Algae, unavoidable companions.

Algae are undoubtedly one of the inhabitants of a reef tank that cause the biggest headaches to hobbyists. In a world idealized by aesthetics, we might wish that these living beings were out of our system, however, this wish makes no biological and ecological sense. Algae are part of the food web of any aquarium, actively participating in the recycling of nutrients and contributing to the maintenance of adequate levels of nitrate, phosphate and CO2 in the water. "Aseptic" aquariums cannot exist.

So far so good, however, it is well known that algae, too often, and especially in the first months of operation of a tank, tend to generate pests, spreading over all available surfaces and harming corals and other invertebrates. A pest occurs when one of the algae specie takes advantage of its competitors (which can be other algae, protists or bacteria, among others) and is able to colonize quickly and unstoppably all the available space. The consequences are bad, not only from an ecological but also from an aesthetic point of view.

Low nutrients, a guarantee of algae absence?

Algae are true survivors in their ecological niche. This implies they can thrive with very little: little light, little nitrate, little phosphate, little CO2, and additionally trace elements, which are almost always available in the necessary concentrations (which are extraordinarily low). We hobbyists have a strong tendency to think that maintaining low levels of inorganic nutrients equates to being less likely to have algae infestations, however, there are frequent and numerous evidence that there is no such relationship, at least in a strictly cause-effect way.

In my experience and that of many other hobbyists, keeping nitrate and phosphate concentrations at permanently low levels (e.g., below 0.06 ppm for phosphate and 3 ppm for nitrate) has implied greater long-term persistence of cyanobacteria, dinoflagellates, and filamentous algae. The reality is that, once bloom is initiated, nutrient reduction is absolutely useless to stop propagation, especially during the initial exponential reproduction phase, which can last for several weeks.

Increase nutrients to favor competition.

In cases where cyanobacteria, dinoflagellates, diatoms and some hair algae pests are started, if inorganic nutrients are too low, the algae that can live in those conditions (which are the majority) will not be subject to competition from other organisms, which thrive in water with higher nutrient concentrations. For example, it is well known that many species of coraline algae spread better when nitrate concentration is high (not the case with high phosphate concentration, because it inhibits the precipitation of calcium carbonate).

The strategy of increasing nitrate and phosphate levels to combat algae pests does not work in the short term, but it does work in the long term and especially as a preventive measure. Once the pest is established, mechanical actions are clearly a good option, e.g., pruning, siphoning or even carefully removing the rocks from the tank and pouring a hydrogen peroxide solution over them.

The concept of tank maturity

The disappearance of algae pests is related to the concept of "mature and stable aquarium". Since I started in the hobby more than 25 years ago, I have heard these two terms all the time, and have always wondered what they really mean. Tank maturity and stability is an emerging quality that has very complex chemical, biological and ecological underpinnings. We can say that aquarium stability is closely related to the stability of the typical parameters that we usually measure, i.e.: concentrations of calcium, magnesium, alkalinity, nitrate, phosphate, potassium, trace elements, etc., but there is something that goes beyond such parameter stability and has a lot to do with the interspecific competition between species of microorganisms and macroscopic organisms that inhabit the aquarium.

When we speak of these "emergent" qualities it is convenient to emphasize that they are based on innumerable chemical and biochemical processes that sustain the metabolism of the organisms that are part of the microcosm. There is absorption and release of nutrients into the water, birth, growth, reproduction, predation and death. Algae, protists and bacteria constitute the first levels of the trophic web, taking care of the recycling of organic and inorganic nutrients in the water column, all of them interacting with each other and with the environment where they live (as an ecosystem itself).

In reference to the concept of maturity I want to propose a hypothesis, which by definition is something that must be demonstrated to be taken as true. The intention of this article is to generate a debate about it. The hypothesis can be expressed as a series of "requirements" for the attainment of the state of maturity, namely:

- Stability of chemical parameters, as mentioned.

- Stability in light intensity and spectrum.

- Diversity of microorganisms: phytoplankton, zooplankton (copepods, for example), autotrophic and heterotrophic bacteria. Diversity is one of the consequences of
stability, as occurs in coral reefs.

