Walne (Conway) medium

Walne's main nutrient solution

per 1 L · usage rate 1 mL per 1 L of medium · storage at 4 °C in dark glass, 6 months

Walne's rare metals solution

per 100 mL · usage rate 1 mL per 1 L of the main nutrient solution · storage at 4 °C, 12 months

Walne's vitamin solution

per 100 mL · usage rate 0.1 mL per 1 L of medium · storage at −20 °C, 6 months

Filter and sterilise the seawater. To the cooled water add 1 mL of the main nutrient solution, and when working with diatoms 2 mL of the silicate solution, then 0.1 mL of the vitamin solution (aseptically, after cooling).

• The main solution combines nitrogen, phosphorus, iron and part of the trace elements in a single bottle — this is the medium's chief practical advantage under production conditions.
• The rare metals solution and the vitamin solution are prepared and stored separately from the main solution.
• In aquaculture the medium is often prepared with untreated but filtered and UV-treated seawater, without autoclaving; in that case the dose of the main solution is increased to 2 mL/L.
• A production-scale solution: it is prepared per litre because in aquaculture the medium is made by the tonne. For laboratory use 100 mL can be prepared by reducing all the weighed amounts tenfold and taking 0.1 mL of the rare metals solution. The water is warmed to 40–50 °C and acidified with 1–2 mL of conc. HCl; otherwise the EDTA and boric acid dissolve incompletely. Turbidity or a brown Fe(OH)₃ precipitate during storage indicates that the solution is no longer usable.
• This solution does not go into the medium directly but into Walne's main nutrient solution. It becomes turbid on storage; to clarify it, add 2–3 drops of conc. HCl.
• Biotin is absent from the classical Walne formulation. The solution is added only to cooled medium and only by sterile filtration.