (Maltsev, Y. et al. 2021)
• Nutrient Source: Providing necessary nutrients for large-scale algae cultivation projects is a challenging task. Algae Production utilizes mostly inorganic fossil-based NPK fertilizers for the same purpose, but the ever-increasing demand and decreased production could lead to fertilizer scarcity in the future.
• CO₂ Supplementation: Traditionally, photoautotrophs obtain their major carbon source from dissolved CO2, which researchers supply by pneumatically feeding ambient air mixed with or without pure industrial CO2 gas.

Researchers develop algae cultivation systems that range from laboratory-scale experimental studies to outdoor massive production, depending on the scale. For laboratory-scale systems, they mostly use borosilicate glass, which is readily autoclavable for aseptic experimental studies. Additionally, researchers can utilize outdoor cultivation vessels that range from a simple open pond to a complex closed photobioreactor, based on the requirements of the algae strain being grown and the desired final product. The flowchart below presents a clear idea of the types of algae cultivation systems, which researchers classify based on the construction scale and the light source they utilize.

Laboratory Scale Algae Cultivation Systems

The successful algae cultivation process starts from a laboratory-scale inoculum development process that involves the transfer of unialgal or even axenic culture of algae from a sterile environment to outdoor non-sterile conditions in a step-by-step manner. This involves the utilization of smaller-level sterile glass liquid vessels like tubes, flasks, and bottles to some liter capacity laboratory scale PBR systems (Borowiak, D., et al.2020). Apart from scaleup, the laboratory-scale algae cultivation system with artificial light illumination can be a highly useful tool for experimentation and research purposes. In order to ensure the system is both efficient and effective, it is important to consider certain factors such as the lighting system and the autoclavability of the equipment.

1. All kinds of glass vessels to grow liquid cultures in a sterile condition

They are preferred over other materials because of their transparency, durability, and ease of cleaning. Here are some common types of glass vessels used in algae cultivation:

Culture Tubes:

Culture tubes have a straighter shape and a smaller volume. These systems are utilized for growing small volumes of algae in a sterile environment, serving as the initial transition point for transferring algal cells from a spent solid growth medium to a fresh sterile liquid medium.

Erlenmeyer Flask:

This is a conical-shaped glass vessel with a narrow neck and a wider base. The narrow neck helps to reduce evaporation and contamination. Researchers commonly use Erlenmeyer flasks for small-scale cultures of algae in the laboratory. These flasks are typically either screw-capped or covered with cotton to ensure the maintenance of internal sterile conditions.

Carboys:

Carboys are large glass containers with narrow necks and wide bases. They are used for growing large volumes of algae in a sterile environment. They are particularly useful for industrial-scale algae cultivation.

Fernbach Flask:

Fernbach flasks have a wider base and a shallower depth than Erlenmeyer flasks. They are used for growing algae cultures that require a lot of aeration, such as those that produce high levels of oxygen.

Bell Jars:

Researchers use bell jars, which are glass domes, to cover Petri dishes or other small glass vessels. These bell jars are employed to establish a sterile environment for growing algae cultures on a small scale.

Regardless of the type of glass vessel utilized, it is crucial to ensure proper cleaning and sterilization before use to prevent contamination of the algae culture. Due to their small sizes, researchers can easily clean, wash, and sterilize them using laboratory equipment. Therefore, basic laboratory glassware has become an important vessel for algae cultivation. As the scale-up process progresses in algae cultivation, larger systems are required to accommodate higher volumes of algae. The required systems for algae cultivation include the following: (Wikipedia contributors. 2023, February 1).  

2. Cylindrical glass vessels

The cylindrical glass vessels, which are bioreactors made of glass, come equipped with the necessary provisions for agitation, temperature control, light illumination, aeration (CO2), and pH. These bioreactors provide a controlled environment for algae cultivation, allowing researchers to optimize the growth conditions and maximize productivity. They are generally of ≤20cm internal diameter making a light path length of 10cm when illuminated from all directions externally as per the standard recommended limit for light penetration. They are mostly illuminated externally with artificial light sources which can be regulated as per the need of the microalgal species growing in it.

