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Pearling - True or False?
Planted tank folk routinely praise their tanks (read as: themselves and their skills), chatting about how soon after the lights turn on their plants start "pearling", and how within so many hours the water looks like champagne. Pearling? What? Champagne? Okay, they do have certain justification in bragging, as pearling is a sign that the plants are photosynthesizing well, producing more oxygen than they can use, and supersaturating the water with oxygen. But just what is going on? And what is it on the boards when some posters dismiss others by saying that they are seeing "false pearling", not the real thing?
This photo was originally taken by nttrbx and the original photo can be found here: http://www.flickr.com/photos/nattarbox/2213920898/
Animal life on this planet relies on oxygen, O2 as the gas, which now makes up ~21% of the Earth's atmosphere. This oxygen gas on which we rely for existence is a by-product of photosynthesis, possibly first developed by cyanobacteria, now done also by algae and higher plants. With land-based plants, we cannot directly see any part of the process. With submerged aquatic plants, under certain conditions we can see gaseous oxygen form bubbles on the underwater leaves. Those (usually tiny) bubbles releasing one after the other and climbing up through the water, shining in the light, can give the appearance of a string of tiny pearls. From this appearance of these tiny strings of bubbles comes the term - pearling. Similarly, the small bubbles on the plant leaves can give the appearance of small and highly iridescent pearls.
Plants generate energy using captured sunlight (or artificial light in most of our tanks) to produce stored and transportable energy. Then they use that energy to produce the organic materials they require for life and growth. To do this they must take in an array of nutrients (dissolved minerals and carbon from CO2 or dissolved bicarbonates). They also require oxygen for many of their metabolic process just as we do, but their light energy capture and conversion to chemical energy produces more oxygen as a by-product than their metabolism requires. Oxygen can be toxic - it is highly reactive. Higher plants and animals require it, but in this case too much of a good thing could be toxic. So plants release their excess oxygen. That release over geologic time is responsible for the oxygen in our atmosphere and the planet as we know it.
Okay, we understand that oxygen gas is released by plants as part of their normal life processes. What is required for us to see this in our tanks? There are several requirements, all of which must be present:
1. A planted tank. Occasionally a blue-green algae (really cyanobacteria)
or true alga coated tank will serve.
The planted tank is generally moderately to heavily planted. This means that a single Amazon Swordplant, even a large specimen, may not be seen to pearl if it is alone in a 180-gallon tank. The reason for this is that plants only produce a certain amount of oxygen, related to the particular plant, its surface area and mass, and the complete array of nutrients and the light that it needs. Freshwater saturates at a dissolved oxygen level of about 8.1ppm at sea level. Water can supersaturate with oxygen, up to something like 150% without overpressure, but in tanks such levels would be rare; levels beyond 120% (~10ppm O2) are unusual in most tanks. For the example given of the single large sword in a large tank, that plant alone would have to produce and release enough excess oxygen to saturate the entire water volume. Below water oxygen saturation, the oxygen produced is dissolved in the water and therefore invisible. It is being produced and released into the surrounding water, but we cannot directly see it. If this same tank were fully planted (looking down on the tank from above with the lights temporarily removed, no substrate would be visible) then under the same nutrient and light conditions we could expect to see pearling. In other words, many more plants and much greater plant mass are contributing to the dissolved oxygen in the water, it is therefore easier to saturate and supersaturate the entire water volume. The same situation could apply if that single very large plant were housed in a 50-gallon tank. With good conditions, that single plant could saturate the water with oxygen. Thereafter O2 produced will be released as visible bubbles and pearling.
The light and nutrients the plants require must also be present in the needed forms and proportions. Anything missing or present in insufficient amounts can stop the process. This is dealing with the concept of limiting factors or minimums and as such is beyond the scope of this article, but we can for present purposes just accept that if something - anything - essential is used up or missing, photosynthesis will stop. The easiest to grasp is light - turn the light off, the process stops. It works much the same way for the other essentials, minerals and bio-available carbon.
Low surface disruption is also important. We have been conditioned to think of aeration - by airstone or current - as adding oxygen to the water and driving off carbon dioxide. The word itself implies that - aeration, adding air.
