Algal Cultivation – Methods, Factors, Feature, Types, Uses

Algal cultivation means growing algae (microalgae or seaweeds) in controlled or partly controlled environments for useful biomass.

The algae are supplied with light, carbon (often CO₂), water and nutrients so growth is promoted.

In open ponds (raceway ponds etc.), cultures are exposed to ambient conditions, and mixing is provided to avoid shading and settling.

Photobioreactors (closed systems) are used so contamination is reduced, conditions can be tightly managed.

Growth rate is affected by light intensity, nutrient concentration, temperature, pH, mixing, and CO₂ supply.

Once sufficient biomass is produced, algae are harvested by methods such as centrifugation, filtration, flocculation.

Biomass is processed to extract desired compounds (lipids, proteins, pigments) or used directly.

Cultivation is used for biofuels, food, feed, pharmaceuticals, wastewater treatment, carbon capture, and more.

Challenges include cost, controlling contamination, ensuring consistent conditions, and energy demands.

Algal Cultivation Methods

Algae are cultivated by these following methods;

1. Open / external systems

• In open ponds, algae are grown in shallow water bodies exposed to natural sunlight, and mixing is provided so that light / nutrients reach the cells.
• Raceway ponds are among the most used open systems, where paddle wheels force water circulation in a loop which keeps algae suspended.
• High-rate algal ponds (HRAPs) are modified raceway ponds with optimized flow, mixing, maybe CO₂ injection; better productivity is sought, but still, they are subject to environmental variation.
• Seaweed (macroalgae) can be cultivated in open sea using ropes, rafts, nets, long lines or bottom planting; seedlings or cuttings are tied to structure and grown in natural waters.

2. Closed / enclosed systems (Photobioreactors, PBRs)

• In photobioreactors, algae are cultivated in transparent vessels so light enters and unwanted contamination is limited.
• Flat-panel PBRs use flat transparent panels (glass or plastic) so that culture remains thin and light penetration is good.
• Tubular PBRs consist of transparent tubes (horizontal, vertical, coiled) through which culture circulates continuously, exposing algae to CO₂, nutrients, and light.
• Plastic bag / flexible PBRs: algae are grown in sealed clear plastic bags (e.g. hanging or laid flat) so that a closed environment is maintained albeit with trade-offs.
• Substrate / biofilm reactors: algae are allowed to adhere or grow on a surface or mesh so that they form biofilms instead of being fully suspended; in these systems harvesting may be easier.

3. Mixed / hybrid / alternative cultivation modes

• Autotrophic mode: algae use light + CO₂ + nutrients for their growth (typical in open and closed systems).
• Heterotrophic mode: algae grow in darkness using organic carbon sources (glucose, etc.) instead of light.
• Mixotrophic mode: algae combine both light and organic carbon uptake modes so that growth is supported by both photosynthesis and uptake of organics.

4. Other considerations / combinations

• Hybrid systems may integrate open ponds with closed PBR stages for better control or productivity.
• In closed systems, harvesting and maintaining sterility, degassing (removal of excess O₂), cooling (in case of overheating) are needed and are designed into the system.
• Light distribution, mixing, CO₂ supply, nutrient diffusion must be well managed in all systems so that growth is not limited by shading or stagnation.¹²³⁴

Open System Algal Cultivation Methods

• Open system algal cultivation is where algae are grown in ponds or natural waters, exposed to ambient weather and environment.
• Open ponds / raceway ponds are shallow loops or channels, in which algae, water and nutrients circulate by paddle wheels.
• In raceway ponds, mixing is provided so that algae do not settle, nutrients are spread, light is more uniformly used.
• High-Rate Algal Ponds (HRAPs) are variants of open ponds, designed to increase growth rate by improved mixing, shallow depth, sometimes CO₂ injection.
• In open systems, natural water bodies (lakes, lagoons) may also be used, with algae growing in existing ecosystems, though control is weak.
• The ponds are typically shallow (e.g. 20-40 cm deep) so that sunlight reaches most algal cells.
• Mixing / circulation devices such as paddle wheels, rotating arms, or impellers are used so flow is maintained.
• CO₂ is sometimes added, or flue gas bubbled, to maintain carbon supply and pH.
• Due to exposure, contamination by unwanted algae, bacteria, protozoa is a major issue, and control is limited.
• Also, temperature, light, evaporation, rainfall influence growth strongly in open systems, and are hard to regulate.
• Harvesting from open systems is harder because the cultures are dilute, so efficient separation (flocculation, filtration) is needed.⁵⁹

