Red plants: interaction between anthocyanins and CO2 and other considerations
By Eduardo Fonseca Jr
2023/01/06
Red plants are probably the biggest desire among planted aquarists. It’s not the time of the dense carpet of Glossostigma anymore, now almost everyone wants to see the aquarium “on fire” with Ludwigias glandulosa and Rotalas H’ra. It’s okay, it’s quite understandable, we all love it. However, the search for the perfect red leads many aquarists to failure because they get lost in the imbalance between excess light and luxury consumption. Every experienced aquarist knows how difficult it is to keep an aquarium with intense red for a long time, walking on the razor’s edge imposed by the high metabolic rate of the system. Just a little failure in fertilization and the tank suffers a delay of weeks or months. Is it worth it? Well, with time, I believe, we will put these extravagances aside and start to prioritize more sustainable and friendly aquariums, but that depends a lot on each person’s profile. A lazy or negligent aquarist is definitely not compatible with red plants. It’s like a newbie or a lazy bodybuilder (one who doesn’t train very regularly) wanting to pump up steroids; it is easier to hypertrophy the heart than the rest. Red plants is more or less like that; you will fry the aquarium with a lot of light intensity, saturate the photosystems light capture limit and force the plants to “tan themselves” by producing blocking pigments above chlorophylls, the much-loved anthocyanins – the red and purple pigments, for those who are coming now. The price of this is imposing a very strong pace on the rest of the system, dangerously increasing the consumption scale of all plants and greatly increasing the chances of having a lower board in the Liebig barrel. If it fails, it orders an algae outbreak that can take weeks to remedy… that’s if you’re good enough to reverse it.
Figure 1 – Ludwigia glandulosa contrasting with the green of other plants.
But let’s assume that the aquarist masters these precepts very well and manages to keep the barrel with high boards. What’s the first thing our friend is going to do? Up the CO2? You got it right! He’s so smart that he’ll raise the CO2 as high as he can to avoid any risk of limiting photosynthesis; that’s really the recommendation. Then he’ll feed the whole line of nutrients properly, he’ll sit back waiting to see the plants redden, and… he might just wait anyway… the plants might not redden, or they might not redden as much as he expected.
I already fell for that and thought that the plants didn’t turn red because I had gotten a bad crop, plants that despite being red species, didn’t have good genes. Well, this can really happen, like orchids that don’t bloom or bloom timidly for years while the neighbors, of the same species, are blooming out of season, but we can only blame bad genetics when we don’t have a history of occurrence of the character in the system in progress. For example, if other plants are reddening in the aquarium, except for that one, once we have ruled out any lighting distribution problems such as dark corners or very high parts outside the light cone, we can suspect individual genetics, otherwise, if two or more species don’t redden, so it’s easier to be a system problem. It is for this reason that it is difficult to set up the aquarium with only one species of red plant; if it doesn’t redden, we don’t know if it’s a system problem or an individual limitation. Here’s a warning to beginners: if you are not an advanced aquarist with real experience with red plants in previous aquaria, always consider that you are failing the system and disregard genetics. Even if that possibility exists, it’s pretty remote if you don’t have experience. And here’s a warning to the experts: plants are not bound to redden even with all conditions met. A typical reaction of a species need not be present in all individuals of that species or present at the same intensity. That’s kind of obvious. Different people don’t all have the same height, different lions don’t all have a dominant black mane, and different birds don’t sing in the same breath, so why should plants redden all the same? But let’s take it easy, the more typical a feature, the lower the chances of the individual not presenting it. When it comes to red plants, for example, observation leads us to believe that this is easier to happen with Rotalas than with Ludwigias, but this is not a subject that I want to expand too much here.
Figure 2 – The greater the light intensity, the easier it is to get red plants through the production of anthocyanins.
Considering all the previously mentioned conditions (intense light, high amount of CO2 and abundant fertilization), what could prevent them from turning red? We can deduce some hypotheses:
1- As already explained in a previous article Carbon dictates consumption, the excess of CO2 tends to occupy all the stock volumes of plant cells with starch, thus expelling other nutrients. It’s like vegetable obesity with the aggravating factor of not being able, eventually, to practice luxury consumption (because there is no stock available). This could lead the plants to show signs of long-term nutrient deficiency, including nitrogen, making them more yellow.
2- A variation of the previous hypothesis, considering that starch can expel from cell vacuoles not only the stored nutrients but also the anthocyanins themselves, making the plants more faded (emphasizing that anthocyanins are stored in the vacuoles of the mesophyll cells and, in this case of stress due to excess light, also in the cells of the epidermis).
3- In the article Carbon dictates the rhythm, it was also described the fact that carbon also works as a driver in the metabolic rhythm in a similar way to light, increasing the consumption of all nutrients. High concentrations of CO2, therefore, could lead the plants to an eventual nutritional deficiency, that is, raising the level of the Liebig barrel to a point of leakage and leaving the plants with a paler color.
4- And finally, the fact that excess CO2 lowers the pH so much (especially when KH is low) to the point of modifying the molecular species of anthocyanins to another subtype that is not red. Let’s study a little more about this:
Figure 3 – On the left, an aquarium with a CO2 peak due to problems with the regulator valve. On the right, the same aquarium after three days of fixing the CO2 problem. Photos courtesy of Luca Galarraga.
