Nutrients. Producer or consumer?

Here we get into the mud, literally into the muck 🤣. Let's talk about the stuff many reefkeepers try to wipe out as if it were the bubonic plague: nutrients.

The coral's secret identity: producer or consumer?

Here we need to stop for a moment and give our brains a little reset.

Do we think of Acropora as an animal that lives on light? Wrong. We have to see Acropora as a biological cooperative between an animal (the polyp) and an alga (the zooxanthella).

The coral as a "producer" (autotrophy)

It is true, the coral produces. Thanks to its symbiosis with dinoflagellates, the zooxanthellae (Symbiodinium), the coral uses photosynthesis to make sugars, glycerol and amino acids.

What science says: as explained by Prof. Yuri I. Sorokin in Coral Reef Ecology (ch. 4), zooxanthellae can supply up to 90% of the energy requirement, in the form of photosynthetic carbon, needed for the coral's basal metabolism.

In practice the coral has a solar panel fitted that pays its bills 🤣 (the energy to stay alive).

But nobody lives on sugar alone… otherwise we would all be obese and malnourished.

1. 1From outside nitrogen (N) and phosphorus (P): zooplankton, bacteria, nutrients in the water
2. 2CO₂ and ammonium the animal's waste feeds the alga
3. 3Sugars and oxygen from the alga to the animal: up to 90% of the energy

Light pays the bills (energy). To grow and colour up, the coral needs bricks: nitrogen and phosphorus.

Sources: [1]

Polyp and zooxanthella: a cooperative that needs nitrogen and phosphorus from outside.

The coral as a "consumer" (heterotrophy)

And here's the catch. Acropora is a carnivorous animal. It has tentacles, nematocysts (stinging cells) and a digestive system.

Zooxanthellae make sugars, but they do not make enough nitrogen (N) and phosphorus (P). To build tissue, copy its DNA and grow, the coral has to "eat" these substances from outside.

What Sorokin says: the coral is an active consumer of zooplankton and, even more incredibly, of bacteria. The coral literally "fishes" nutrients out of the water. If the water is at "absolute zero", the coral lacks the bricks to build the house, even if it has all the light in the world.

Nutrients: the internal cycle

The coral is a perfect recycling machine. The animal's waste (CO₂ and ammonium) is used by the alga for photosynthesis. In return, the alga gives the animal oxygen and nutrients.

Why do we need NO₃ and PO₄ in the water? Because this cycle is never 100% closed: something is always lost. As documented in the study in Nature (2025), when external nutrients are too low the coral starts breaking down its own proteins to feed the algae. In practice, it eats itself.

The truth about the Zeovit system and the "zeros" on the tests

At this point someone, like me, might think: so how does Zeovit (one of the most valid and widespread methods) work at all?

Because the aim of Zeovit is to limit the growth of the algae (zooxanthellae) to make the coral tissue transparent and show off the colours.

Reducing the zooxanthellae reduces internal competition for N and P, but increases the coral's dependence on heterotrophy. So, to keep the animal from dying, you have to add heterotrophic food in quantity (amino acids, coral vitalizer, etc.).

At this point there are two paths, put very simply:

• NO₃ and PO₄ in the water: a "pantry" that is always open (inorganic feeding).
• ULNS method (Zeovit): close the pantry and feed several times a day (organic feeding).

Open pantry

• Inorganic feeding
• NO₃ and PO₄ always present in the water
• The coral takes what it needs, when it needs it
• More zooxanthellae: darker colours if you overdo it

Closed pantry

• ULNS, for example Zeovit
• NO₃ and PO₄ close to zero
• Food and amino acids several times a day
• Fewer zooxanthellae: transparent tissue and colour

There is no right or wrong. There is the balance of your tank, your life and your commitments.

Two ways to feed: an open pantry or a closed pantry.

In short

The coral is a producer as far as energy (sugars) goes, but a voracious consumer as far as matter (proteins and skeleton) goes. If we do not give it nutrients, one way or the other, it also stops making coloured pigments, because fluorescent proteins are made of… protein! And proteins are made of nitrogen.

No food, no colour.

There is no right or wrong, no one way better than the other. It is not better to keep NO₃ and PO₄ at zero or to keep them high: there is the balance each reefkeeper finds for their own tank, their life and their commitments!

Sources cited by the author

• Sorokin, Y. I. (1995). Coral Reef Ecology. Springer. (Ch. 5: Heterotrophic nutrition of corals).
• Nature Scientific Reports (2025). Nutrient limitation and coral bleaching susceptibility.
• Skomal, G. Saltwater Aquariums For Dummies. (Section: The Symbiotic Relationship).

Who wrote this article

Sources

1. NOAA National Ocean Service (2026). Corals Tutorial: What is Zooxanthellae?. National Oceanic and Atmospheric Administration.
2. Houlbrèque F., Ferrier-Pagès C. (2009). Heterotrophy in tropical scleractinian corals. Biological Reviews 84(1): 1-17.
3. WoRMS Editorial Board (2026). Symbiodiniaceae Fensome, Taylor, Norris, Sarjeant, Wharton & Williams, 1993. World Register of Marine Species (WoRMS).