Problem Dinoflagellates and pH

Dinoflagellates are widespread in nature, and vary considerably in their habits. Some are free floating, photosynthetic organisms and are classic phytoplankton. Others can become symbiotic photosynthetic organisms living inside corals, clams and other marine organisms (i.e., zooxanthallae). Some dinoflagellates are parasites on fish; still others are predators. These are often larger than typical dinoflagellates (up to 2 mm long), and they move through the water consuming smaller organisms. Some dinoflagellates are bioluminescent, and others release toxins (e.g., red tide toxins) that can travel all the way up the food chain to humans. Previous articles on all types of dinoflagellates can be found here and here.

This article focuses on one other type of dinoflagellate that can become the nemesis of reefkeepers. These are photosynthetic dinoflagellates that attach to surfaces. In some aquaria these gooey, snot-like masses of organisms can coat everything in sight, from the aquarium's walls to the corals it contains. Not only are they unsightly, but they can smother other organisms and sometimes can kill from afar by releasing toxins into the water.

In particular, this article focuses on one of the traditional ways of treating problem dinoflagellates with elevated pH.

The sections are:

Identification of Dinoflagellates

The first step toward developing an appropriate treatment regimen for a particular pest is to properly identify it. Unfortunately, that is far easier said than done, and ordinary aquarists may have to settle for the idea that a pest "might be" X, so they should consider trying Y and Z treatments. While problem dinoflagellates have certain identifying characteristics, other organisms look somewhat similar, including cyanobacteria, some types of algae, diatoms and bacteria. While some treatments may apply to many of these pests (such as reducing their available nutrients), others are more specific. Treating with elevated pH, for example, is not generally useful in treating these other pests.

The problem dinoflagellates encountered in reef aquaria are often brown, although they can also be almost colorless, green, yellow/green or rust colored. They form masses that coat surfaces such as the tank's walls, rock and sand. The coating often becomes filled with oxygen bubbles during the day as the organisms produce O2 during photosynthesis. The mass is often described as gelatinous, slimy, snotty or gooey. That part of the description may be the best way to distinguish it from other typical reef aquarium pests, although other organisms in the ocean (such as chrysophytes) have a similar appearance.

Snails seem to be especially prone to suffering from dinoflagellate toxins, so if you have pests such as dinoflagellates and notice that the snails seem to be moribund (near death, not moving, etc.), that may help finger dinoflagellates as the pests, although other pests can also produce toxins. Fish and other organisms that eat the dinoflagellates can die from their toxins as well.

Figures 1 and 2 show some typical organisms in reef aquaria that may be dinoflagellates, although they have not been identified as such by an expert. In any case, these have the typical look of trapped gas bubbles in a slimy coating on surfaces.

Figure 1. An infestation in the aquarium of Reef Central member Old Salty that may be dinoflagellates.

Figure 2. An infestation in the aquarium of Reef Central member Rays that may be dinoflagellates.

Dinoflagellates and Elevated pH

One of the ways that problem dinoflagellates have been treated is with elevated pH. The suggestion is often to raise the pH to 8.4 or higher. Later in this article I'll give specific suggestions about pH target levels and how to raise it. Before getting to that, however, it is worthwhile to consider how and why raising pH might impact dinoflagellates more than the aquarium's other organisms.

There are at least two possible reasons that problem dinoflagellates may respond to elevated pH. Because the exact species that become problematic in reef aquaria have not been identified, it can be difficult to look to the scientific literature on dinoflagellates for clues. Nevertheless, there are two clear possible reasons that problem dinoflagellates may respond to elevated pH when other organisms in the aquarium may not.

In one study that supports the general idea that some marine dinoflagellates may respond negatively to pH increases, Japanese scientists were investigating the effects of dumping steel-making slag into the ocean.1 The slag apparently contains substantial amounts of nutrients, and can drive the growth of organisms such as diatoms. In fact, in their studies the slag increased the growth of the diatom Skeletonema costatum considerably. The growth of the dinoflagellate Alexandrium tamarense, however, was reduced by the slag's addition and the researchers attributed this effect to the increased pH that came along with the slag.

It should be noted, however, that one species of dinoflagellate, the planktonic toxin producer Alexandrium catenella,2 was found to grow optimally at pH 8.5 in lab cultures. So raising pH is not a panacea for all dinoflagellate species that might be a problem.

