To many, this post is akin to “how to give fleas to your dog,” or “how to grow ragweed.”  Bryopsis is a benthic (substrate-associated) genus of algae that most aquarists dread seeing in their tanks.  Bryopsis can thrive in reef aquaria, to the point that it can smother and kill corals.  Many methods of eradication have been explored, including near-toxic levels of magnesium, and the anti-fungal medication fluconazole.

E. clarki feeding on Bryopsis; the red alga, Botryocladia, is in the background.

On the other hand, I really want the stuff.  As far as I can tell, it is the favorite food of Elysia clarki.  E. clarki will eat other macroalgae, such as Penicillus, Avrainvillea or Halimeda, but, given the choice, they will go straight for Bryopsis and stay there.  Further, they have consistently produced eggs only at times when they have had unlimited availability of Bryopsis.  One of my primary goals is to maintain a self-sustaining colony of slugs, so I very much want a constant, adequate supply of algae.

I’d be perfectly happy if I could count on the outbreaks of local aquarists to supply my needs.  Unfortunately, sources have been spotty.  My primary source, Justin, who had a beautiful outbreak in a 500-gallon tank, unfortunately managed to get his infestation under control.  He swears the controlling factor for him is the level of phosphate; if PO₄ levels creep up, he gets an outbreak.  Others have provided quality material as well, but not as consistently. Yet others have offered species that turn out not to be Bryopsis.  The take home lesson is that people who are trying to eradicate something are not the most dependable source.

The whole gang enjoying the new batch of algae provided by a local retailer. 4/17/16.

How hard can the stuff be to grow?  It is a pest, after all.  Like any aquatic plant, it needs light, nutrients, and circulation.  It gets a little more difficult when you try to provide for the plants’ requirements on a consistent basis.  As far as I can tell (and PLEASE email or comment if I am wrong), nobody has published specific requirements of Bryopsis for PAR (photosynthetically-available radiation), macronutrients (e.g., carbon, nitrogen, phosphorous), or micronutrients (like iron, copper and cobalt).  When marine aquarists have outbreaks, much of the nutrient load is derived from fish food (well, really fish poop), which is not measured.  In fact, most reef aquaria don’t have Bryopsis outbreaks, which suggests that the unlucky few have managed to combine all the right ingredients.  For a constant supply of Bryopsis, I need to provide for all of its needs.

If I wanted to grow phytoplankton, the single-cell, free-floating relatives of Bryopsis, I would be in luck.  Because of the important role of phytoplankton in our oceans, and because there is a growing industry using them to generate hydrocarbons from sunlight, growing conditions have been explored and published at length.  The most popular formula is Guillard’s F/2, so named because it consists of his formula F diluted to 50% (Guillard & Ryther, 1962, Can. J. Microbiol. 8:229), is readily available through suppliers such as Florida Aqua Farms, or even Ebay.  The formula provides a balanced blend of nitrogen and phosphorous (with an option to add silicon if you want to grow diatoms), along with a mix of trace elements and vitamins.  Mix it up, add a starter of the phytoplankton of your choice, bubble in some CO₂ as a carbon source, and you’ll soon have a thriving culture of green water.

I started dosing the Florida Aqua Farms version (“Plant Fuel Too”) almost a year and a half ago, using the complete dry mix (minus silicates).  The Bryopsis started to look darker and happier pretty quickly.  Perhaps unsurprisingly, there was also a phyoplankton bloom that turned the water green and generated a layer of green scum on the surface of the water.  Also, the added nutrients seemed to increase levels of nuisances like cyanobacteria (a.k.a., red slime algae). That would have all been tolerable, but the biggest sticking point was that Bryopsis would go through cycles of boom and bust, which is not conducive to long-term management of a slug colony.  I needed to get the right balance of nutrients that would get the Bryopsis to grow maximally and out-compete the species I don’t want.

With all of the possible factors (e.g., light, circulation, temperature, nutrients) to be within the right ranges, there were a lot of variables to play with:

• Levels of light seemed to be within the range I observed in tanks with steady outbreaks, so I left that alone.
• Bryopsis always grows well in the areas with the strongest water flow (e.g., pump outlets).  I am not sure why, maybe it keeps other algae from settling on it, but I increased the flow to ridiculous levels in the 15-gallon primary culture tank by combining a power filter Aquaclear AC70), powerhead (Hydor 2450) and wavemaker (MaxSpect Gyre XF130) that were all rated for tanks 3-5 times the size.
• I plunged back into the literature to try to find papers examining the nutrient requirements of benthic macroalgae, rather than phytoplankton.  After a little digging, I found that species related to Bryopsis demand much more carbon and nitrogen, and are not as dependent on phosphorous (e.g., Atkinson and Smith, 1983, Limnol. Oceanogr. 28:568; Pedersen and Borum, 1997, Mar. Ecol. Prog. Ser. 161:155).