Factors influence

• Light availability / intensity is key — if light is weak or uneven, growth slows, and shading in deep water layers is a problem.
• Water depth matters — deeper ponds require more nutrients, and light fails to reach lower zones, so optimum depth is kept shallow.
• Nutrient concentrations (N, P, trace elements) influence growth; deficiency or imbalance reduces biomass yield.
• CO₂ / carbon source supply is crucial — diffusion from air may be insufficient, so supplementation or bubbling helps maintain carbon availability.
• Temperature (of the pond) affects metabolism and enzyme activity; when temperature is too low or too high, growth is impaired.
• pH / alkalinity control is needed, because CO₂ uptake, nutrient forms, and cell physiology depend on pH.
• Mixing / circulation is necessary — without mixing, cells settle, nutrients / light are not well distributed.
• Dissolved oxygen accumulation may inhibit photosynthesis if not relieved; gas exchange is a factor.
• Contamination / invasion by unwanted species is easy in open systems, so species competition influences yield.
• Meteorological / environmental factors (light changes, rain, evaporation, wind) strongly influence cultivation and must be anticipated.
• Seasonal variation / day-night cycle impose fluctuations, making consistent year-round output difficult.

Types of Open System Algal Cultivation Methods

• Raceway ponds – Shallow, oval or looped channels are constructed, and water is circulated by paddle wheels so algae stay suspended and exposed to light.
• High-Rate Algal Ponds (HRAPs) – These are shallow raceway-style ponds (0.1–0.4 m deep) with increased mixing and aeration, sometimes CO₂ injection, and often used in wastewater treatment + algal growth.
• Natural water bodies / lagoons / lakes – Algae are grown in existing ponds, lakes, lagoons, or natural basins; control is low, environmental factors dominate.
• Artificial ponds / shallow ponds – Man-made ponds (lined with plastic, concrete, earthen) are used, with nutrient and water supply managed, but still open to atmosphere.
• Inclined / cascade systems – Water flows over a sloped surface or cascade, gravity driven, with retention basins and recirculation (less common)⁶⁷

Characteristics features

• Shallow / Low Depth ponds are used so that sunlight can penetrate most of the water column, and self-shading is reduced.
• Large Area Coverage is typical, many square meters or hectares are involved, to produce more biomass with low cost per unit area.
• Exposed to Ambient Environment, so temperature, light, rainfall, wind, evaporation affect the cultivation directly.
• Simple Construction & Operation is possible, with minimal capital and lesser technical complexity required.
• Mixing / Circulation is provided (by paddle wheels, impellers, or flow) so that algae do not settle, nutrients are distributed, and light exposure is improved.
• Vulnerability to Contamination is high, because unwanted algae, bacteria, protozoa or predators can enter easily.
• Limited Control over Conditions (temperature, pH, light intensity) is a drawback, because external fluctuations dominate.
• Dilute Cultures are common, as high cell density causes shading and reduces productivity; so cultures are often low concentration.
• Evaporation & Water Loss occur, especially in hot climates, which require replenishing water or managing concentration.
• CO₂ Exchange with Atmosphere happens naturally, but is inefficient; supplemental CO₂ may sometimes be added to boost growth.
• Easy Scale-up is allowed, because extending ponds is easier than scaling closed reactors.
• Cost-Efficiency (low capital / operating cost) is a strong feature, making open systems favorable where conditions permit.⁸¹⁰

Advantages

• It is cheap to build and operate, because few structures and low complexity are required.
• The system can be scaled up easily; large land areas can be used so output is increased.
• In open systems, natural sunlight is fully used, so energy for artificial lighting is saved.
• Operating costs are low, because pumping, control systems, and maintenance are simpler than closed reactors.
• Water and nutrients (sometimes wastewater) can be used so resource reuse is possible, reducing raw material cost.
• Because the method is similar to natural growth, it is more forgiving for some species which tolerate fluctuations, and contamination is less disastrous in some contexts.¹¹¹²¹³

Limitations

• It is hard to maintain high productivity, because light, nutrients, CO₂ supply are limiting and often non-ideal.
• The culture is vulnerable to contamination / invasion by unwanted algae, bacteria, protozoa or predators which degrade yield.
• It is difficult to control temperature, pH, light intensity in open systems, since external weather and day/night cycles dominate.
• Evaporation / water loss is significant, especially in hot or dry climates, leading to concentration changes or need for water addition.
• Cultures stay dilute / low cell density, because if density becomes high, light penetration is poor and self-shading occurs.
• CO₂ supplied by atmosphere diffuses slowly, so carbon limitation is common unless additional CO₂ is added, which raises cost.
• Large land area is required, so space demand is big and site cost / land use become issues.
• Output is strongly seasonal / weather-dependent, so year-round production is hard in many locations.
• Poor light utilization efficiency occurs, since many cells remain in shade and light is wasted.
• Finally, strain limitation is found: only robust species tolerant to fluctuations can survive in open systems, restricting choices.¹⁴¹⁵¹⁶

