Algal Nutrient Solution

An algal nutrient solution, also called an algal culture medium or growth medium, is a carefully defined mixture of dissolved salts, trace metals and sometimes vitamins that supplies what algae need to grow, apart from light and carbon dioxide. Wikipedia describes it as a mixture of chemical salts and water, or seawater, that differs from agricultural fertilizer in being designed for aquatic use and having a more precise composition.

Nutrient solutions are the quiet foundation of almost everything people do with microalgae and many seaweeds. Research labs use them to keep reference strains alive for decades. Hatcheries use them to grow the tiny algae that feed oyster and clam larvae. Biotechnology firms use them to grow algae for pigments, food supplements and experimental fuels. Water engineers study the reverse problem, the way nutrients from sewage and farm runoff turn lakes and coastal seas into overgrown, oxygen-starved water.

This article explains what algae need, the chemistry behind the recipes, the history of the best-known media, how solutions are prepared and used at different scales, how nutrient choices affect what algae produce, and the environmental side of algal nutrition.

What algae need to grow

Light, carbon and water

Most cultivated algae are photosynthetic. They use light energy to turn carbon dioxide and water into organic matter. A nutrient solution does not supply light or, usually, carbon, but it has to be designed around them. Wikipedia's algaculture article notes that photosynthesis requires CO₂ and that an external CO₂ supply is usually needed for fast growth, because the small amount dissolving from air cannot keep up with a dense culture.

Light is often the real bottleneck. In open ponds, according to the same article, light penetrates only the top 3 to 4 inches (76 to 102 mm), depending on algal density. Algae can use only about a tenth of the light in direct sunlight, though shading by surface cells can actually help deeper cells. A nutrient solution can make a culture grow dense enough that light runs out, at which point extra nutrients do nothing.

Some algae can also grow heterotrophically, without light, by consuming sugars or other organic carbon. Media for these algae add an organic carbon source.

Macronutrients

The major nutrients, needed in relatively large amounts, are:

• Nitrogen, for proteins, nucleic acids and chlorophyll. Media usually supply it as nitrate, sometimes as ammonium or urea.
• Phosphorus, for nucleic acids, cell membranes and the energy-carrying molecule ATP. Usually supplied as phosphate.
• Silicon, for diatoms, which build their cell walls from silica. Supplied as silicate. Most other algae do not need it.
• Potassium, magnesium, calcium and sulfur. Freshwater media add these as salts. Seawater already contains them in abundance.

Wikipedia's algaculture article summarizes the basic list: nitrogen, phosphorus and potassium are generally necessary, while silica, iron and trace elements can limit growth, and all nutrients must be dissolved in forms the algae can take up.

Trace metals

Algae need small amounts of iron, manganese, zinc, copper, cobalt and molybdenum, and some species benefit from selenium, nickel or other elements. These metals are cofactors for enzymes in photosynthesis, respiration and nitrogen metabolism. Iron is the most important and the most troublesome, because in oxygenated water at neutral pH it quickly forms insoluble oxides that algae cannot use.

Vitamins

Many algae, especially marine species, cannot make all their own vitamins. Standard marine media add three: vitamin B12 (cyanocobalamin), thiamine (vitamin B1) and biotin. Freshwater cyanobacteria often need none, which is why some freshwater recipes omit vitamins entirely.

Temperature and pH

A nutrient solution works only within the right physical conditions. Wikipedia's algaculture article notes that most species grow best in water between roughly 15 °C and 35 °C. The pH of the medium affects which chemical form nutrients take, how much CO₂ stays dissolved, and whether metals precipitate. Recipes therefore specify a target pH, such as 7.4 for BG-11 after cooling and CO₂ equilibration, or 7.8 to 8.2 for autoclaved seawater media.

The chemistry of balance

The Redfield ratio

In 1934 the American oceanographer Alfred Redfield compared the nitrate and phosphate in seawater samples from the Atlantic, Indian and Pacific Oceans and the Barents Sea with the composition of plankton collected on voyages of the research vessel Atlantis. He found a remarkably consistent ratio. When nutrients are not limiting, most phytoplankton contain carbon, nitrogen and phosphorus in a molar ratio of about 106:16:1. Deep ocean water carries nitrate and phosphate in a similar N:P ratio of about 16:1.

