Microalgae Streaking and Inoculation: Aseptic Lab Protocol

Microalgae are microscopic algae that cannot be seen with the naked eye and require magnification of 100-400 times for identification. They are classified into 9 divisions based on their pigments and are phototrophic (using light for energy), though some can also be heterotrophic. Microalgae growth follows distinct phases: (1) Lag phase - initial adaptation period, (2) Exponential phase - rapid growth when nutrients are abundant, (3) Stationary phase - growth slows as nutrients deplete, (4) Death phase - cell death when nutrients are exhausted. Harvesting for valuable compounds typically occurs during the stationary phase.

Microalgae are primary producers that convert water and carbon dioxide into biomass and oxygen through photosynthesis, serving as valuable sources of bioactive compounds (lipids, proteins, carbohydrates, carotenoids, vitamins) for nutrition and aquaculture applications; successful microalgae culture requires careful management of key environmental parameters including light intensity (with blue and red wavelengths being most effective), pH (maintained between 7-9), temperature (optimal range 20-24°C), salinity, and aeration, along with proper nutrient enrichment using macronutrients (nitrate, phosphate, silicate) and micronutrients (vitamins and trace metals), while growth dynamics follow five distinct phases: lag phase, exponential growth phase, declining growth rate phase, stationary phase, and death/crash phase.

Photosynthesis converts water and carbon dioxide into sugar and oxygen using photopigments like chlorophyll a, b, and c. Microalgae exhibit two cellular organizations: prokaryotic cyanobacteria (blue-green algae) with simple designs, and eukaryotic algae with true nuclei and compartmentalized organelles. Major groups include green algae (evolutionary origin of land plants), dinoflagellates (flagellated with diverse ecological roles), diatoms (glass-shelled with planktonic and benthic forms), cryptophytes (complex internal structures), and haptophytes (small green with potential nutritional value). Each group varies dramatically in morphology, ecology, and utility for aquaculture applications.

This section covers the different cultivation systems, growth phases, and complete production flow in microalgae cultivation. Three main cultivation systems exist: discontinuous (batch) where nutrients are added once and entire culture is harvested after 7-8 days; semi-continuous where partial harvesting with nutrient replenishment reduces cycle time to 3-4 days; and continuous where constant nutrient input and harvest maintain steady-state conditions. The choice depends on scale, contamination risk, and production requirements. Microalgae cultivation follows a characteristic growth curve with distinct phases: lag phase (day 0) where microalgae adapt to new conditions; acceleration phase where growth begins; exponential phase where optimal growth and nutritional value occur, ideal for harvesting; deceleration phase where growth slows; stationary phase where density stabilizes; and death phase where cells begin dying. For production, harvesting occurs during the exponential phase (around day 7) when microalgae have optimal nutritional value. The complete production flow involves: maintaining stock cultures on agar plates; selecting healthy colonies for inoculation; initial cultivation in test tubes (7 days); transfer to 150mL flasks (7 days); transfer to 12L bottles (7 days); transfer to 18L volumes; and final scale-up to 500L tanks. Each transfer serves as inoculum for the next phase. Cultures should be renewed after 4-5 repiquings to maintain vigor.

Optimal microalgae growth requires about 100 micro einsteins per meter squared second of light (sunlight provides 3,000, which is too bright). Temperatures above 100°F kill algae. After about 8 days, brine shrimp cysts hatch and swim in the culture, visible against the grid. The seshi stick uses optical density to measure biomass: fold it on the dotted line, submerge until the black and white target disappears, then use the provided graph to determine cells per milliliter. This allows tracking cell density over time and creating growth curves to monitor culture development.

The streak plate technique dilutes cells to obtain isolated colonies. Steps include: sterilizing the loop in blue flame until orange-hot, cooling by touching agar (no sizzling), collecting a single colony, performing primary streaks gently, flaming between sets, turning the plate to intersect lines, and creating final vertical/zigzag patterns. The loop must be cooled between transfers to prevent killing organisms. Only the primary streak receives inoculum—the technique relies on progressive dilution.

