For all media, and especially seawater media, it is advisable to autoclave the phosphate and iron separately and to add it to the medium after autoclaving to avoid the precipitation of insoluble salts. If possible Teflon vessels rather than glass vessels should be used for autoclaving seawater to avoid precipitation.
Where vitamins are required, these should be filter-sterilised separately and added to the medium just before inoculation.
For the culture of diatoms, media which do not contain Si should be supplemented with 0.05 g.L-1 Na2SiO3.9H2O. Some diatoms, as for example Skeletonema costatum, have a very high Si requirement and will require Si supplementation to maintain good growth.
If required, media may be solidified with 1-1.5% agar.
If seawater is used as a base for the medium (i.e. f/2 medium) this should be collected as far off-shore as possible to reduce the possible effects of contaminants (organic substances, pesticides etc.) due to land-runoff or rivers. For best results the seawater should be ‘aged’ by keeping in a cold room (approx. 3oC) in the dark for at least one week, and then stirring with about 2g.L-1 activated charcoal powder for 30 min before filtering through Whatman No. 1 filter paper. Small volumes of seawater media should be filter-sterilised rather than autoclaved to avoid possible changes in salinity. Larger volumes can be sterilised by autoclaving, pasteurisation1 or with UV light. Detailed description of sterilization methods can be found in Kawachi and Noél (2005) If a fully