The Redfield ratio is the starting point for thinking about algal nutrition. It has been extended to other elements:

The ratio is a guide, not a law. Wikipedia notes that phytoplankton grown under nitrogen or phosphorus limitation show N:P ratios anywhere from 6:1 to 60:1, depending on growth rate, nutrient scarcity and species. A later global median for particulate C:N:P has been put at 163:22:1.

Limiting nutrients

The idea of a limiting nutrient follows naturally from fixed ratios. If algae need nitrogen and phosphorus in a roughly fixed proportion, then whichever runs out first stops growth, regardless of how much of the other is left. Wikipedia's eutrophication article notes that phosphorus is the limiting nutrient in most freshwater ecosystems, nitrogen is the primary limiting nutrient in marine and coastal waters, and estuaries can be limited by either or both.

A culture medium is often designed so that no nutrient is limiting until the culture is very dense, or so that one particular nutrient runs out by design, for example in experiments on nitrogen starvation.

Keeping metals in solution: chelators

The trace metal problem has a chemical solution. Chelators are molecules that wrap around metal ions and hold them in solution. Nearly all modern media include EDTA (ethylenediaminetetraacetic acid) or a related compound alongside the iron and other metals. The National Center for Marine Algae and Microbiota (NCMA) notes that the original f medium used a commercial ferric chelate called ferric sequestrene, which NCMA has replaced with the sodium salt of EDTA plus ferric chloride. BG-11 uses citric acid and ferric ammonium citrate for the same purpose, plus a small amount of EDTA.

Chelators also buffer metal availability. Copper and zinc are essential at low levels but toxic at higher ones. Holding them in complexes keeps the free ion concentration low and stable. This idea is taken furthest in research media such as Aquil, an artificial seawater medium whose preparation and chemistry were described by Price and colleagues in 1989, which is designed so that the free concentrations of trace metals can be controlled precisely for studies of how metals affect phytoplankton.

A short history of algal culture media

From plant nutrition to algal media

Systematic nutrient solutions came first from plant science. In 1933, Dennis Hoagland and W. C. Snyder developed the hydroponic recipe now called Hoagland solution. Hoagland and D. I. Arnon revised it in 1938, cutting the trace elements to those generally accepted as essential: boron, manganese, zinc, copper, molybdenum, iron and chlorine. Arnon revised it again in 1950. With more than 21,000 Google Scholar citations, according to Wikipedia, it is among the most widely used standard formulations in plant science.

Hoagland solution is designed for land plants such as tomatoes and peppers, and Wikipedia's algal nutrient solution article mentions it as an example of a nutrient solution. Wikipedia's Hoagland article notes that diluted to a quarter or a fifth of full strength, it suits lower-demand plants such as lettuce and aquatic plants. Specialized algal media, however, developed along their own line.

Guillard's f medium and f/2

The most famous marine algal medium came from Robert Guillard and John Ryther, who described "f medium" in a 1962 paper on marine planktonic diatoms in the Canadian Journal of Microbiology. Guillard later published a half-strength version, f/2, in a 1975 chapter on culturing phytoplankton to feed marine invertebrates. NCMA calls f/2 a common and widely used general enriched seawater medium for coastal marine algae, especially diatoms.

f/2 is an "enriched seawater" medium. It starts with filtered natural seawater, which already provides the major salts, and adds nitrate, phosphate, silicate, trace metals and vitamins. The name reflects its strength: f/2 is half the concentration of the original f medium.

L1 and K media

In 1993, Guillard and P. E. Hargraves published L1 medium, which is based on f/2 but adds further trace metals. NCMA describes it as a general-purpose marine medium for coastal algae and now uses L1 and its derivatives in place of f/2. NCMA's recipe sheet carries a candid note: the intended final chromium concentration in the 1993 paper was 10⁻⁹ molar, but the paper's directions for the chromium stock produce 10⁻⁸ molar. NCMA has used the 10⁻⁸ version for about fifteen years for consistency, and Guillard corrected the error in 2003. Small inconsistencies like this are a reminder that media recipes are working documents, not sacred texts.

NCMA also distributes K medium and modified K medium for oceanic algae.

