Plankton on the menu: the dynamics of plankton feeding

In recent decades our understanding of how coral reefs feed has changed radically: from a simplified view of the coral as a purely autotrophic organism we have moved to a complex model of facultative mixotrophy.

This debate, which often comes up in discussions between hobbyists and professionals, now finds precise answers in the scientific evidence published up to the first quarter of 2025. The gap between those who claim phytoplankton is absent from natural reefs and those who promote dosing it in the tank is not just a matter of opinion: it reflects a deep misunderstanding of biological "drawdown" and of the Island Mass Effect (IME).

Coral reefs are extremely productive ecosystems, capable of photosynthetic production of 5 to 20 g of carbon per square metre per day, regardless of the nutrients in the surrounding waters. This "Darwin's paradox" is explained by the corals' ability to recycle nutrients internally and actively capture proteins and fats from plankton. Research published in 2025 in journals such as Nature Communications and Nature Microbiology indicates that corals' resistance to heat stress and acidification is directly linked to their ability to combine an autotrophic diet with a heterotrophic one.

Reef plankton: who is there and how much

Reef plankton is not a single block, but a mosaic of organisms that differ in size, origin and function. Yuri Sorokin, one of the leading authorities in coral ecology, divides it into three categories: bacterioplankton, phytoplankton and zooplankton.

Bacterioplankton, the foundation of the chain

Sorokin shows that bacterioplankton is often the largest component by quantity in reef waters. On healthy reefs bacterial biomass can reach 15-60 mg of carbon per m³, 4 to 6 times more than in the surrounding oligotrophic ocean waters. These bacteria do not just float as single cells: they form aggregates of coral mucus and detritus of 3-10 µm, available both to fine filter-feeders and to polyps.

Here are Sorokin's historical data (bacteria in millions per ml · biomass in mg C/m³ · production in mg C/m³ per day):

• Heron Island, outer reef: 0.35 · 20.7 · 20.2
• Heron Island, lagoon: 0.81 · 41.3 · 43.3
• Lizard Island, fringing reef: 1.28 · 26.2 · 12.2
• Tarawa Atoll, lagoon: 1.5-3.0 · 30-80 · 25-50
• Open ocean waters: 0.1-0.3 · 1-5 · 2-5

The comparison shows that the reef is a real "biological reactor", enriching the water that flows over it with microbial biomass: far from sterile water.

Open sea (1-5)5 mg of carbon per m³

Heron, outer reef20.7 mg of carbon per m³

Lizard, fringing reef26.2 mg of carbon per m³

Heron, lagoon41.3 mg of carbon per m³

Tarawa, lagoon (30-80)80 mg of carbon per m³

Over the reef the water holds up to 10 times more bacteria than the open sea: it is a biological reactor.

Bacterial biomass in the water: the reef versus the open sea. Data from Y. Sorokin, Coral Reef Ecology.

Phytoplankton and the island mass effect

The idea that phytoplankton is missing where corals live often comes from observations in inner lagoons, where biological filtration (the so-called wall of mouths) is so intense that algal cells are removed almost instantly. Satellite studies and field sampling published in 2025, however, confirm that the vast majority of coral atolls and islands produce an increase in phytoplankton of up to 86% in the surrounding zones compared with the open ocean.

The phenomenon, the Island Mass Effect, is due to deep nutrient-rich water rising up (localised upwelling), caused by currents striking the reef structure. The phytoplankton produced in this way is the base of the food chain for demersal zooplankton and sessile filter-feeders.

The main groups of reef phytoplankton:

• Diatoms (Nitzschia, Chaetoceros, Thalassionema): high primary productivity, silica recycling.
• Dinoflagellates (Gymnodinium, Peridinium, Ceratium): mixotrophy, possible seasonal blooms.
• Picocyanobacteria (Prochlorococcus, Synechococcus): nitrogen fixation, key biomass.
• Filamentous cyanobacteria (Trichodesmium): enrichment in organic detritus and nitrogen.

The coral as a predator

Scleractinian corals have a sophisticated arsenal for getting food from outside. The 2025 literature has made it clear that taking up nitrogen and essential fatty acids from plankton is vital for biomineralisation and tissue repair, especially under stress.

How they capture plankton

Capture with tentacles: the nematocysts stun the prey, mainly zooplankton between 0.2 and 2 mm. This is the main method in large-polyp corals (LPS) such as Euphyllia, Favia or Fungia.

Mucus net and ciliary currents: the coral's surface is covered with cilia that beat in coordinated waves, pushing a film of particle-laden mucus towards the mouth. It is an extremely efficient mechanism for picoplankton and nanoplankton (phytoplankton and bacteria).

Mesenterial filaments: some corals, such as Galaxea fascicularis, push out mesenterial filaments to digest prey outside their body.

1. 1Capture with tentacles nematocysts stun zooplankton of 0.2-2 mm; typical of LPS
2. 2Mucus and ciliary currents cilia push a film of mucus with picoplankton and nanoplankton to the mouth
3. 3Mesenterial filaments as in Galaxea fascicularis: they digest prey outside the body

The coral does not wait for food: it goes and gets it in three different ways.