Our thinking on aeration has been conditioned by a particular case, which I'll label Case 1: the situation in which the tank oxygen is less than saturated (below or well below 8.1ppm O2), and/or the carbon dioxide is elevated (beyond the ~5ppm CO2 equilibrium with air at sea level). In that case, generally because in-tank biological processes (fish, bacteria, etc.) have depleted the oxygen and supplemented the carbon dioxide, surface disruption by strong surface current or by airstone (which functions by also providing surface currents) can blow off the excess CO2 and increase the deficient O2. This is the situation we normally visualize when we think and speak about aeration. But in the case of heavily or fully planted tanks, with strong lighting, complete required mineral supplements and added CO2, increased aeration by pump or airstone can blow off the higher-than-equilibrium levels of CO2 needed in such tanks. That can make the CO2 the limiting factor and slow or stop photosynthesis.
First consider the situation we'll call Case 2A: the fully planted tank used as an example earlier, with strong lighting and all required nutrients (including higher than air-equilibrium CO2): the tank is likely to be supersaturated with O2 and pearling. This is the conventional high light, mineral and CO2-supplemented plant tank. The dissolved CO2 levels are supplemented to levels 3- to 5-fold (or slightly more) air-equilibrium to support high rates of photosynthesis. That photosynthesis level will produce much more oxygen than the plants and fish require, and even more than the water will dissolve, resulting in O2 supersaturation and visible pearling.
To convert Case 2A to Case 2B: Start an airstone, or a pump with venturi at the surface or at least causing surface rippling, and the dissolved O2 will drop from ~10ppm to ~8 ppm and the CO2 will drop from wherever supplement level it is set down toward ~5 ppm or below. Photosynthesis will slow as well (from the CO2 reduction the plants become carbon-limited), and pearling will no longer be visible, due both to carbon limitation and to the lack of oxygen saturation. In this case both O2 and CO2 have been blown off, because both were initially beyond their air-equilibrium levels.
Under the ongoing conditions of still having the rest of the nutrients available and strong light, aeration may well support yet another case we'll call Case 2C: CO2 dissolving in the water from the air - if the plants have pulled CO2 below air-equilibrium levels - say down to 2-3ppm. Then CO2 from the air can dissolve in the water, boosting the dissolved CO2 toward the air-equilibrium level. The fine distinctions at low these CO2 levels are very difficult to measure, but the possibility is valid.
If we have the situations defined as Case 2B (or 2C), we can produce another equilibrium as Case 2D: If the CO2 feed is increased sufficiently to maintain the Case 2A higher concentration of dissolved CO2 (i.e., the input rate equals the blow-off rate, but the dissolved CO2 levels are maintained at 3-5x air-equilibrium), then the level of photosynthesis will not be carbon-limited and can continue. However, the plants are unlikely to produce enough oxygen to maintain supersaturation of that gas, so visible pearling will not be likely. The O2 production rate may be approximately what it was before aeration increased (i.e., Case 2A), but the O2 blow-off makes visible pearling impossible at the same oxygen production level. So we have high O2 production, but no visibility of that production due to excess O2 blow-off (or aeration). We are compensating for the blow-off of CO2, but not for the blow-off of O2.
This whole concept seems quite difficult for many hobbyists to grasp. Some current is needed in high light tanks to bring dissolved nutrients and CO2 to the plants (if they are not readily available to the roots and are materials normally taken up by the roots, which some but not all are). But surface disturbance can blow off CO2 easily, O2 as well, just as it can absorb either from the air. The blow-off or solution is of either gas is relative to the concentration of the gas in the water, and the concentration of the gas in the air, the solubility of the gas (CO2 is much more soluble than O2, but also at much lower concentration in air), the temperature, and the current-imposed interaction between the water and the air. If you read through these cases until you see and understand the differences and dynamics involved, you should not be confused by oxygenation and gas exchange again (I sincerely hope). Strongly lighted tanks meeting all other mineral requirements need elevated levels of CO2 to maintain high levels of photosynthesis. We supplement that by DIY or pressurized gas to satisfactory levels. We also have sufficient circulation to provide nutrients and dissolved CO2 to the plants without excessive surface disruption. Anything that changes that balance can interfere with the photosynthetic process - such as increasing aeration as discussed in the cases above. Strongly lighted tanks meeting all mineral and carbon requirements will have high levels of photosynthesis. This will be evident to the viewer by visible bubbling from plant surfaces provided that the water is saturated to supersaturated with oxygen. It is the supersaturation that makes the pearling visible, not just O2 production and release. The same level of photosynthesis can operate without pearling if the water is not saturated or supersaturated with oxygen.
