Do you have a freshwater fish tank? Do you notice brown scum coating the glass or other items in the tank? You probably have diatoms growing in your tank! Diatoms exude a polysaccharide gelatinous-like coating in which they reside and can form long end-to-end chains. This chain is what looks like brown slime in the tank. If you are careful, you might be able to sample a “tube” of diatoms to mount on a slide and view with your Foldscope. Can you see the diatoms in the tube?
In the laboratory, many species of diatoms, especially the centrics, can be difficult to grow. However, there are “weed” species that are easily grown, and these taxa are considered to be nuisance or invasive species, depending on where you find them and in what quantity. One case in point is the freshwater diatom Didymosphenia geminata which has become an invasive in the Pacific Northwest. Some species of Nitzschia, a freshwater diatom genus, reproduce at a fast rate and may be a cause of some algal blooms. A fast growing marine diatom genus containing species that are toxic is Pseudonitzschia. These diatoms are capable of producing domoic acid that, if ingested, can affect the neurological system.
Diatoms are sensitive to particular environmental conditions and will grow optimally with the right amount of light, temperature and nutrients. Diatoms feed on phosphorus and nitrogen compounds and incorporate silica into their intricate shells. There are many micronutrients that are important in diatom growth as well. The particular amounts of nutrients and micronutrients are tailored to particular diatom species. There are products that are supposed to enhance the growth of diatoms in marine tanks—do these products work? If you have a marine tank, you could give them a try (for example, https://nualgiaquarium.com/nano-silica-diatoms/ ). There are also kits available for sale for growing different kinds of algae, including diatoms, but growth is dependent on specific growth media and light and temperature conditions (for example, http://www.algaetoday.com/algae_biofuels/algae_nutrients). Growing diatoms can be challenging and require some experimentation on your part. Your best bet for growing freshwater diatoms is a not very clean fish tank. Otherwise, collect diatoms and other algae at your nearest pond, river, or puddle, or scrape some algae and diatoms off of rocks and other wet materials near a pond or river. There are plenty of places to find diatoms in nature!