- Sufficient" concentrations of nitrate and phosphate in the water so that all species have "food" so that none of them can take advantage due to their metabolism
adapted to "extreme" situations, for example, cyanobacteria that are able to take advantage of the nitrogen dissolved in the water, without the need to directly assimilate

- Real competition between all aquarium organisms, especially microorganisms: bacteria, phytoplankton and zooplankton. This competition could be summarized in several

o Competition for space. This is probably one of the most relevant. Once a surface (live rock, substrate, etc.) is colonized by one species, it becomes more difficult
for another to occupy that spot (therefore newly set up aquariums are so prone to algae pests).

o Competition for inorganic nutrients. The availability of ammonia, nitrate, phosphate, CO2 and trace elements in the water is variable and depends on
consumption and release by organisms. For example, a functional heterotrophic bacterial biofilm will remove nitrate and phosphate on its surface, made
more difficult for algae to assimilate.

o Grazing and predation. Algae are subjected to grazing by zooplankton, for example, copepods feeding on dinoflagellates. Surgeonfish grazing at diatoms and
hair algae on the rocks or tank walls limit their growth.

o Chemical interaction between organisms. I will elaborate on this point in more detail in the next paragraph for clarity. I am convinced that it is the most
relevant factor of them all.

Chemical interaction between tank inhabitants.

The hypothesis that I present in this article is that the chemical interaction between micro and macro-organisms that inhabit the aquarium has a great relevance in the robustness of a tank against destabilizing events that are potentially triggering algal pests. I will now present some evidence that, in my opinion, can justify this hypothesis:

- A tank maintained with a very small population of corals and other invertebrates (relative to the net water volume) is usually much more prone to permanent algae pests.

- Water changes make cyanobacteria and dinoflagellate infestations worse, and I believe this is due to two factors primarily:

o Nutrient dilution.

o Dilution of allelopathy compounds that inhibit the growth of dinoflagellates and cyanobacteria. It is striking the fact that a bloom of dinos, cyano and even
hair algae, are initiated in a "synchronized" way at different spots of the tank, at exactly at the same time. This suggests the existence of "chemical signals" or, at
least, significant changes in the water concentration of certain inhibitory compounds released by competing species. It is relatively easy to find scientific papers
that support this evidence.

- Organic carbon dosing is very effective in controlling dinoflagellates such as Ostreopsis, Prorocentrum and Amphidinium, provided that nitrate and phosphate are not allowed
to drop to undetectable values during treatment. Heterotrophic bacteria generate natural algaecide compounds that inhibit dinoflagellate growth.

- The use of "miracle" products such as commercial chemical algaecides (e.g., heavy metals), can eradicate the pest, but often make a big hole in the food web, so that another
invasive species is able to occupy the available niche after the death of the previous one. In this way, pests follow one after another in the tank: cyanobacteria,
dinoflagellates,filamentous, etc.

Conclusion for discussion.

The hypothesis presented is that the chemical interaction between the organisms that inhabit a reef tank is probably the most important factor that allows the ecosystem to "self-regulate" its algae populations, so that the probability of a single species taking advantage of the others is quite low. This would therefore be the most determining factor in terms of the robustness of an aquarium. Almost all algae species are desired. What is undesired by the hobbyist is a pest of any one of them. I am convinced that this chemical interaction plays a decisive factor in determining the concept of "mature aquarium". One question would be: What percentage of the total "maturity" of the tank could be attributed to this factor if any?
About author
Angel Cegarra is the author of the book “Reefkeeping Fundamentals”, where he presents a fascinating journey through the science and technology that govern life inside a reef tank. From nutrient recycling processes, through coral biology, project initiation, feeding techniques and maintenance of water chemistry. The book includes world class reef aquariums description along with priceless advice on how they were planned and how they are kept healthy and vibrant. Covering the biology of coral reefs and relating it to the biological, chemical, and physical parameters considered when creating a reef aquarium. Home aquarist moving into an area of knowledge that was once the exclusive domains of scientists.

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