This type of system is either mechanically or pneumatically agitated (or with the help of both options) depending upon the shear tolerance and aeration requirement of the microalgae in consideration. They provide optimal conditions for laboratory-scale studies to monitor the effect of light, temperature, and nutrients on algal biomass production as well as their primary and secondary metabolites. They can construct these cylindrical bioreactors to hold different volumes, ranging from milliliters to liters. However, for laboratory-scale studies, it is always operationally feasible to use cylindrical bioreactors with volumes less than 1L.

3. Glass Flat Panels

Glass Flat Panels are another essential type of laboratory-scale algae cultivation system. These panels serve as a thin clamber holding algae between two plates that offer a wide surface area for maximum light exposure for photosynthesis. The light path length through a flat panel is ≥10cm preferred when illuminated from one side. This could be doubled if illuminated from both sides. To ensure accurate experimentation, the panels must be able to withstand the conditions of the cultivation process, including exposure to artificial light and autoclaving.

Glass Panels for algae cultivation have the ability to withstand high temperatures and pressures during autoclaving. Therefore, researchers construct most flat panel systems using borosilicate glass materials and stainless-steel framing to create sturdy models for frequent sterilization processes. Due to its larger surface area, the flat panel system earns recognition for its exceptional light illumination capability. Researchers favor it for conducting experiments that examine the effects of varying light intensities and types on algal growth, lipid production, nutrient consumption rate, and photoinhibition studies.

Open Systems

An open system of algae cultivation involves the growth of algae in open shallow water streams which could be originating from the natural system or artificially prepared. In this system, researchers can cultivate algae in natural water bodies such as lakes, rivers, and oceans, as well as in artificial ponds constructed from concrete, plastic, pond liners, or a variety of materials. The open system of algae cultivation is simple and cost-effective, making it an attractive option for commercial production of algae-based products.

1. Water Lagoon

A lagoon, a type of aquatic ecosystem, is characterized by a shallow body of water that is separated from the open ocean using natural barriers such as sandbars, barrier islands, or coral reefs. An Australian company uses this algae cultivation system. Cognis Australia Pty Ltd is a well-known company that specializes in producing β-carotene from Dunaliella salina harvested from hypersaline extensive ponds located in Hutt Lagoon and Whyalla. These ponds are primarily used for wastewater treatment, and the production of D. salina is a secondary benefit (Spolaore, P., et al. 2006, Curtain, C. 2000, Campbell, P. K., et al. 2009).

2. Open Sea Cultivation (Seaweed)

Open sea cultivation is a method of cultivating seaweed in the open ocean, as well as on a coastal line in shallow water. The seaweed farming industry serves commercial needs for various products such as food, feed, pharma chemicals, cosmetics, biofuels, and bio-stimulants. Seaweed extracts act as bio-stimulants, reducing biotic stress and increasing crop production. Additionally, it presents opportunities for creating animal and human nutrition products that can improve immunity and productivity. Open ocean seaweed cultivation is an eco-friendly technology that doesn’t require land, fresh water, or chemicals. It also helps mitigate the effects of climate change by sequestering CO2.

The open sea cultivation method involves the use of rafts or ropes anchored in the ocean, where the seaweed grows attached to them. This method is widely used for commercial seaweed farming, as it allows for large-scale production and harvesting. The process of open sea cultivation of seaweed involves several steps. First, the intender identifies a suitable site in the ocean based on factors such as water depth, temperature, salinity, and nutrient availability. Once they choose a site, they anchor ropes or rafts in the water and attach seed pieces of seaweed to them using specialized equipment. The seaweed is then allowed to grow for several months, absorbing nutrients from the water and sunlight through photosynthesis. (Peteiro, C., et al. 2014).

