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Polar ice Diatoms, Dr. Gordon Taylor
It has been long held that silica in reef tanks is bad, very, very bad. We avoid beach and play sand like it’s poison because it is believed that silica will cause an undesirable diatom bloom: brown glass, brown gook on rock and substrates, and unhappy reefkeepers. But what if we could control this system and use it to our advantage…say, to increase nutrient export? Let’s take a look at some old data, some new data, and a controversial hypothesis regarding the intentional dosing of silica to our reef aquaria.
Diatoms take many shapes (the photo above, by Dr. Gordon Taylor of the School of Marine and Atmospheric Sciences, my academic “home”, shows a collection of polar diatoms). It is well established in biological oceanography that diatom blooms occur when seasonal upwelling of nitrates combines with longer sunlight and warmer spring waters. These diatoms can exist as benthic coatings, like the brown coating we see in a newly cycling tank, but the bulk of them in the ocean are in the water column. Once the diatom bloom commences, nitrate levels drop precipitously until they can no longer support growth, and the diatoms are either eaten by zooplankton or fall to the bottom of the sea. One thing that is unique to diatoms is that they build shells out of silica, or “tests”, which are symmetrical and some of the most beautiful creatures you will find.
The Old Data: Because we have all learned the dogma that “silica is bad”, silica levels in our tanks are typically way below the level found in reefs (Randy Holmes-Farley, 2003 ). As a result diatom growth is silica-limited, so the diatoms are not performing one of their useful tasks—nitrate removal. The diatoms that do exist in established tanks are not only found as benthic coatings but throughout the water (many commercial phytoplankton products contain diatoms).
The New Data: Dr. Ken Feldman, a chemist at Penn State University, gave a lecture at MACNA XXI concerning skimmer efficiencies and skimmate production and quality. As part of the study he dried down and analyzed the minerals contained in typical skimmate, and largely there were no surprises—carbon, nitrogen, phosphorus. But one thing that struck me at the time was an unexpectedly high concentration of silicon. Because the skimmate was rinsed to eliminate all dissolved minerals, the silicon could only have been part of an organism—and here’s where the hypothesis comes in.
The Hypothesis:
- Skimmers are particularly good at removing planktonic diatoms, hence the high silicon concentration in skimmate.
- Diatoms in reef tanks are growth-limited by a lack of silica, but are good nitrate utilizers.
- We can dose silica to increase the diatom growth rate, the diatoms will utilize the nitrates in the water then get skimmed out, thus giving us a new way to control nutrient export.
Now of course this is merely a hypothesis and will need to be tested to prove its worth, but some anecdotal evidence leads me to believe silica dosing might be beneficial and at least do no harm. Dr. Holmes-Farley dosed silica into his tanks with no ill effect—in fact, he reported that the film that did aggregate on the glass was green and easier to remove than what we usually scrape off. “The Grumpy Old Reefer” reports dosing silica as well, with no ill effects. The added benefits of silica dosing may be interesting as well—sponges also depend on silica, so they may also grow faster. We will have to compare skimmate and nutrient levels from tanks with added silica to control tanks to see if there is a measureable benefit
So…who’s going to be the first to give Silica Dosing a whirl?
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This thing has 5 Comments
Greetings All !
Nice post, Christine. I enjoy reading your perspective immensely. It is fascinating to watch the anecdotal-based “data” of the cyber-herd acclaimed “gurus” that has driven the reefkeeping hobby online discussion for the last 15 years begin to crumble around the edges, isn’t it?
“So…who’s going to be the first to give Silica Dosing a whirl?”
All of us who are interested in the sustained culture of non-photosynthetic corals and their allies, and … FWIW … if we’re going to talk “firsts” at recognizing the potentials of dosing silicates , I’m afraid we’re going to have to set the WayBack machine back at least three decades.
Less technology, more biology.
Less anecdotal correlation, more causality.
The idea of dosing silica in a reef is so interesting to me. I remember a time when our R.O. membranes and Phosphate absorbers all had Silicate reducing qualities that were explicitly marketed. You definitely don’t want to dose Si to a new reef but it makes total sense in an established reef: Not only will Si spur diatom growth for nutrient export, but it could also be just one more link in the food web that we need to become better at growing Non Photosynthetic Corals. Great brainstorming Chris, thanks for sharing.
I find it strange that people do not like the idea of using silica in their tanks. I have been using silica sand in my tanks now for almost 10 years. Makes great media for frag system and its cheap (free except for going and getting it). I have even been using base rocks (limestone) with SiO4 bands inside of the rock and find it does just as well as the expensive base rock sold in most shops.
I think it’s easier to take in black and white issues like bacteria=good, silica=bad for most people in the hobby. Its the gray areas that takes further analysis to weigh out the pros and cons that surprisingly not more people dive into. A lot of people rely on a handful of “advanced” hobbyists to dictate the course of action in the hobby instead of looking to advance it themselves. It’s great that you’re taking concepts like these and getting the dialogue going to advance misunderstood aspects of the hobby!
Hi Christine,
Silica dosing was a practice for a short period in the mid 90s, but the fear was that an undesirable outbreak would occur. The more we come to appreciate nutrient flux in a system it is entirely likely that we will be removing silicon (assimilated by planktonic diatoms) as fast as we are adding it.
My personal thoughts would run towards a balanced silicon/carbon additive and essentially kill two birds with one stone.
P.