In one study of the effect of pH (8.0 to 9.5) on a natural marine planktonic community of organisms that contained dinoflagellates,3 the initially collected dinoflagellates did not grow well at any pH, which the researchers attributed to low nutrients in the cultures. This result suggests that reducing nutrients may be a useful tactic, but does not bear on whether pH is a suitable method. In a second study,4 researchers noted a correlation between planktonic dinoflagellate blooms and high pH, suggesting that high pH does not inhibit these species of dinoflagellates.

Elevated pH and Availability of Carbon Dioxide

The first possible mechanism whereby dinoflagellates may respond negatively to pH relates to their acquisition of carbon for photosynthesis. All photosynthesizing organisms need to take up carbon dioxide in some fashion in order to use it to make organic molecules. In a previous article I detailed many of these mechanisms, and they include a variety of different ways of taking carbon dioxide or bicarbonate/carbonate from the water and into the organism.

As the pH is raised at constant carbonate alkalinity, the amount of carbon dioxide in the water declines. A rise in pH of 0.3 units implies approximately a 50% reduction in the available carbon dioxide, but not a significant decrease in bicarbonate (or carbonate). Some organisms are known to suffer considerably from this loss in available carbon dioxide, particularly those that do not use bicarbonate or carbonate. Some species of macroalgae, for example, can photosynthesize only 18% as fast at pH 8.7 as they do at pH 8.1, while others do just as well at the higher pH.

So the question here is whether the problem dinoflagellates have this same response or not. As mentioned above, the exact species that are a problem in reef aquaria have not been identified, and even if identified, have probably not been studied with respect to their pH response. From the literature, some dinoflagellates can take up carbon dioxide only as carbon dioxide, while others can use bicarbonate.

Two marine dinoflagellates, Amphidinium carterae Hulburt and Heterocapsa oceanica Stein, demonstrate active uptake of carbon dioxide (or carbonic acid), but not bicarbonate. Because this mechanism is fundamentally limited in its effectiveness, it has been speculated that these organisms may be CO2-limited in their natural environment.5 These species would likely be stressed considerably if the pH of a reef aquarium containing them were raised substantially. On the other hand, three marine bloom-forming (red tide) dinoflagellates, Prorocentrum minimum, Heterocapsa triquetra and Ceratium lineatum,6 have been shown to take up bicarbonate directly, with bicarbonate accounting for approximately 80% of the carbon dioxide they use in photosynthesis. It is believed that these dinoflagellates are not carbon limited in photosynthesis due to their efficient direct bicarbonate uptake mechanisms, so they may not be overly stressed (by this mechanism) by raising the pH to levels achievable in a reef aquarium.

Dinoflagellates' Internal pH

Organisms typically have strong control of their internal pH regardless of small changes in the external pH. Internal cellular pH is often near pH 7. The green alga Chlorella saccharophila, for example, has an internal pH of 7.3 that does not change across the external pH range from pH 5 to pH 7.5. As the pH drops below 5, however, its internal pH begins to drop and falls to 6.4 when the external pH reaches 3.0.7

The reason that organisms control their internal pH so strongly is that the rate of many different biochemical processes depends on pH. Enzymes, for example, catalyze reactions, and their ability to do so nearly always depends on the pH. So, in order to ensure that the myriad chemical reactions taking place inside cells operate at desirable rates, organisms keep their internal pH from fluctuating. If their internal pH strayed significantly from "normal" for that organism, chemical imbalances are likely to arise and the organism can be significantly stressed.

As an aside, the primary reason that I believe that small sudden pH changes do not stress most reef aquarium organisms, as long as the pH does not move outside the normal pH range that is acceptable to them, is because of this strong internal pH control. For example, I do not believe that a sudden rise in pH from 8.1 to 8.4 is any more stressful for most marine organisms than is a stable continuous pH 8.1 or 8.4.

So, back to dinoflagellates. A recent report in the literature suggests that at least one species has unusually poor internal pH control and consequently showed poor growth as the external pH changed from its "optimal" level.8 Two marine dinoflagellates, Amphidinium carterae Hulburt and Heterocapsa oceanica Stein, were shown to stop growing as the pH dropped from 8 to 7. When the external pH was reduced from 8 to 7, the internal pH of A. carterae dropped from 7.92 to 7.04 (H. oceanica's dropped from 8.14 to 7.22). The researchers attributed the change in internal pH as the cause of the reduced growth. While this experiment involves a pH reduction rather than an increase, and while it is not likely the same species that infests some reef aquaria, it does show that changes in dinoflagellates' internal pH may make them susceptible to changes in external pH that do not as strongly impact other types of organisms.