3. Raceway Ponds

One of the most common and economic methods of large-scale algae cultivation is in open raceway ponds, which offer several advantages over other cultivation methods. An open raceway pond is a shallow, rectangular-shaped pond used for the cultivation of algae. Because it is designed to circulate water in a continuous loop or raceway, allowing algae to grow in a controlled environment. Open System is a low-cost method of algae cultivation, and it is relatively easy to construct and maintain. Researchers typically line the pond with a synthetic material, such as polyethylene (HDPE) or polyvinyl chloride, to prevent the loss of water and nutrients. They also equip the pond with paddlewheels or other types of mechanical devices to provide mixing and aeration (Klein, B., & Davis, R. 2022).

4. High Rate Algal Ponds (HRAPs)

High-Rate Algal Ponds (HRAPs) are a type of open algae cultivation system that has gained popularity in recent years due to their efficiency and low cost of operation. HRAPs (High Rate Algal Ponds) are shallow ponds, usually ranging from 0.1 to 0.4 meters deep, utilized for cultivating algae. Researchers equip these ponds with a paddlewheel or another type of mechanical agitation system to provide mixing and aeration, thereby facilitating algae growth. The use of HRAP systems is also recommended in wastewater treatment involving algae (Mehrabadi, A. et al. 2015, The Local Government Association-LGA of South Australia).  

5. Revolving Algal Biofilm Reactor (RABR)

The use of revolving disks of polystyrene to produce algal biofilm to reduce nitrogen and phosphorus in municipal wastewater was originally described in the 1980s. (Przytocka-Jusiak et al., 1984). The goal of this research was to create a rotating algal biofilm (RAB) growing method that microalgae producers could use to easily harvest biomass. Algal cells grew on the surface of a substance that alternated between a nutrient-rich liquid phase and a CO₂-rich gaseous phase. Scraping biomass from the connected surface saved costly harvesting operations like centrifugation. Cotton sheets outperformed all other attachment materials in terms of algal growth, durability, and cost-effectiveness. Harvest frequency, rotation speed, and CO₂ levels were further tuned in a lab-scale RAB system.

The water content of the algal biomass from the RAB system was comparable to that of centrifuged biomass. When compared to a control open pond, an open pond raceway retrofitted with a pilot-scale RAB system resulted in significantly higher biomass productivity. The research indicates that the RAB system is an efficient algal culture system for convenient biomass harvesting and increased biomass productivity (Gross, M., et al. 2013).

RAB Systems by Gross-Wen Technologies:

Currently, industries primarily perform algal cultivation in open ponds or photobioreactors, where they suspend algal cells and harvest them through flocculation and centrifugation. They recently developed a novel attachment-based Revolving Algal Biofilm (RAB) culture system that enables easier biomass harvest and increased biomass productivity. The goal of this study was to assess the efficacy of the RAB system at the pilot scale (durability, algae growth, and shape). The RAB system was successfully tested for a year at a greenhouse plant in Boone, Iowa, USA. The RAB increased biomass productivity by 302% on average when compared to a typical raceway pond, with a maximum biomass productivity (ash free) of 18.9 g/m2-day achieved. The vertical RAB outperformed the triangle RAB in terms of productivity. The research reveals that the RAB, as an efficient algal growing method, has a high potential for commercialization. (Gross, M., & Wen, Z. 2014)

Productivity:

Gross-Wen Technologies have devised a Revolving Algal Biofilm (RAB) growth system, where they bond algal cells to a flexible material that undergoes rotation between liquid and gas phases. In this work, they created different configurations of the RAB system at pilot size by retrofitting the attachment materials to a raceway pond with a 2000-L capacity and an 8.5 m2 footprint area, as well as a trough reservoir with a 150-L capacity and a 3.5 m2 footprint area. The trough-based RAB system has a maximum productivity of 46.8 g m-2 day-1.

The RAB system shows higher water evaporation loss, the specific water consumption per unit of biomass generated was only 26% (raceway-based RAB) and 7% (trough-based RAB) of that of the control pond. This research demonstrates that the RAB system is an efficient algal culture system with a high potential for commercially producing microalgae with high productivity and efficient water use. (Gross, M., Mascarenhas, V., & Wen, Z. 2015).