Anthocyanin is a pigment that varies its molecular form according to the pH. It can assume basically five different forms, each with a typical color, and at pH 6 to 7 it takes on a purple color, becoming more bluish when higher than 7 and more colorless when lower than 6. Excess CO2, by keeping the pH too low, could quickly fade the plants, however this is a direct effect of the pH drop and not the CO2.
We could easily suspect this hypothesis, especially if the aquarium already had the plants very reddish and, due to some carelessness with the CO2 injection, the pH dropped too much and the plants quickly faded. It is a chemical fading process, not a physiological one, so it must occur acutely, more quickly. In other words, the anthocyanins are there, but in a more colorless molecular configuration. If the aquarist controls the CO2 injection or raises the pH a little and the plants redden again in a few days, this hypothesis is confirmed.
Figure 4 – Molecular configurations of anthocyanin as a function of pH and their respective colors.
The other hypotheses are very valid, but they should take a little longer to occur, since they would cause more chronic problems. Inducing a healthy system to nutritional deficiency by excess CO2, as well as expelling anthocyanins from the vacuoles, takes much longer, but can be the cause of an aquarium that has not yet managed to pronounce intensely red plants. It is worth remembering that these other hypotheses, all related to the filling of the vacuoles by starch, have nothing to do with the pH, which leads us to lose the redness of the plants due to the excess of CO2 even with pH under control, which is what happens with aquariums with CO2 above 60ppm with KH of 8 or more. Contest tanks end up easily fitting this profile.
So what’s the recommendation?
Well, once again we are faced with what I tend to call the “short blanket”; to get intense red plants we need to impose rhythm, but this conduct, in turn, can bring the problem of lowering the pH too much (consider less than 6.4 a problem), which can be easily corrected by increasing the KH without decreasing CO2 injection. This would remedy the acute effect of the pH drop on anthocyanins. However, high CO2 concentrations can bring about the chronic effects we’ve already seen. It is difficult to prescribe a purely rational conduct under these conditions, but, once we are aware of these mechanisms, we can act on them, perhaps trying to maintain the high rhythm without raising CO2 so much, keeping it at around 40ppm (and running the risk of having black beard algae) or maybe, who knows, not imposing the high rhythm during the whole life of the aquarium, but only in some periods more or less synchronized with the peaks of the vegetal form before pruning, photographs for contests, etc. This makes more sense to me, as a high-performing system is not only harder to maintain, but it ages sooner and accumulates a lot of problems in the long run. I would also recommend using plants that are more naturally red, such as Ludwigias, which just need good quality lighting without so much intensity to get them very red, instead of those that need more light stress to redden, such as Rotalas. In the case of systems that must live for a longer period of time, beyond that foreseen to participate in a competition, it is also advisable to accept slightly less intense red tones in favor of a more sustainable general health of the aquarium.
Now let’s level some things up…
It is not because a very low pH is more likely to discolor the anthocyanins that we are going to try to make our aquariums alkaline. At pH above 7 anthocyanins become more unstable and tend to degrade. We shouldn’t change our position on aquarium parameters because of a premise; remember the aquarium is the old story of the short blanket. What we need is to understand the mechanisms and the LIMITS of these mechanisms; we need to find the middle path and stay on track. If we move too far from the snake, we fall into the hole on the other side.
Another factor that we must take into account is the temperature, so that it degrades faster above 25oC and even faster if the pH is above 7. Therefore, it is advisable to allow aquariums with many red plants to be cooler.
And finally, the issue of metals when associated with anthocyanins. I think it’s a perfect time to dismantle a huge myth about iron’s supposed role in reddening plants. I already wrote about this in the article Red plants: nutrients or lighting, but a few more details are in order here; metals can associate with anthocyanins in vacuoles and generate chelated compounds that change the colors of anthocyanins, the so-called metal-anthocyanins. Some of the metals that have this property include aluminum, zinc, magnesium, copper and iron among others, including heavy metals. These bonds make anthocyanins more stable against temperature denaturation and less reactive to pH changes.
Figure 5 – Molecular scheme of anthocyanin binding to metal ions forming a chelated compound.
The Fe3+ ion, which is what interests us, when binding with anthocyanins at a pH greater than 5.5, stabilizes them in a blue color, but that does not mean that this is what highlights the red in the plants (perhaps we could think of the junction in the typical purple anthocyanins of pH 6 to 7 with the bluish chelated anthocyanins, resulting in a perception of red or purple), because there is no evidence that plants very well nourished with Fe manage to chelate the anthocyanins significantly in quantity or, if they do, if this reaction significantly changes the color of the plants. What we can say with more confidence is that plants well nourished with Fe may have more stable colors (how much more stable?), but not necessarily more intense. Everything is mere speculation arising from a rather poor inductive reasoning, if that idea originates from these assumptions. It would be necessary to test this hypothesis in the light of what we already know in different aquariums in order to be able to accumulate enough experience to attest that Fe definitely intensifies the production of anthocyanins or rather, that it makes the plants redder. For the time being, I consider myself within the scientific right to dismiss this theory as a fraud, as there is, to date, no solid experience from aquarists reporting that plants actually redden with iron or fail to redden without it.
As we have seen, there are many variables involved in the reddening of plants and we can explain, at least in part, why some aquarists succeed and others do not despite fundamental conditions being met. I believe that the information in this article can raise more objective observation points in order to help reference people’s experiences and thus refine our practical knowledge on this and other subjects.