Closed System Algal Cultivation Techniques

• Closed system algal cultivation uses enclosed vessels (photobioreactors, PBRs) so that culture is isolated from outside environment, and conditions are more precisely managed.
• In photobioreactors, light is provided (natural or artificial) and algae are held in transparent or translucent containers, with nutrient and gas supply controlled.
• In closed systems, mixing / circulation is induced (by pumps, bubbling, air‐lifting) so that cells are kept suspended, gas exchange (CO₂ in, O₂ out) is promoted.
• Tubular reactors are common: algae culture flows through transparent tubes (horizontal, vertical, or coiled), light enters through tube walls, gas exchange is done along tubes or via degassing units.
• Flat panel reactors are used: flat plates of glass or plastic hold a thin layer of culture to ensure good light penetration, and mixing or bubbling keeps culture moving.
• Plastic bag / flexible reactor systems exist: culture is held in sealed bags or flexible containers, with controlled inputs of CO₂, nutrients, light, etc.
• Biofilm / surface reactors: algae grow attached to surfaces (films) inside enclosed reactors rather than fully suspended, reducing energy needed for mixing.
• In closed systems, sterility / contamination control is easier, because the environment is sealed and external invaders are blocked (or limited).
• Conditions like temperature, pH, light intensity, nutrient concentration, CO₂ level can be manipulated precisely (and monitored), so growth is more consistent.
• Because of control, higher cell densities / productivity can be achieved compared to open systems (though cost is higher).¹⁷

Factors influence

• Light intensity / distribution is critical, because if light is too weak or uneven, inner cells suffer shading (optical depth matters)
• Nutrient concentration and composition (nitrogen, phosphorus, trace elements) must be well balanced, as deficiency or excess impair growth or cause unwanted byproducts
• Temperature affects metabolic / enzymatic rates, so deviation from species’ optimum slows growth or causes stress
• CO₂ supply / carbon dioxide concentration is a factor: dissolution, transfer, and delivery must match demand, else carbon becomes limiting
• Gas management is needed: O₂ accumulation must be removed (degassing), and CO₂ must be maintained, else photosynthesis is inhibited
• Mixing / circulation or turbulence is important so that cells receive light / nutrients and avoid settling, stagnation or gradients
• Optical path length and cell density interplay: too dense culture reduces light penetration; optimal path must be designed
• pH must be regulated, because CO₂ uptake, metabolic activity, and nutrient forms are pH‐sensitive; drift in pH harms growth
• Species / strain characteristics matter: different algal strains vary in light tolerance, nutrient demands, shear sensitivity, etc.
• Shear stress / mechanical damage is a concern: mixing, pumping or flow can damage delicate algal cells if not designed properly.
• Fouling / biofilm on internal surfaces can interfere with light penetration and mass transfer, so cleaning / maintenance is a factor.
• Heat / thermal gradients in reactor (hot spots, cooling needs) must be controlled to avoid stress zones.
• Scale / geometry effects: as reactor size increases, uniformity, mixing, light, gas transfer become harder to maintain.²¹

Types of Closed System Algal Cultivation Techniques

• Photobioreactors (PBRs) are closed vessels in which light, nutrients, temperature, gas exchange are controlled so algae growth is managed in isolation from outside environment.
• Flat-panel PBR is constructed of flat glass/plastic panels with a high surface-area to volume ratio; used so light penetrates well, and mixing ensures uniform conditions.
• Tubular PBR consists of transparent tubes (coiled, looped, horizontal or vertical) through which algal culture flows, exposing cells to light and CO₂ while minimizing contamination.
• Plastic Bag / Flexible PBR uses clear bags (vertical/horizontal) to grow algae in a closed bag environment, often at small scale or experimental setups.
• Porous Substrate / Biofilm Reactor employs surfaces (foams, meshes, films) inside enclosed reactors where algae attach and grow as films, rather than being suspended.
• Internally Lit PBR has light sources inserted inside the reactor (or internal illumination) so that dense cultures get light inside, and side walls need not be fully transparent.
• Vertical / Alveolar Panel Reactors place reactor panels upright or with alveolar structure to optimize light capture in vertical direction, often used in urban or constrained spaces.
• Hybrid / Combined Systems may integrate closed PBR modules with semi-open or other modules (mixing closed & open strategies) to balance productivity, cost, and control.¹⁸¹⁹

