A technical guide to culture media for kelp gametophytes and sporophytes — Provasoli, f/2, what each nutrient does, and how to mix, sterilise and dose it.
Everything a seaweed farm becomes — every hectare of forest on every longline — passes first through a beaker of nutrient solution the size of a coffee cup. The microscopic gametophytes that make kelp seedstock (see Kelp Gametophytes: The Hidden Generation) cannot forage for their own nitrogen and iron the way an ocean forest does; in the culture room, the water you mix is the entire world they eat from. Get the medium right and cultures hum along quietly for months. Get it wrong and you learn, slowly and expensively, that seaweed farming starts with chemistry.
This guide covers what a kelp hatchery medium must contain, the classic recipes (Provasoli, f/2, and their working descendants), and the practical craft of mixing, sterilising and dosing them.
A culture medium replaces three things the ocean normally provides: macronutrients (nitrogen and phosphorus, plus silicon for the diatoms you hope to exclude), micronutrients (trace metals such as iron, manganese, zinc, copper, cobalt and molybdenum), and vitamins (notably B12, thiamine and biotin for many microalgae). For kelp gametophytes and early sporophytes the essentials that most often limit health in practice are:
The broader ocean context for these elements is covered in The Ocean's Nutrient Engine and the stoichiometry that underpins all of it in The Redfield Ratio. Culture media are, in essence, Redfield made controllable: the same elements, delivered at hatchery concentrations, with a chelator standing in for the ocean's organic soup.
Almost every kelp hatchery medium in the world is a descendant of a small family of formulations developed for phycology labs in the mid-twentieth century.
The workhorse of kelp culture. Erich von Stosch and later Luigi Provasoli's enriched seawater formulas deliver nitrate and phosphate at modest concentrations, a full trace-metal suite bound with EDTA (often with an iron citrate component), a soil or seawater extract for unknown factors, and vitamins. Its defining virtue for macroalgae is moderation: nutrient spikes timed to support growth without turning a culture dish into an algal soup of contaminants. Many kelp hatcheries run "half-strength PES" for gametophyte maintenance and fuller-strength during sporophyte induction, because hungry, slightly lean gametophytes are often healthier gametophytes.
Guillard's f-series media (f/2 is half-strength "f") were built for marine microalgae — diatoms and flagellates — at much higher nutrient concentrations, with a silicate addition for diatoms. For kelp you generally omit silicate (feeding your enemies feeds epiphyte diatoms) and dilute liberally; f/10 to quarter-strength f/2 variants are common working media for sporophyte nursery phases when rapid biomass is wanted and contamination pressure on a clean line is low.
Most production hatcheries converge on something like: seawater + nitrate + phosphate + iron/EDTA + a simple trace-metal and vitamin stock, at concentrations borrowed from PES — because the fewer exotic components you rely on (soil extract is fundamentally unreproducible), the more consistent your cultures run year to year.
Whatever recipe you adopt, never weigh powders into every batch of medium. Make concentrated stocks:
A finished working medium is then: filtered seawater + measured shots of each stock, diluted per your recipe. Write the recipe as a checklist pinned above the bench, including units — nearly every hatchery horror story begins with a misplaced decimal.
The medium is only as good as the water it enriches. Hatchery Water Quality covers source selection; for media work, the chain is typically:
Clean work habits — flaming or sterile-tip discipline, clean benches, quarantine of new cultures — matter more than any single equipment purchase; they are the daily practice behind the systems described in Setting Up a Seaweed Hatchery.
Two dosing philosophies coexist:
The dose rate itself is read off the biology, not the recipe: pale cultures, fouling ratios, and growth per unit light guide whether you feed more, less, or simply refresh. Hatchery Light Cycles pairs directly with this — nutrient dose divorced from photoperiod and temperature is numbers without context.
Some growers do use hydroponic-style N-P-K fertilisers at very dilute rates for large-volume nursery water, and it can work — but purity, chelation and trace-metal suites are inconsistent between products and batches. For gametophyte culture, where a hundred grams of tissue underpins your whole season, dedicated stocks from reagent salts are cheap insurance.
Often yes, or at least "yes" is the safe answer — B12, thiamine and biotin appear in most successful long-term gametophyte media, and some strains decline without them. Leaving them out to save money is a false economy that usually reveals itself months later as a slow, unexplained fade.
Commonly every two to four weeks at low density under maintenance lighting, more often as density rises or during induction. But the culture answers the question, not the calendar: turbidity, colour and contamination checks set the real schedule.
Historic enrichment media added soil extract to supply unknown organic factors; many kelp lines genuinely grew better with it. Most production hatcheries abandon it because it is unreproducible — every batch of soil is different — preferring defined media that behave the same in June as in January.
Filtered natural seawater is standard for production scale both for cost and because it carries background organics kelp uses. Artificial seawater is a legitimate research and quarantine tool where water quality is unreliable, but it is more expensive per litre and less forgiving of recipe mistakes.
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