The streak plate technique is a fundamental microbiology method used to isolate individual bacterial colonies on an agar plate by spatially diluting bacteria across multiple sectors; a single bacterial cell undergoes binary fission to form a visible colony containing approximately one million bacteria, and this technique allows researchers to determine if a culture is pure or contaminated by observing whether multiple distinct colony types appear, with each sector showing progressively lighter growth until isolated colonies form in the final sector.

Isolation streaking is a microbiology technique that uses progressive dilution to create individual bacterial colonies on an agar plate; the process involves dividing the plate into quadrants and making zigzag streaks where each subsequent streak crosses the previous one, ensuring that later streaks contain fewer bacteria and allowing for the isolation of pure colonies after incubation.

To successfully streak for single colonies on a petri dish, use a disposable inoculating loop to spread cells across the plate in progressively diluting regions, replacing the loop between each streak to ensure cells are spread far enough to isolate individual colonies; always store petri dishes upside down to prevent condensation from dripping onto the agar and smearing the cells.

Streaking is a dilution method used in microbiology to isolate pure bacterial colonies by progressively diluting bacteria across a petri dish through repeated sterilization of the inoculation loop and spreading bacteria in multiple zones, allowing individual bacterial cells to grow into isolated colonies that can be identified and studied.

Microalgae products range from low-value biomass to high-value compounds. Low-value products include carbohydrates, lipids, and biomass with large market sizes. High-value products include pigments (carotenoids, astaxanthin), omega-3 and omega-6 fatty acids, and amino acids, used in cosmetics, nutrition, and biomedicine. Advantages include 10-100 times higher productivity than traditional crops, efficient carbon capture, high lipid content for biodiesel, and ability to grow on non-arable land using wastewater. Challenges include high production and extraction costs requiring innovative solutions.

Beyond omega-3 fatty acids, algae produce valuable carotenoids (pigments with antioxidant properties) and phytosterols (compounds that lower cholesterol levels). One particular microalgae species, Pavlova lutherii, produces omega-3 fatty acids, carotenoids, and phytosterols together at concentrations up to 5% of its dry weight—making it exceptionally valuable for health applications. These compounds represent premium market opportunities that can subsidize lower-margin fuel production, creating economically attractive business models for algae cultivation.

Carotenoids are lipophilic components soluble in lipids, found in higher plants and microalgae, with proven anti-inflammatory and anti-cancer activities representing a billion-dollar global market. Beta-carotene, produced by Dunaliella salina, serves as vitamin A precursor and is preferred over synthetic forms for its better bioavailability. Astaxanthin, produced by Hematococcus pluvialis through stress-induced thickening of cell walls, costs $2500/kg and offers UV protection and therapeutic benefits for cancer and inflammatory diseases, crossing blood-brain barriers without eye crystal formation. Canthaxanthin enhances poultry liver vitamin E content. Lutein, essential for retinal and lens health, reduces disease risk when present at significant levels in human serum.

Spirulina and chlorella are excellent dietary sources of carotenoids lutein and zeaxanthin, which are essential for eye health. One teaspoon of these microalgae provides clinically significant amounts of these nutrients. Lutein and zeaxanthin protect the retina from UV radiation damage and oxidative stress from free radicals. Scientific evidence shows these carotenoids help prevent age-related macular degeneration and cataracts, and may also improve visual acuity in conditions such as diabetic retinopathy.

This section covers the discovery of carotenoids during microalgae research for anti-cancer compounds. Carotenoids are liquid pigments belonging to the terpene family, typically containing 40 or 45 carbon atoms, appearing in red, orange, and yellow colors. Over 1,000 carotenoids have been identified. Examples include lycopene (tomatoes), beta-carotene (carrots), astaxanthin (microalgae), and fucoxanthin (brown seaweed). Some carotenoids contain complex structures like epoxy functions and glycosylation, making them difficult to synthesize chemically and requiring purification from biological sources. Carotenoids serve multiple biological functions: photoreception for vision and photosynthesis, phototropism, membrane stabilization, antioxidant activity against reactive oxygen species, photoprotection against high light, allelopathy, chemoattractant functions, and regulation of cell cycle and metabolism. Retinoic acid, a carotenoid derivative, is important in embryonic development.