Characteristics features

• The culture is enclosed so that outside contamination is reduced and ambient interference is limited (sterility more possible).
• Precise control is maintained over temperature, light intensity, pH, nutrient concentrations, CO₂ supply, so conditions are optimized.
• High surface-area to volume ratio is achieved (for example in flat panels, tubes) so light can reach more cells.
• Mixing or circulation is enforced (by pumps, bubbling, airlift) so that cells do not settle, and nutrients, gases are uniformly delivered.
• The illumination path is short in many designs so that shading / light attenuation is minimized.
• Oxygen accumulation is controlled (degassing units or gas exchange) to prevent inhibition by high dissolved O₂.
• Culture density can be higher, because controlling factors allows more cells per unit volume than open systems.
• Variation by external weather is minimized; fluctuations in light, temperature, wind do not directly impact the culture.
• Material / construction is transparent (glass, clear plastic) or includes internal lighting so light can penetrate to cells.
• The system is more complex, thus design, operation, maintenance require more technical input.
• Scalability is harder: modules must be replicated or scaled carefully so that control is maintained.
• Fouling / biofilm formation on internal surfaces is possible, which reduces light penetration unless cleaned.
• Energy demand is higher (for pumps, sensors, cooling/heating, mixing) because of the needs of control.
• Capital cost is high for setup because of equipment, sensors, control systems, sealed vessels.²⁰

Advantages

• It is easier to maintain sterility / low contamination, because closed walls block many external invaders.
• Culture conditions (temperature, pH, nutrient concentration, CO₂ supply) can be precisely controlled, so growth is more predictable and optimized.
• High cell density / biomass productivity is possible, since light, mixing and gas exchange are engineered to reduce limitations.
• Water loss by evaporation is minimized, because systems are sealed or enclosed.
• It allows growth of sensitive or special strains (which cannot survive in fluctuating external conditions) under tailored conditions.
• Better utilization of light is made possible by design (short light paths, surface-area to volume optimization) so internal shading is reduced.
• Fluctuations due to weather, wind, rain etc are buffered, so external variability doesn’t directly influence culture.
• Closed systems support continuous / semi-continuous operation for longer periods, rather than short batch cycles only.
• It is possible to reuse / recycle media more reliably, because the risk of contamination is lowered and conditions are stable.
• The product (algal biomass, compounds) tends to be higher purity / consistency, since fewer external species interfere.

Limitations

• It is expensive to build, because sealed vessels, sensors, control systems are required which raise capital cost.
• The operating costs are high, because energy is needed for lighting, mixing, cooling/heating, monitoring.
• Scalability is limited: when systems grow in size, control and uniformity become harder and costs increase non-linearly.
• Light penetration / shading is a problem: in dense cultures, inner cells get little light, so growth is limited (self-shading).
• Fouling / biofilm formation on reactor surfaces occurs, which blocks light and needs cleaning / maintenance.
• Oxygen accumulation can occur, and high dissolved O₂ levels may inhibit photosynthesis / growth.
• Temperature control is harder in extremes (very hot or cold climates), and thermal gradients in reactors may stress cells.
• Design complexity is high, and maintenance / monitoring burdens are large, requiring skilled personnel.
• CO₂ delivery and gas exchange are challenging: achieving efficient transfer without losses, and removing excess gases, is technically demanding.
• Cell shearing / mechanical damage may happen due to mixing, pumping, flow stresses, especially for fragile algal strains.

Factors influencing Algal Growth

• Light Intensity & Quality — algae require sufficient light (but not too strong) and the spectrum matters; if light is weak or too deep water, growth slows.
• Nutrient Availability & Ratio — nitrogen, phosphorus, trace metals must be present in proper amounts and ratios; limitation of any can restrict growth.
• Temperature — each algal species has an optimal temperature range (eg 20-30 °C for many); if too cold or too hot, metabolic activity is reduced.
• Mixing / Circulation & Turbulence — motion ensures cells move, nutrients, gases are distributed, prevents settling and boundary layer limitation.
• Carbon / CO₂ Supply — algae need carbon (often from CO₂); if CO₂ is insufficient, growth is limited.
• pH & Alkalinity — pH affects nutrient speciation, enzyme activity, CO₂ availability; drift in pH harms growth.
• Self-Shading & Optical Depth — in dense cultures, inner cells get less light; optimal density / depth balance is needed.
• Dissolved Oxygen / Gas Build-Up — too much O₂ can inhibit photosynthesis; gas exchange must relieve excess.
• Species / Strain Traits — different algal strains differ in tolerance to light, nutrients, shear stress, etc.
• Environmental / External Factors — in open systems especially, weather, day/night cycles, seasonal change influence growth strongly.

