Astaxanthin Production from Haematococcus Pluvialis Algae

Carotenoids are synthesized from all-trans-lycopene through the MEP pathway, where beta-rings are introduced at both terminal ends to form beta-carotene, the most abundant carotenoid and precursor to vitamin A, while alpha-carotene forms when an epsilon-ring is introduced at one end before beta-ring addition; these carotenoids can be further converted to xanthophylls through hydroxylation reactions, with their biosynthesis being regulated by light and darkness conditions in photosynthetic organisms.

Carotenoid biosynthesis begins with the formation of phytoene from two molecules of geranylgeranyl diphosphate. The pathway involves multiple enzymatic steps including phytoene desaturase, ζ-carotene desaturase, and carotene isomerase. These enzymes catalyze the conversion of phytoene through various intermediates to form different carotenoids. Carotenoids are tetraterpenoids composed of 40 carbon atoms, derived from 8 isoprene units (each containing 5 carbons). Isoprene is a 5-carbon compound that serves as the building block for terpenoids. The biosynthesis of carotenoids occurs through the non-mevalonate pathway (MEP pathway) in chloroplasts, which is distinct from the mevalonate pathway used for other terpenoid biosynthesis.

Carotenoids serve multiple functions in photosynthesis: (1) They absorb light at different wavelengths than chlorophyll, expanding the range of light energy that can be captured for photosynthesis; (2) They act as antioxidants, protecting the photosynthetic apparatus from damage by free radicals; (3) They protect chlorophyll from photodamage by absorbing excess light energy that could otherwise damage the photosynthetic machinery. Different carotenoids are associated with different photosystems: beta-carotene is dominant in photosystem I, while xanthophyll is dominant in photosystem II.

Carotenoids are accessory pigments that assist chlorophyll in photosynthesis. They include carotene (orange), xanthophyll (yellow), and lycopene (red). Carotenoids absorb light wavelengths that chlorophyll cannot absorb efficiently and transfer this energy to chlorophyll. They also protect chlorophyll from photooxidation damage and contribute to plant coloration, which aids in pollination.

Photosynthetic pigments are substances that absorb light energy for photosynthesis. Plants contain two main groups: chlorophylls and carotenoids. Chlorophyll a (C55H72O5N4Mg) is the primary pigment found in all photosynthetic organisms except photosynthetic bacteria, while chlorophyll b (C55H70O6N4Mg) is found in most plants and algae. The key structural difference is that chlorophyll b has a formyl group instead of a methyl group. Chlorophyll molecules consist of a porphyrin ring with a central magnesium atom coordinated with four nitrogen atoms, and two hydrocarbon tails that anchor the molecule in the thylakoid membrane. When light energy strikes chlorophyll, the central magnesium atom releases two electrons, initiating photosynthesis. Carotenoids are accessory pigments that appear orange, yellow, or brown, classified as lipids composed of isoprene units. They are classified into carotenes (hydrocarbons) and xanthophylls (containing oxygen). Carotenoids absorb blue and violet wavelengths, transfer energy to chlorophyll a, protect against photooxidative damage, and contribute to flower and fruit colors that attract pollinators and seed dispersers.

Secondary metabolite production is inversely proportional to cell growth and division. Higher cell division rates correlate with lower secondary metabolite production, while lower growth rates correlate with higher production. This occurs because secondary metabolites are produced during the stationary phase when cell growth has ceased. Media nutrient depletion and toxic compound accumulation create stress conditions that trigger cytodifferentiation and enzyme induction for secondary metabolite formation. The production strategy follows a two-phase approach: Phase 1 focuses on biomass production using optimized growth conditions, while Phase 2 focuses on secondary metabolite production using different conditions to maximize accumulation.

High nitrogen levels (NH4+) significantly reduce secondary metabolite (terpene) production in plants. Research shows that nitrogen levels above certain thresholds can cause 30-60% reductions in terpenes. Conversely, nitrogen deficiency triggers stress responses that increase secondary metabolite production. The three key strategies for terpene production are: (1) faster drying to preserve terpenes, (2) Emerson effect implementation for rapid light intensity increase, and (3) strategic nutrient manipulation (nitrogen tapering). Entering bulking phase earlier (around day 17 instead of day 24-25) provides advantages because plants can be pushed harder during transition. Starting day one of flower at high light intensity (1200 ppfd) with high EC (10 EC) and 25% dryback maximizes the transition period. After bulking, nitrogen should be removed (starvation) while maintaining other nutrients, signaling the plant to shift from biomass production to secondary metabolite production.

Research shows that lipid and carotenoid production in microalgae increases under stress conditions such as nitrogen or phosphate deficiency, light stress, and salinity changes. However, very few studies have examined the effects of low pH or changing pH on microalgae. The challenge is balancing optimal growth with high metabolite content, as carotenoids are stress-induced antioxidants produced to protect cells from environmental damage.