Does Raising the pH "Cure" Dinoflagellates?

Raising the pH appears to help in some cases of problem dinoflagellates. In some cases when the pH is raised quite a bit (e.g., 8.6-8.8 or higher), the effect can be dramatic and rapid (within a few days), but if the pH is later reduced to normal, the dinoflagellates can return.

I recently polled reef aquarists and found that most respondents either had never had dinoflagellates, or had them but never specifically treated for them (presumably most of these latter aquarists did not have severe outbreaks). Of those reporting that they had specifically treated for dinoflagellates, about half treated with elevated pH and half in other ways. Of those who did elect to treat with elevated pH, half of them described themselves as successful and half not (although the numbers in each case were small).

Is the variable result that aquarists observe due to different species of dinoflagellates? Or did some have organisms other than dinoflagellates? Did some not raise the pH high enough or for long enough? I don't know the answers. The reports on the usefulness of pH are mixed, and those who have problem dinoflagellates should consider trying it, but they may not find it successful in all cases. Patience may be an important factor, and combining the elevated pH with other methods (e.g., reduced nutrients, manual removal, etc.) may be the best bet.

Does High pH Reduce the Likelihood of Dinoflagellates?

If one way to treat problem dinoflagellates is to raise pH, then it stands to reason that such problems could be less likely to occur in reef aquaria whose pH is naturally high. Many reef aquarists who use limewater to supply calcium and alkalinity operate tanks with the pH on the high end of "normal" (i.e. 8.3 to 8.5). My system is a case in point. Other aquaria that use high pH two-part calcium and alkalinity additive systems (such as B-ionic or my DIY Recipe #1) may also have their usual pH on the high end of normal.

Do these aquaria have a lower incidence of dinoflagellate problems? I've never had such problems in more than 10 years, but that says little about whether pH was responsible. I recently surveyed 112 aquarists about their experiences with dinoflagellates, as well as the typical daily maximum pH that they encounter. The results are shown in Figure 3, which shows a slightly lower incidence of reported dinoflagellates at higher pH (above 8.2) than at lower pH (below 8.2). However, due to the difficulties in accurately measuring pH, and in identifying dinoflagellates relative to cyanobacteria and diatoms, I would not suggest that these data constitute strong evidence of such a relationship.

Figure 3. The fraction of respondents reporting dinoflagellate problems (black) and no dinoflagellate problems (red) in reef aquaria as a function of the daily maximum pH. The pH maximum and the incidence of dinoflagellates was self-reported by 112 aquarists who chose to respond. The results are normalized to add up to 1.0 for both cases.

How to Treat Problem Dinoflagellates

Here's a series of actions besides raising pH that may help aquarists to deal with problem dinoflagellates.

1. Reduce available nutrients in the water. These include nitrate and especially phosphate. In a severe case, the concerns with driving phosphate too low may be minor compared to the dinoflagellates (and their toxins). In addition to the usual ways of reducing nutrients (skimming, growing macroalgae, deep sand beds, etc.), aquarists should consider very aggressive use of granular ferric oxide (GFO). Putting a larger than normally recommended amount into a canister filter or reactor, and changing it every few days, may help. Don't bother to measure the phosphate level, because the goal is to have it well below normally detectable levels (say, 0.02 ppm).

2. Reduce the photoperiod to four hours per day. This may help to keep the dinoflagellates under control, but by itself will not usually eradicate them.

3. Use more than normal amounts of activated carbon, and possibly ozone, to deal with toxins that the dinoflagellates may be releasing. This may allow snails and other organisms to survive while the dinoflagellates are still at nuisance levels.

4. Manually siphon out as much of the mass of dinoflagellates as possible. Daily removal would be preferable to keep populations at a reduced level.

