BG-11 (Blue-Green medium No. 11)

A5 + Co trace element solution

per 100 mL of solution · usage rate 1 mL per 1 L of medium (sufficient for 100 L) · storage at 4 °C in dark glass, 12 months

Dissolve NaNO₃ in 800 mL of water → add K₂HPO₄ → MgSO₄·7H₂O → citric acid → ferric ammonium citrate → Na₂EDTA → Na₂CO₃ → 1 mL of A5 + Co solution → last of all, with vigorous stirring, add CaCl₂·2H₂O → make up to 1000 mL → adjust the pH to 7.1 → autoclave at 121 °C for 15 min.

• CaCl₂ is added last and only to a dilute solution: when added to a concentrated mixture containing phosphate and carbonate it produces a white turbidity of Ca₃(PO₄)₂ and CaCO₃, irreversibly removing phosphorus and calcium from the medium.
• Molar equivalents of the main components: NaNO₃ — 17.6 mM; K₂HPO₄ — 0.18 mM; MgSO₄ — 0.30 mM; CaCl₂ — 0.24 mM.
• For solid medium use 10–15 g/L agar. The agar is autoclaved SEPARATELY in a double volume of water, the medium is prepared at double strength, and the two are combined at 45–50 °C — autoclaving them together causes hydrolysis of the agar and cloudy plates.
• For plates add 0.3 g/L Na₂S₂O₃·5H₂O: the thiosulfate binds the products of thermal decomposition of agar, which are toxic to cyanobacteria, and markedly improves plating efficiency.
• Dissolve the salts one at a time, in the order given; H₃BO₃ dissolves slowly — warming to 40 °C is permissible. Cobalt is added from a primary stock solution: a weighed amount of 4.94 mg cannot be measured on an ordinary balance. Primary Co stock solution: 494 mg Co(NO₃)₂·6H₂O in 100 mL of water, take 1 mL. Without cobalt the solution is simply called "A5". The appearance of a precipitate or turbidity indicates that the solution is no longer usable.