Stock solutions and salts

In our recipes we refer to “working stocks” and “primary stocks”. Working stocks are those whose aliquots contribute directly to making the final media. Primary stocks are normally made where several single substance solutions are then combined to form the working stock, eg. CuSO4.5H2O and ZnSO4.7H2O are two of the primary stocks used to make up the Trace Metal working stock in f2 medium.

Culture Collections and scientists undertaking experimental research will use chemicals that are at least Analytical Reagent Grade, both to reduce trace contaminants that may be harmful to some microalgae and also to ensure experimental rigour.  While we suggest all stock or starter cultures be grown with AR grade chemicals it is understandable that in mass culture applications (> 20 – 50 L) , particularly for aquaculture, these chemicals may be too expensive when bought in bulk quantities.

Stock solutions are made up by accurately weighing the prescribed amount of nutrient and dissolving in a specified volume of distilled water, if possible in a volumetric flask. Some nutrients will readily dissolve, others need heat and stirring to fully dissolve. In contrast vitamin stocks are heat sensitive and should not be subjected to heat treatment and should also be kept in the dark. Failure to fully dissolve the primary stocks of some nutrients such as EDTA can lead to gross precipitation when these stocks are combined to make the media.

Nutrients come with different salts and hydration. For example, while copper and zinc may be two desired active constituents they are readily obtained from suppliers with either SO4 or Cl2 salts (ie CuSO4 or CuCl2 and ZnSO4 or ZnCl2). Some nutrients also come with different hydrations, ie the .nH2O suffix. Substituting one form for another may have no effect on the growth of some microalgae species, but it can lead to poor growth in others and also lead to unwanted and time consuming precipitation problems as the overall ratio of salts in the medium has changed. Therefore deviating from the prescribed recipes is to be avoided and ordering the correct form is recommended.