In order to separate the variables a bit better, I started using Florida Aqua Farms original “Plant Fuel,” which contains all of the trace elements and vitamins, but no nitrogen or phosphate. That would allow me to play with levels of trace elements, N, and P separately. I also added a source of carbon.  Because white vinegar is cheap, always available, and is readily broken down into CO₂ by bacteria, it seemed like a good choice.

By adjusting the relative input of all four ingredients (C, N, P and trace elements), combined with very high flow in the algae culture tank, I was able to get significantly better growth.  The final improvement was something that I had not expected to have such an impact: rebuilding the system.  Within about a week of the big move, the algae was thriving on the racks of plastic eggcrate I was using as a substrate.  I suspect that the 50% water change and improvements to circulation in the new system played a large part in the improved growth.

Bryopsis growing on eggcrate in 15 gallon algae culture tank. 3/2/17

By growing on plastic racks, Bryopsis cultures can be moved to slug tanks to be consumed, and then removed for regrowth.

Bryopsis-covered plastic rack in 20L slug tank. 3/23/17

So we’re making forward progress.  We are controlling the growth of food algae to the point where it seems to be sustainable.  As a side benefit, the other macroalgae are also thriving.  For example, the three deep green Penicillus in the photo below are new sprouts from the ratty old plant in the center.

Three new Penicillus plants surrounding the faded parent taken from the wild. 4/11/17

At the moment (and likely to change) the system receives major elements at the (molar) ratio of 150:30:1 C:N:P.  This is roughly in the range reported for other macroalgae, and does not support much growth of cyanobacteria or phytoplankton.  In other words, it looks like I am providing nutrients at about the rate at which the are being consumed by the desired algae.

• N: dosing KNO₃ (13.9% N); stock = 80 grams/liter (g/l), diluted to 20 g/l; dosing 80 milliliters/day (ml/d); that equals 0.222 g/d of N, or 0.0159 moles/day
• P: Dosing KH₂PO₄ (22.8% P); stock = 11.6 g/l; diluted to 1.45 g/l; dosing 50 ml/d; 0.0165 g/d or 0.00053 mol/d
• C: from acetic acid: 5% solution (50 g/l); 48 ml/d; C = 40%; 0.96 g/d; 0.08 mol/d
• K: from KNO₃ AND KH₂PO₄:  0.64 g/d or 0.021 mol/d
• Guillard’s F/2 trace element and vitamin solution: diluted 1:20, dosing 100 ml/d;
• I have also started adding calcium hydroxide (CaOH₂, kalkwasser) to the water used to make up for evaporation to maintain Ca.

There is still plenty of room for improvement.  The priority for the moment is eliminating or reducing dinoflagellates in the algae culture tanks.  They do not seem to cause too much trouble in the slug tanks, but they form a slimy film on the Bryopsis in the culture tanks, presumably reducing growth, competing for nutrients, and producing toxins.

Dinoflagellates at low magnification. They can be seen as strings of brown dots surrounding the green algae branches. 4/11/17

At higher magnification they are kind of pretty.  Based on the shape, size and general appearance, it looks a lot like Ostreopsis ovata (Faust and Gulledge, 2002, Contr. U.S. Natl. Herb. 42: 1-144).  Like many dinoflagellates, O. ovata produces toxins, some of which are toxic to mice (Nakajima et al., 1981, Okinawa Bull. Jpn. Soc, Sci. Fish. 47: 1029).  The effects on slugs or other invertebrates is not known.

DInoflagellate from Bryopsis tank, 400X magnification. Gold color is from densely packed chloroplasts.  4/6/17.

At the moment, I have reduced the photoperiod (the time the lights are on), and am rinsing the Bryopsis racks in clean artificial seawater periodically to reduce the population.  At some point, it might be necessary to use harsher methods, such as antibiotics (metronidazole seems to affect dinoflagellates somewhat specifically) or starting a clean culture of Bryopsis and preventing entry of other species using UV sterilization.

For now, there is plentiful food for the slugs, which is a step forward.