Now we have the requirements met, everything in balance, everything needed is provided. The lights turn on for the tank and photosynthesis begins. Oxygen is produced in excess and released by dissolving into the water. Soon the water is saturated and then supersaturated. No more oxygen can dissolve. Tiny bubbles begin to appear on and under the leaves of the plants - these are pure oxygen. Soon they start releasing and rising to the surface - we have visible pearling. This is almost by definition a strongly positive thing. But is it the best of all possible worlds? Ready for a dash of heresy? Not necessarily. It all depends on what you want from your planted tanks. If you want high light, strongly growing plants, champagne-like water all afternoon and evening, then yes, this is your aim point, the proof of your accomplishment. But do remember that you are required to maintain that to me knife-edged balance of all nutrients and lights and plant type and mass by chronic supplementations, trimming and dividing, general fiddling in and about the tank. The product may be worth the effort to you - pure personal choice here, not better or worse. You might opt for moderate lighting, minimal supplementation and minimal pruning/division/replanting and see much less pearling. Is this bad? No it is not. It is a personal choice. In my own opinion, the current USA style is that anything worth doing is worth overdoing. For myself, I'll take multiple planted tanks to make my fish look better and hopefully to be better rather than one or a few displays requiring hours of attention per week. But my choice is not right or wrong either - it is just that - a personal choice. On filtration, I overdo. On plants, I do not overdo. Well, perhaps just a bit on number of planted tanks, but not on the time per tank requirement. Perhaps I'm just greedy for tank numbers rather than higher performance in a smaller number.
Do not allow yourself to be buffaloed by any particular school of thought. The high light, CO2 supplement, frequent fertilization, routine pruning group will insist that you need x light, y CO2 levels, z multiple supplements, etc. The "natural" group will insist on soils, no filters, natural light, not even any water changes. There is a very broad world in between those varied extremes. Once again, there is no one true path. There are many valid techniques from which you can learn. But it is a learning process, and only you can say where on the spectrum of planted tank procedures you will be, and want to be at home and comfortable.
Okay, the above hopefully made the real thing a bit clearer. What on earth is false pearling? There are to me at least three forms, one is clearly false, one may or may not be false, and the other I do not know the answer - on that one you can make up your own mind.
Clearly false pearling is due to dissolved gases in the water well beyond air equilibrium. Think of wintertime, when ground water is coldest in our municipal water systems. The intake water is colder, and the distribution network is colder. The pressures used to distribute the water are likely to be the same or quite similar. Cold water holds more dissolved gas. The network is pressurized; the gas is not going anywhere. You draw a glass of water and sit it on the counter. It starts warming up, and it is no longer under pressure. Gas bubbles form on the sides of the glass. They are not moving so that is not pearling. But you are doing a partial water change on your tank. You mix hot and cold water to get the desired temperature and add that to your tank. Your tank has current from pumps, fish, perhaps even airstones. Many small bubbles start forming wherever they find a surface or particle. Some of them pull loose in the current; some are on and pulled loose from plant leaves - Hey! Look! The plants are pearling! Sometimes even jagged rocks look like they are pearling. OOPS, probably not real. The bubbles are nitrogen gas (almost 80% of air); oxygen (just over 20% of air) and tiny amounts are the low-concentration (CO2) and rare gases.
However, if your water is something like a hypothetical KH ~10, pH 8.0 after aging and/or aeration, and measures at pH 7.3 right out of the cold tap, you may have the second and "maybe" case. There is sufficient CO2 dissolved in your cold tap water to have a dissolved CO2 level of ~15ppm, more likely from limestone aquifers than simple dissolved air. But if there is dissolved air in that water, even dilution with hot tap, and dilution again into your tank may - just may - give saturated O2 in the water, and enough boost in photosynthesis from the dissolved CO2 to provide true pearling with O2 saturation in part supported by the tap water dissolved gas. But how can you tell against the background of false pearling from the dissolved air? If you suspect that this is real in your tank, confirming the tank KH and pH while it is going on, and the tap pH to show acid suppression of the cold water pH by dissolved CO2, could give you enough data to at least argue the point.
The third and last case I am familiar with is debatable and needs hard
data for resolution. A broken stem or broken leaf (especially with large
veins) may sit there "pearling" - releasing a string of small
bubbles. Obviously gas is coming out or forming the bubbles. Is it O2?
Is it CO2? Is it disrupted gas transport, or do plants ever transport
gas as gas and not dissolved? My suspicion would be that it is O2, previously
dissolved in the plant's sap and being transported elsewhere, but now
coming out of solution. But I have no data to support that position
at all. Is this "true pearling"? Lacking data, I'll give a
tentative no, saying it is "false pearling" because it is
so localized and not tied directly to the usual photosynthetic process
and tank parameters. You get to select and justify your own answer to
Robert T. Ricketts, a.k.a. RTR
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