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Diatoms are single-celled phytoplankton with walls made of silica. They are one of the most common contaminants within a kelp hatchery, as their unique shape allows them to pass through some of the filtration steps, entering tanks via the seawater source or during the cleaning stages of the wild sorus blades. Diatoms are able to rapidly reproduce and quickly outcompete young kelp blades, particularly for substrate space on the seed string. 

Diatoms can come in many unique shapes, but most commonly in the GreenWave hatchery we see the elongated, symmetric “pennate” diatom form, which appear as ovate specks on the seed string when looking under the microscope. If diatom abundance is too high or the proper remediation steps are not taken, the spool(s) can be overtaken by diatom growth and they will be visible with the naked eye as dark brown splotches on the spool. 

During the first two weeks, the kelp is particularly susceptible to diatoms, as the spores and gametophytes are just starting to establish their presence on the seed string. To fully prevent this risk, tanks can be treated with a germanium dioxide (GeO₂) solution, a treatment that prevents silica uptake, stopping cell wall growth of the diatoms¹. A stock solution of 250 mg of GeO₂ per liter of deionized (DI) water is recommended to be added to your culture tanks at the concentration of 0.4-2 mL^2,3 GeO₂ stock solution per liter of tank water. GreenWave works on the lower end of that range (0.4 mL GeO₂ solution per L tank water) because GeO₂ is known to also slow down kelp growth. It’s recommended to only add the solution for the first two weeks of a culture for this reason. 

Should any diatoms appear later in the season, GeO₂ can be used again to help control an outbreak. Diatoms can be very difficult to eliminate once they enter your hatchery, so starting treatment sooner, rather than later, is best.

¹Lewin, J. 1966. Silicon metabolism in diatoms. V. Germanium dioxide, a specific inhibitor of diatom growth. Phycologia 6:1­ 12.
²Forbord et al. 2012. Development of Saccharina latissima (Phaeophyceae) kelp hatcheries with year-round production of zoospores and juvenile sporophytes on culture ropes for kelp aquaculture. Journal of Applied Phycology. 24:393-399. SOI 10.10007/s10811-011-9784y.
³Redmond, Sarah et al. (2014) New England Seaweed Culture Handbook: Nursery Systems. Connecticut Sea Grant CTSG‐14‐01. 92 pp. PDF file.