How to Treat Problem Dinoflagellates: Elevated pH

In order to treat problem dinoflagellates with elevated pH, I'd recommend keeping the pH at 8.4 to 8.5 until they are gone. The pH can be as high as 8.6 without causing too much stress on anything else. The process may take weeks. In desperation (i.e. if nothing else works), allow the pH to go even higher.

pH is best raised by adding calcium hydroxide, either as limewater (kalkwasser; calcium hydroxide or "lime" dissolved in freshwater), or as a lime slurry. Bear in mind that aeration will tend to lower the pH, so if maintaining high pH is difficult, reducing aeration may help a bit. pH naturally drops at night, so be sure to measure pH in the early morning as well as later in the day. Alternative methods to save on clothing products like Ackermans Specials.

As a general guideline, adding the equivalent of 1.25% of the tank's volume in saturated limewater will raise the pH by about 0.66 pH units. That increase may be more than desired all at once, but that volume, or more, spread out over the course of a day may be necessary to maintain high pH.

If you are limited by low evaporation and cannot add enough limewater, use a slurry of lime. For example, 1-2 level teaspoons of calcium hydroxide can be made into a slurry by mixing with one cup of RO/DI (reverse osmosis/deionized) water (not tank water). Stir it up and dump it into a high flow area away from delicate organisms. Adding one level teaspoon of solid lime this way into a 100-gallon aquarium will raise its pH by about 0.3 pH units. This process may need to be repeated several times a day to keep the pH high.

Don't worry about raising calcium or alkalinity with this method. The higher pH will accelerate calcification by organisms and abiotic precipitation. Beware that you may eventually clog pumps, impellers and intakes this way, and you might get white precipitates on surfaces (that is usually okay for a short term treatment and does not usually harm corals).


Dinoflagellates are a nasty problem that have driven some aquarists to consider leaving the hobby. Treatments often take a considerable period of time, and are not always effective. Nevertheless, the best known ways to treat problem dinoflagellates are to reduce nutrients and to raise pH, especially with limewater.

Good luck and happy reefing!

If you have any questions about this article, please visit my author forum on Reef Central.


1. Effect of steel-making slag addition on growth of the diatom Skeletonema costatum and the dinoflagellate Alexandrium tamarense. Suzuki, Masami; Yamamoto, Tamiji. Graduate School of Biosphere Sciences, Hiroshima University, 1-4-4 Kagamiyama Higashi-Hiroshima, Japan. Tetsu to Hagane (2005), 91(10), 783-787. Publisher: Iron and Steel Institute of Japan.

2. Environmental and nutritional factors which regulate population dynamics and toxin production in the dinoflagellate Alexandrium catenella. Siu, Gavin K. Y.; Young, Maria L. C.; Chan, D. K. O. Department of Zoology, University of Hong Kong, Hong Kong. Hydrobiologia (1997), 352 117-140.

3. Effects of high pH on a natural marine planktonic community. Pedersen, Maria Fenger; Hansen, Per Juel. Marine Biological Laboratory, University of Copenhagen, Helsingor, Den. Marine Ecology: Progress Series (2003), 260 19-31. Publisher: Inter-Research.

4. Co-occurrence of Dinoflagellate Blooms and High pH in Marine Enclosures. Hinga, K.R. Marine Ecology Progress Series MESEDT, Vol. 86, No. 2, p. 181-187, September 10, 1992.

5. Source of inorganic carbon for photosynthesis in two marine dinoflagellates. Dason, Jeffrey S.; Huertas, I. Emma; Colman, Brian. Department of Biology, York University, Toronto, ON, Can. Journal of Phycology (2004), 40(2), 285-292. Publisher: Blackwell Publishing, Inc.

6. Inorganic carbon acquisition in red tide dinoflagellates. Rost, Bjoern; Richter, Klaus-Uwe; Riebesell, Ulf; Hansen, Per Juel. Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany. Plant, Cell and Environment (2006), 29(5), 810-822.

7. Effect of external pH on the internal pH of Chlorella saccharophila. Gehl, Katharina A.; Colman, Brian. Dep. Biol., York Univ., Downsview, ON, Can. Plant Physiology (1985), 77(4), 917-21.

8. Inhibition of growth in two dinoflagellates by rapid changes in external pH. Dason, Jeffrey S.; Colman, Brian. Department of Biology, York University, Toronto, ON, Can. Canadian Journal of Botany (2004), 82(4), 515-520.

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Problem Dinoflagellates and pH by Randy Holmes-Farley -