Closed System- Photobioreactors (PBRs)

Closed systems or photobioreactors (PBRs) are commonly used in algae cultivation systems due to their numerous advantages over open pond systems. These systems are more efficient in terms of land and water usage and also provide greater control over environmental conditions, such as temperature, light, and nutrient supply. Two common types of closed PBRs are tubular and flat panel systems, but there are various designs and models suggested by many for the cultivation of different microalgae on a mass scale (Znad, H. 2020).

1. Tubular PBRs

Tubular PBRs consist of long, transparent tubes that are either vertically or horizontally oriented. Algae are grown inside the tubes, which are typically made of glass or plastic. The tubes are arranged in a helical or serpentine pattern to increase surface area for light exposure. The tubing can be either continuously or intermittently circulated to promote mixing and prevent stagnation. Researchers generally use tubular PBRs (Photobioreactors) for high-density cultivation and find them well-suited for species that require high light intensity. Controlling the temperature in tubular PBRs is a challenging task, typically accomplished by externally sprinkling deionized water. This sprinkling allows the tubes to cool down, subsequently reducing the temperature of the culture circulating inside them (Torzillo, G., & Chini Zittelli, G. 2015).   

2. Flat panel PBRs

Flat panel PBRs (Photobioreactors) consist of researchers stacking a series of flat, transparent panels on top of each other to create a thin layer of liquid between them. They grow algae in this thin liquid layer while continuously circulating it to promote mixing and prevent stagnation. The panels, typically made of glass or plastic, can be arranged in various configurations to optimize light exposure. Researchers generally use flat panel PBRs for low-to-medium density cultivation and find them well-suited for species that require lower light intensity and maximum surface area for optimal light exposure. They control the temperature in the flat panel PBR system by cooling down the culture in the reservoir chamber using a chilled water jacket and by sprinkling cold water on the flat panel surface (Sierra, E. et al. 2008).

3. Plastic V-Shaped Bag

V-shaped plastic bags are commonly used in closed systems of algae cultivation for several reasons. These bags are made from high-density polyethylene (HDPE) and are designed to hold algae cultures in a closed environment, providing an ideal environment for algae growth. V-shaped plastic bags are effective for growing a variety of algae species, including Chlorella, Spirulina, and Nannochloropsis (Chen, Y. P., et al. 2021).

Chlorella vulgaris exhibited a higher growth rate and biomass yield when cultivated in V-shaped plastic bags compared to other-shaped plastic bags. Different designs of plastic bags based PBR are developed by sealing the plastic bags at different places that generated, flat bottom hanging plastic bags, V-shaped hanging plastic bags, horizontally laying plastic bags that serve the kind of flat PBR system, etc. Many plastics bag-based design is proposed but few are utilized on a commercial scale due to their productivity. The operation of plastic bags is tedious as they need to be replaced after every use to maintain sterility, which is a laborious task for large-scale facilities (Wang, B., et al. 2012, Huang, Q., et al. 2017).

Conclusion:

Algae cultivation technology is becoming an important commercial and economical asset in the algae mass cultivation industry. As the importance of different algal products is growing, their maas cultivation demand makes it necessary to improve and define newer cultivation concepts for significant production. The cultivation methods greatly influence the productivity of particular algae species and eventually, it affects the profit from the cultivation. Therefore, implementing a more economical, operationally feasible, sturdy, easy-to-maintain, and scalable algae cultivation system is becoming a critical aspect of microalgae cultivation practices.

For the growing market of microalgae-based products variety of commercial PBR systems are now coming into the market proving significant progress in this sector. For macroalgae or seaweed, open sea cultivation is becoming a way of the future for their cultivation, and lot many inventions from improving seaweed anchoring, maximum nutrients & CO2 absorption to reimplantation of lost coral reef is happening. Ultimately, the availability of plenty of coastal lines all over the world would play a very essential role in developing new concepts of sea farming of algae.

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