BG-11 for cyanobacteria

For freshwater cyanobacteria, often called blue-green algae, the standard is BG-11. NCMA's recipe sheet traces it to Allen and Stanier's 1968 paper on unicellular blue-green algae, with later modifications published in 1979 and by Watanabe in 2005. It notes that BG-11 has an exceptionally high concentration of nitrate and phosphate, and that while the original recipe has no vitamins, vitamins should be added for many eukaryotic algae. The Culture Collection of Algae and Protozoa (CCAP) also lists a nitrogen-free variant, BG11 0, for nitrogen-fixing cyanobacteria, which can take nitrogen from the air.

Bold's Basal Medium

For freshwater green algae, a widely used recipe is Bold's Basal Medium. The University of Texas Culture Collection of Algae (UTEX) and CCAP both distribute it, along with variants such as Bold 3N, which triples the nitrate, and 3N-BBM+V, with vitamins added.

A growing family

Culture collections now maintain dozens of recipes for different groups of algae. CCAP's media list includes, among many others, Jaworski's medium, Walne's medium for aquaculture, TAP (Tris-Acetate-Phosphate) medium widely used for the green alga Chlamydomonas, and artificial seawater formulations. Wikipedia's algal nutrient solution article also mentions Chu 13, one of a series of media named after their developer. The World Federation for Culture Collections lists over 500 registered algal culture collections, according to Wikipedia's algaculture article.

Classic recipes in detail

The tables below give final concentrations in the prepared medium, taken from the recipe sheets published by NCMA (Bigelow Laboratory) and UTEX. They are given to illustrate how media are built, not as a substitute for the collections' own instructions, which include stock preparation and sterilization details.

f/2 medium (marine)

Prepared in filtered natural seawater, according to NCMA:

NCMA advises leaving out the silicate if the alga does not need it, because silicate enhances precipitation in the medium.

L1 medium uses the same major nutrients and vitamins as f/2 but a richer trace metal mix, adding selenium, nickel, vanadium and chromium at around 10⁻⁸ M each.

BG-11 medium (freshwater, cyanobacteria)

From the UTEX recipe, prepared in distilled or deionized water:

NCMA's modified version lists 1.5 g of sodium nitrate per liter, the same 17.6 mM, with 0.175 mM phosphate, and a trace mix (called A5 + Co) of boric acid, manganese, zinc, copper, molybdenum and cobalt. Its final pH is 7.4 after cooling and CO₂ equilibration. For agar plates, UTEX adds sterile sodium thiosulfate at 1 mM.

Bold's Basal Medium (freshwater, green algae)

From UTEX:

Bold 3N uses three times as much nitrate stock, giving 8.82 mM sodium nitrate, and adds a metal solution, soil water extract and vitamin B12.

Comparing the recipes

The recipes differ widely, and the differences reflect the algae they were built for.

The Hoagland figures are calculated from Wikipedia's listing of 210 ppm nitrogen and 31 ppm phosphorus, divided by atomic masses of 14.0 and 31.0.

Worked example: nitrogen to phosphorus ratios

These ratios are calculated from the tables above.

These numbers show that culture media are not designed to mimic the Redfield ratio. f/2 is moderately nitrogen-rich relative to it. BG-11 is extremely nitrogen-rich, which suits cyanobacteria grown to high density. Bold's Basal Medium is phosphate-rich, partly because its two phosphate salts also act as a pH buffer. Media designers balance many goals beyond stoichiometry, including buffering, avoiding precipitation and keeping cultures healthy through long storage.

Worked example: which nutrient runs out first in f/2?

This example is hypothetical and assumes the algae follow the Redfield ratio exactly, which real cultures do not.

f/2 contains 882 µM of nitrate nitrogen and 36.2 µM of phosphate phosphorus. Under Redfield stoichiometry of C:N:P = 106:16:1:

• The nitrogen could support fixation of 882 × 106 / 16 ≈ 5,840 µM of carbon, or about 70 mg of carbon per liter (5,840 µM × 12.0 g/mol).
• The phosphorus could support 36.2 × 106 ≈ 3,840 µM of carbon, or about 46 mg of carbon per liter.