Different Harvesting Techniques of Algae

The harvesting of algae is done by these following methods;

• Centrifugation — cultures are spun at high speed so that cells (denser) settle out, and supernatant is removed.
• Filtration / membrane separation — algae are retained by filters or membranes while water passes through, often micro- or ultrafiltration.
• Flocculation / coagulation — chemicals or biological agents are added so that individual algal cells aggregate into larger flocs which are easier to settle or filter.
  • Auto-flocculation: cells spontaneously aggregate (e.g. by pH shift)
  • Bio-flocculation: use of microbes or biopolymers to cause aggregation
  • Chemical flocculation: use of alum, ferric salts, polymers to destabilize charge and aggregate cells
• Sedimentation / gravity settling — allow algal cells or flocs to settle under gravity over time, sometimes after flocculation to speed settling.
• Flotation / air‐bubble flotation — bubbles are introduced so algae attach to bubbles and float to surface, then scraped off.
• Electro-coagulation / electro-flotation — electric fields are used to destabilize cells / generate bubbles to aggregate or float algae.
• Hybrid / combined methods — two or more techniques are used in sequence (e.g. flocculation + centrifugation, or flocculation + filtration) to improve efficiency or lower cost.

Applications of Algal Cultivation

• Biofuel / Energy — Algal biomass is converted into biodiesel, biogas, bioethanol etc, because they accumulate lipids, carbohydrates and other energetic compounds.
• Food / Nutraceuticals / Supplements — Algae are used as dietary supplements, protein sources, or for extraction of Omega-3 lipids, pigments, vitamins.
• Animal / Aquaculture Feed — Algal biomass serves as feed or feed additive in aquaculture (fish, shrimp) or livestock, improving nutrition.
• Pharmaceuticals / Cosmetics / Pigments — Algae are cultivated for bioactive compounds, dyes, pigments (carotenoids, phycobiliproteins) used in cosmetics, medicinal products.
• Wastewater Treatment / Bioremediation — Algae remove nutrients (nitrogen, phosphorus) or contaminants from wastewater, thus cleaning water while producing biomass.
• Carbon Capture / CO₂ Sequestration — Algal cultivation fixes CO₂ from air or flue gas, reducing greenhouse gas and converting carbon into biomass.
• Fertilizers / Biofertilizers / Soil Amendment — After processing, algal residuals are used as fertilizers, soil conditioners, or nutrient sources in agriculture.
• Biorefinery / Platform Chemicals — Algal biomass is used as feedstock to extract multiple products (biofuels, chemicals, polymers) in an integrated biorefinery concept.
• Carbon / Nutrient Recycling — Algae help recycle nutrients and carbon in circular systems (e.g. coupling with waste streams) so resources are reused.

Economic and ecological importance of Algal Cultivation

Economic importance

• Algal cultivation generates biomass that can be converted to biofuels, reducing dependence on fossil fuels and creating new energy markets.
• Algae serve as nutraceuticals / food / feed additives, offering proteins, lipids, vitamins, pigments for human / animal consumption.
• In agriculture, algae are used as biofertilizers / soil conditioners, supplying nutrients and plant growth factors.
• By treating wastewater / effluents, algae help recover nutrients and reduce treatment costs, turning a waste stream into resource.
• Algal cultivation supports carbon capture / sequestration, thus contributing to climate mitigation and generating value for carbon credits.
• Extraction of industrial products (pigments, pharmaceuticals, bioplastics) from algae adds high value, enhancing viability of algal farms.
• Seaweed farming especially contributes to local economies and livelihoods, via exports, job creation, and coastal aquaculture development.

Ecological importance

• Algae produce oxygen via photosynthesis and form the base of many aquatic food chains, supporting biodiversity.
• Carbon dioxide is absorbed and converted to biomass, helping reduce greenhouse gas concentration (carbon sink).
• Algal cultivation in wastewater / polluted water helps remove nutrients / contaminants, mitigating eutrophication and water pollution.
• By recycling nutrients and connecting water / carbon cycles, algae contribute to circular economy and more sustainable ecosystems.
• Seaweed farms (macroalgae) can foster coastal habitat restoration, protection of shorelines, and nutrient cycling in marine systems.
• Ecological resilience is supported, because algal systems can buffer environmental changes, and provide ecosystem services in aquatic systems.

Difference between Open System and Closed System Algal Cultivation Techniques