All secondary metabolites originate from common precursors: glucose (from photosynthesis) and acetate. The biosynthesis pathways include: (1) Phenylpropanoid pathway - from phenylalanine (producing phenolic compounds like flavonoids, coumarins, and tannins); (2) Mevalonate pathway - from acetate (producing terpenoids and saponins); (3) Shikimate pathway - from amino acids (producing alkaloids). Plants synthesize secondary metabolites as responses to environmental pressures including biotic factors (interactions with other plants, animals, insects, microorganisms) and abiotic factors (soil composition, water availability, atmospheric gases, circadian rhythms, seasonality, altitude). Plants cannot migrate like animals, so they produce compounds to improve survival conditions: bitter compounds to deter herbivores, pigments to protect against UV radiation, allelopathic substances to inhibit competing plants, and compounds to attract pollinators or seed dispersers.

This extensive section covers plant responses to environmental stress and secondary metabolite biosynthesis. Photoperiodism is the ability of plants to measure day and night length, with short-day plants requiring short light periods and long dark periods for flowering, while long-day plants require long light periods and short dark periods. Hydrogen peroxide plays multiple roles in plant defense: it is directly toxic to pathogens, leads to hydroxyl radical formation, induces biosynthesis of salicylic acid and benzoic acid, and is produced during incompatible plant-pathogen interactions. The nitrogenase complex is the enzyme responsible for atmospheric nitrogen fixation, consisting of dinitrogenase (FeMo-cofactor containing protein) and dinitrogenase reductase (iron protein), with the FeMo-cofactor being the active site where N2 binding occurs. Salinity stress involves high concentrations of sodium, chlorine, and calcium ions in soil, with plants adapting by sequestering these ions into vacuoles to prevent cytoplasmic toxicity. Secondary metabolites include terpenoids (protect against herbivores, synthesized via mevalonic acid pathway and MEP pathway), phenolics (protect against UV radiation, synthesized via shikimic acid pathway and mevalonic acid pathway), alkaloids (nitrogen-containing compounds), and pyrethroids (insecticidal compounds). Terpene biosynthesis follows a specific carbon chain pattern: isopentenyl pyrophosphate (5 carbons) combines to form geranyl pyrophosphate (10 carbons, monoterpenes), farnesyl pyrophosphate (15 carbons, sesquiterpenes), geranylgeranyl pyrophosphate (20 carbons, diterpenes), and phytanyl pyrophosphate (25 carbons, triterpenes).

Algae are currently grown in two main systems: open raceway ponds (20cm deep with paddle wheels) which are cheap but vulnerable to contamination, and closed photobioreactors (PBRs) which provide controlled environments but have high manufacturing costs and energy requirements. PBRs currently have 5-10 times better yield than open ponds. Algal agronomy is in its infancy - unlike land plants, we lack understanding of growth characteristics, quality control, crop protection, and disease management. Understanding algal biology is essential for both effective production and containment, as releasing organisms into natural environments could be deleterious.

Closed photobioreactors include horizontal tubular, vertical column, and flat panel systems. Comparison shows: tubular PBR offers maximum biomass yield and light exposure but highest O2/CO2 accumulation problems and medium maintenance; flat panel PBR provides good yield but highest manufacturing costs and lowest scale-up potential; column PBR balances performance with lower costs and easier scaling. Key differences from open systems include: space requirements (high vs low), water loss (high vs low), CO2 loss (high vs low), oxygen management (spontaneous outgassing vs controlled exchange), temperature control (variable vs precise), shear forces (low vs high), contamination risk (high vs low), biomass quality (variable vs reproducible), and production flexibility (limited vs high).

Algae cultivation involves growing photosynthetic microorganisms for biofuel, pharmaceuticals, cosmetics, food, feed, and wastewater treatment. Algae range from microscopic microalgae to large seaweeds, converting sunlight, CO2, and nutrients into biomass rich in lipids, proteins, carbohydrates, pigments, and vitamins. Cultivation systems are classified as open or closed: open systems are basic and inexpensive but face contamination risks; closed systems provide better control but require higher investment. Open methods include open ponds (15-30 cm deep for hardy species), raceway ponds with paddle wheel mixing, high rate algal ponds with CO2 injection and wastewater recycling, and open sea farming using longlines, rafts, and nets. Closed systems employ photobioreactors (PBRs)—transparent vessels enabling precise control. PBR types include flat panels (high surface-to-volume ratio but biofilm issues), tubular reactors (continuous circulation for commercial production), plastic bags (small-scale experiments), and porous substrates (biofilm growth for water conservation). Cultivation strategies include monoculture (homogeneous biomass for specialized applications) and mixed cultures (enhanced resilience and bioactive diversity). Five key environmental factors govern growth: temperature (20-30°C optimal), light exposure, nutrient availability (N, P, Fe, trace elements), oxygen management (de-gassing prevents toxicity), and odor control (prevents anaerobic conditions).

Bioprocess engineering is an interdisciplinary branch of chemical engineering that combines biology, chemistry, physics, and engineering principles to design, optimize, and operate processes involving biological materials such as microorganisms, enzymes, plants, and animal cells for producing pharmaceuticals, biofuels, food, chemicals, and other bioproducts; the field encompasses upstream processing (fermentation using recombinant DNA and metabolic engineering technologies) and downstream processing (product recovery and purification), with key applications in pharmaceuticals, biofuel production, food and beverage industries, environmental biotechnology, and biodegradable materials, emphasizing sustainability through renewable resources, waste minimization, and improved energy efficiency compared to traditional chemical engineering approaches.