On these assumptions, phosphate would run out first, after about 46 mg of organic carbon per liter had been produced, leaving a third of the nitrate unused. In practice algae can stretch phosphorus by changing their composition, as the 6:1 to 60:1 range of observed N:P ratios suggests, so real cultures may grow further. But the calculation shows how a recipe sets a ceiling on biomass, and why changing a single ingredient can shift which nutrient controls growth.

Preparing nutrient solutions

Stock solutions

Weighing micrograms of cobalt chloride for every liter of medium would be impractical and error-prone. Labs instead make concentrated stock solutions of each component, or groups of components, and add small volumes. In NCMA's f/2 recipe, for example, sodium nitrate is kept as a 75 g/L stock and 1 mL is added per liter of medium. The trace metals are made up as a mixed solution from primary stocks, and the vitamins as a separate solution stored cold. UTEX notes that preparing stock solutions is both convenient and necessary to avoid errors from weighing tiny amounts.

Order of addition

Order matters, because some combinations precipitate. NCMA's L1 trace metal instructions say to dissolve the EDTA first and bring it to pH 8.0 to 8.4 so it is fully dissolved, then add the ferric chloride and make sure it too is fully dissolved, before adding the other metals. UTEX's general advice is to wait for each component to dissolve before adding the next.

Water quality

The base water matters as much as the additives. UTEX stresses that the quality of water, including natural seawater, is important, and that natural seawater and fresh water should come from an unpolluted source. NCMA makes all its seawater media, except one, in natural seawater. Laboratory "dH₂O" can mean distilled, deionized or ultrapure water, and trace contamination from water, glassware or salts can matter in experiments on trace metals.

Sterilization and pH

Most media are sterilized by autoclaving, which heats them under pressure. Autoclaving has side effects. UTEX notes that it drives carbon dioxide out of media that lack carbonate buffers, making the medium very alkaline soon after it comes out of the autoclave, and recommends waiting about 24 hours for gas equilibrium before inoculating. NCMA says autoclaved seawater media should return to a pH of 7.8 to 8.2 once they have cooled to room temperature. Heat-sensitive vitamins are sometimes added after autoclaving, as in UTEX's Bold 3N recipe, where vitamin B12 is added once the medium has cooled.

Solid media

For keeping strains on plates or slants, media are solidified with agar. UTEX's recipes call for 15 g of agar per liter, or a 1.5 percent agar medium. In the BG-11 agar procedure, the nutrient solution and the agar are autoclaved separately at double strength, cooled to 45 to 50 °C, and combined.

From flask to pond: nutrient solutions at scale

Laboratory cultures

At small scale, algae grow in flasks, tubes or bottles of sterile medium under lamps. Cultures are kept as pure as possible. Wikipedia's algaculture article notes that growers generally prefer monocultures, that pure cultures are often obtained by serial dilution followed by microscope checks, and that selective conditions can exclude competitors. Its example is the alga Dunaliella, which thrives in extremely salty water that keeps most competing organisms out.

Raceway ponds

At large scale, the most common system is the raceway pond: a shallow, oval channel in which a paddlewheel pushes water around a continuous loop. Wikipedia describes raceways as among the most common and economical methods for large-scale cultivation, usually lined with HDPE or PVC to limit water and nutrient loss. They are open to the environment, so they are vulnerable to contamination and offer no control of temperature or light. Wikipedia's algal nutrient solution article adds that raceways are cheaper but low-yield.

The modern raceway owes much to William J. Oswald (1919 to 2005), a civil engineer at the University of California, Berkeley, who developed shallow, mixed "high rate" ponds for treating wastewater with algae. A source cited on Wikipedia credits his high rate pond system with more than 90 percent of the world's commercial microalgae production. Oswald also developed the Advanced Integrated Wastewater Pond System, in which wastewater passes through deeper facultative ponds, then high rate ponds, then maturation ponds.

Photobioreactors

Photobioreactors are closed systems of transparent tubes, panels or bags in which algae grow with light supplied from outside. Wikipedia notes that in a photobioreactor the grower must supply all nutrients, including CO₂. Photobioreactors can run as batch cultures or continuously. In continuous operation, productivity peaks when the exchange rate, the time to replace one culture volume, matches the algae's doubling time. Wikipedia's nutrient solution article describes photobioreactors as more productive than raceways but mostly used at small scale and costly in energy.

Nutrients from wastewater

At large scale, buying laboratory-grade salts makes little sense. Algae can instead draw nitrogen and phosphorus from wastewater, which is the idea behind Oswald's systems and much later research. Wikipedia's nutrient solution article mentions work on growing algae with flue gas as a CO₂ source and wastewater as a nutrient source. Using wastewater cleans the water and grows biomass at the same time, but it brings contamination, variable composition and regulatory questions about what the biomass can be used for.

Harvesting

Once algae have used up their nutrients, they must be separated from the water. Wikipedia lists microfiltration and microscreens, flocculation, sedimentation, flotation and centrifugation. Some algae clump together, or autoflocculate, when the CO₂ supply is interrupted. Chemical flocculants include chitosan, which is effective but expensive, and the cheaper alum and ferric chloride. Wikipedia notes that flocculation is often too costly for large operations.

Nutrients and what algae make

Nitrogen starvation and oil

The choice of nutrient solution affects not just how much algae grow, but what they contain. The best-known example comes from the US Department of Energy's Aquatic Species Program, which ran from 1978 to 1996 and studied algae as a source of fuels. According to Wikipedia, the program's scientists collected about 3,000 algae species and identified 51 as potentially high-value strains.

One of its key findings concerned nutrient starvation. Depriving algae of nutrients such as nitrogen increased the amount of oil per cell, but it slowed cell growth so much that total oil production fell. As Wikipedia puts it, higher levels of oil in the cells were more than offset by lower rates of cell growth. This trade-off between composition and growth rate remains central to algal biotechnology.

The Roswell ponds

The same program built outdoor pond systems of 1,000 square meters each in Roswell, New Mexico. Their target was 50 grams of algae per square meter per day, which was reached on some days. Careful pH control allowed more than 90 percent utilization of the CO₂ supplied, but low temperatures reduced yields. The program's July 1998 close-out report concluded that algal biodiesel would be cost-effective only if petroleum diesel prices doubled from 1998 levels. Over about twenty years, the program cost $25.05 million in total.

Growth phase

Wikipedia's algaculture article notes that algae in their exponential, or log, growth phase generally have higher nutrient content than senescent cells. Hatcheries and food producers who grow algae as feed therefore harvest them while nutrients are still available and growth is fast.

Food and feed

Wikipedia's nutrient solution article points to protein-rich algae as food, citing the red seaweed Pyropia tenera, and to the ability of algae to grow without competing with crops for farmland. Purple laver (Porphyra and its relatives) is perhaps the most widely domesticated marine alga, and spirulina, a cyanobacterium, has a long history as food in East Africa and pre-colonial Mexico, according to the algaculture article.

The environmental side: eutrophication

When nutrients escape

The same nutrients that grow algae in a flask can cause harm in lakes and seas. Eutrophication is the buildup of nutrients in a body of water, which drives excessive growth of algae and other organisms. When the algae die, bacteria decompose them and use up dissolved oxygen. Wikipedia's eutrophication article names the main human sources as sewage, industrial wastewater and fertilizer runoff.

Evidence that phosphorus controls lakes

One of the most famous experiments in ecology confirmed phosphorus as the key nutrient in freshwater. In 1973, at Canada's Experimental Lakes Area, researchers divided Lake 226 in two with an impermeable curtain. Both halves received nitrogen and carbon, but only one received phosphorus. Only the phosphorus-fed half developed a large algal bloom. Phosphate detergents, which had been a major contributor to eutrophication, were phased out in the 1970s, after which sewage and agriculture became the dominant phosphorus sources.

Dead zones

In coastal waters, where nitrogen is usually limiting, nutrient pollution produces hypoxic "dead zones." Wikipedia's eutrophication article notes that the World Resources Institute has identified 375 hypoxic coastal zones worldwide, and that the Gulf of Mexico has a seasonal anoxic zone of more than 5,000 square miles, fueled by nutrients carried by the Mississippi River. The Redfield ratio has itself been used to compare the Mississippi with the northern Gulf in studies of this hypoxia.

Toxins and odors

Algal blooms also bring water quality problems. Wikipedia's algaculture article notes that cyanobacteria can produce odorous compounds, MIB and geosmin, detectable at parts-per-billion levels, and that some produce toxins that have caused drinking water problems. Decaying blooms can generate hydrogen sulfide and ammonia, causing odors and killing aquatic life.

Handling spent media

For anyone running algae cultures, these facts have a practical side. Spent nutrient solutions, especially nitrogen-rich ones such as BG-11, are concentrated sources of the same nutrients that cause eutrophication. Large operations need plans for recycling or treating their water rather than releasing it. Wikipedia's nutrient solution article lists environmental impact, through blooms and eutrophication if algae and their nutrients are mismanaged, among the main concerns of algal cultivation.

Limits and practical problems

Precipitation

Media are supersaturated with some combinations of ions. Phosphate can precipitate with calcium and iron, and silicate encourages precipitation, which is why NCMA recommends leaving it out when not needed. Precipitates remove nutrients from solution and can foul equipment.

pH drift

As algae photosynthesize, they remove CO₂ from the water, and the pH rises. Dense cultures can reach high pH quickly, which changes nutrient chemistry and can stop growth. Large systems control pH by adding CO₂, as the Roswell ponds did.

Contamination

Rich media feed bacteria, fungi and competing algae as well as the intended strain. Open ponds are highly vulnerable. Wikipedia notes that Chlorella farmers often re-inoculate ponds frequently, before invaders can multiply.

Cost and resources

At commercial scale, nutrients are a real cost. Wikipedia's nutrient solution article lists production costs that remain higher than fossil fuels, along with processing and resource availability, as the main barriers to algal fuels. Using waste streams for nutrients helps, but it adds variability.

One medium does not fit all

Every species has its own needs. A medium that grows one diatom beautifully may fail with another. Culture collections publish the medium they use for each strain, and those recommendations are the best starting point.

Choosing a medium

For anyone starting to grow algae, the choice of nutrient solution follows from a few questions about the organism and the goal.

Fresh water or seawater? Marine algae need the major salts of seawater, so they are grown in enriched natural seawater such as f/2 or L1, or in an artificial seawater base. Freshwater algae grow in recipes built on distilled or deionized water, such as BG-11 or Bold's Basal Medium. Using the wrong base usually kills a culture outright, because the cells cannot cope with the change in salinity.

What kind of alga? Diatoms need silicate. Cyanobacteria usually need no vitamins but tolerate, and often benefit from, the very high nitrate of BG-11. Nitrogen-fixing cyanobacteria can be grown without combined nitrogen, in a medium such as BG11 0. Many marine flagellates need vitamin B12. Culture collections list a recommended medium for each strain they hold, and that recommendation is the safest starting point.

What is the purpose? Long-term maintenance of a strain favors modest nutrient levels that keep cells alive without crashing into dense, stressed cultures. Biomass production favors richer media, such as Bold 3N rather than Bold's Basal. Experiments on metal toxicity or limitation call for chemically defined artificial media such as Aquil, in which trace metal availability is controlled. Experiments on nutrient starvation use media with one nutrient deliberately lowered or removed.

Liquid or solid? Liquid media suit growth and harvesting. Agar plates and slants suit isolation of single colonies and storage. Some species grow poorly on agar and are kept only in liquid.

What scale? A few liters of laboratory medium cost little, but thousands of liters do not. At pond scale, growers switch to agricultural-grade nutrients or wastewater and accept less precise chemistry in exchange for lower cost.

What happens afterward? Planning for disposal or recycling of spent medium is part of choosing it. A nitrate-rich medium used at large scale is, in effect, a concentrated fertilizer once the algae are removed.

Common misconceptions

"Nutrient solution is just fertilizer in water." Fertilizers and algal media share ingredients, but Wikipedia notes that algal nutrient solutions are designed for aquatic use and have a more precise composition. Garden fertilizers often lack the chelated trace metals and vitamins many algae need, and may contain forms or amounts of nutrients that do not suit algae.

"More nutrients always means more algae." Growth stops when any one requirement runs out, including light and CO₂. In dense cultures, light usually becomes limiting before nutrients do. Extra nutrients beyond that point add cost and pollution risk without adding biomass.

"Seawater already contains everything marine algae need." Seawater supplies the major salts, but its nitrate, phosphate, iron and vitamins are often too low for dense cultures. That is why f/2 and L1 are called enriched seawater media.

"All media should follow the Redfield ratio." The Redfield ratio describes the average composition of ocean plankton, not an optimal recipe. Standard media range from an N:P ratio below 2:1 in Bold's Basal Medium to roughly 100:1 in modified BG-11.

"Starving algae makes more oil." Starvation raises oil per cell, but the Aquatic Species Program found it slowed growth so much that total oil output fell.

"Hoagland solution is an algae medium." It is a hydroponic solution for land plants. Diluted, it can grow aquatic plants and some algae, but algal culture collections use their own media.

Frequently asked questions

What is the most common algal nutrient solution?

For marine algae, f/2 and its successor L1 are among the most widely used. For freshwater cyanobacteria, BG-11 is standard. For freshwater green algae, Bold's Basal Medium and its variants are common.

What does f/2 mean?

It is half-strength "f medium," described by Guillard and Ryther in 1962. Guillard published the f/2 version in 1975.

Do all algae need silicate?

No. Diatoms need it to build their silica cell walls. Most other algae do not, and NCMA recommends omitting silicate when it is not needed because it encourages precipitation.

Why do recipes include EDTA?

EDTA is a chelator. It binds iron and other trace metals so they stay dissolved and available to algae, and it buffers the free concentration of metals that are toxic at higher levels.

Why do media need vitamins?

Many algae, particularly marine species, cannot synthesize vitamin B12, thiamine or biotin and must take them from the water. Standard marine media add all three. Many cyanobacteria need none.

Can I make an algal medium at home?

Simple media can be prepared with basic lab equipment, but accurate results require pure chemicals, careful weighing through stock solutions, clean water and sterilization. Culture collections such as UTEX and NCMA sell premixed media and kits for people without a lab.

Why does my medium become very alkaline after autoclaving?

Autoclaving drives out dissolved CO₂. In media without carbonate buffers, the pH rises sharply. UTEX recommends waiting about 24 hours for the medium to re-equilibrate with air before inoculating.

How are nutrients supplied in large ponds?

By dosing agricultural-grade fertilizers or using nutrient-rich wastewater, with CO₂ added to sustain growth and control pH. Laboratory-grade media are usually too expensive at that scale.

How do algal nutrients relate to water pollution?

The nutrients that feed algal cultures are the same ones, nitrogen and phosphorus, that cause eutrophication in lakes and coastal waters. Spent media and wastewater from algae farms need proper management.

Summary

An algal nutrient solution is a recipe for life in water: nitrogen, phosphorus, often silicon, a careful sprinkling of trace metals held in solution by chelators, and for many species a few vitamins, all dissolved in clean fresh water or natural seawater. The best-known recipes, Guillard's f/2 and L1 for marine algae, BG-11 for cyanobacteria and Bold's Basal Medium for freshwater greens, grew out of decades of trial, error and occasional correction, and each reflects the biology of the algae it was designed for.

Behind them lie a few big ideas: the Redfield ratio of 106:16:1, the concept of a limiting nutrient, and the chemistry of keeping metals available without poisoning the cells. The same ideas explain why nutrient choices change what algae produce, why scaling up is expensive, and why the nutrients that make algae thrive in a flask can choke a lake or a coastal sea when they escape.

Sources

• Algal nutrient solution (Wikipedia)
• Algaculture (Wikipedia)
• Redfield ratio (Wikipedia)
• Hoagland solution (Wikipedia)
• Eutrophication (Wikipedia)
• Aquatic Species Program (Wikipedia)
• William J. Oswald (Wikipedia)
• Algae media recipes, National Center for Marine Algae and Microbiota (Bigelow Laboratory)
• NCMA f/2 medium recipe sheet
• NCMA L1 medium recipe sheet
• NCMA BG-11 medium (modified) recipe sheet
• BG-11 Medium, UTEX Culture Collection of Algae
• Bold Basal Medium, UTEX Culture Collection of Algae
• Bold 3N Medium, UTEX Culture Collection of Algae
• Media recipes, Culture Collection of Algae and Protozoa (CCAP)