New discoveries: isotopes and biomarkers

A breakthrough comes from a study by the Leibniz Centre for Tropical Marine Research (ZMT) carried out in Eilat and published in December 2025. The researchers showed that traditional methods based on carbon isotopes seriously underestimate the contribution of heterotrophic feeding: much of the carbon eaten is expelled or quickly respired and does not stay in the tissues. Nitrogen isotopes and certain fatty acids, on the other hand, stay in the tissues and reveal that corals depend on plankton far more than was thought.

Estimated contribution of autotrophy and heterotrophy:

• Stylophora pistillata: autotrophy 60-80%, heterotrophy 20-40% (marker: nitrogen isotopes and fatty acids).
• Acropora hyacinthus: autotrophy 70-85%, heterotrophy 15-30% (marker: particulate organic matter, POM).
• Deep-sea corals (Keratoisis): autotrophy 0%, heterotrophy 100% (marker: boron isotopes and biomineralisation pH).

Stylophora: autotrophy60–80 %

0100 %

Stylophora: heterotrophy20–40 %

0100 %

Acropora hyacinthus: autotrophy70–85 %

0100 %

Acropora hyacinthus: heterotrophy15–30 %

0100 %

Even corals with zooxanthellae get up to 40% of their food from plankton; deep-sea ones like Keratoisis get 100%.

Dosing phytoplankton in the aquarium

In a closed system there is no natural supply of ocean plankton. Intense mechanical filtration (skimmer, filter socks, rollers) and UV sterilisers continually remove microfauna and single-celled algae, creating an environment that is clear but may be biologically sterile and short of food for many filter-feeders.

Feeding the chain: the main benefit of phytoplankton is not necessarily feeding all the corals directly, but supporting the base of the tank's food pyramid. It feeds the zooplankton (copepods, amphipods, rotifers), which in turn is perfect food for corals and for demanding fish such as mandarins (Synchiropus) or pipefish (Syngnathidae). Phytoplankton also supplies long-chain polyunsaturated fatty acids (PUFA) such as EPA and DHA, the plant-based "fish oils", essential for cell membranes, growth and coral colour.

Competition with nuisance algae: contrary to what you might think, live phytoplankton can help reduce NO₃ and PO₄. The growing cells take them up for photosynthesis, like a floating biological filter. When phytoplankton is eaten by copepods or corals, the nutrients become animal biomass or are exported, instead of leaving the field open to cyanobacteria, dinoflagellates or filamentous algae.

Live, not preserved: the 2025 consensus is clear. Live phytoplankton is far better than preserved pastes or dry products: live cells stay in suspension for a long time, while dead ones sink to the bottom and break down, releasing phosphates and ammonia. Many filter-feeders (such as Tridacna) reject non-living particles as pseudo-faeces.

Plankton and bleaching: when corals suffer heat stress they expel their zooxanthellae and turn white. In this phase their autotrophic capacity is zero: without outside food they starve within a few weeks. Research by Ohio State University published in February 2025 shows that corals fed plenty of zooplankton in advance survive and recover much better.

In 2025 the UZELA (Underwater Zooplankton Enhancement Light Array) was also presented, an underwater device that uses specific lights to draw zooplankton towards the colonies, increasing feeding by 10 to 50 times. In nature too, under stress, the difference between life and death for a coral often depends on how much plankton it has available.

Deep-sea corals

For completeness, let's look at deep-sea corals (Lophelia pertusa and other corals without zooxanthellae), which live in total darkness. Martin Hovland puts forward the "hydraulic theory": these corals do not depend only on the rain of particles from the surface (marine snow), but are often associated with seabed seeps of hydrocarbons and methane. There, the chemical energy rising from the subsoil feeds bacterial mats and microorganisms that become a constant food source, independent of photosynthesis at the surface. Further confirmation of the importance of microorganisms and bacterioplankton in coral biology.

Biomineralisation under stress

Boron and its isotopes (δ¹¹B) are used by scientists to reconstruct the pH of past and present oceans. Studies on bamboo corals (Keratoisis) published in Geochimica et Cosmochimica Acta indicate that these organisms can actively adjust the pH of the calcifying fluid to favour calcium carbonate precipitation, even when ocean pH is lower.

This adjustment takes a great deal of energy (ATP). Heterotrophic feeding supplies the nutrients to power this "proton pump". In the aquarium, then, phytoplankton and zooplankton are not just there to make the coral "grow", but to give it the biochemical strength to withstand the swings in alkalinity and pH typical of closed systems.

Effects of live phytoplankton:

• Alkalinity (KH): indirect stabilisation, by using up organic acids → less effort to calcify.
• Phosphates (PO₄): biological uptake and lock-up → less inhibition of aragonite crystals.
• Carbon dioxide (CO₂): used up by day through photosynthesis → higher pH inside the polyp.
• Fatty acids (HUFA): supplied from outside through ingestion → intact membranes and more resistance to heat.

Why are there conflicting ideas?

The opposing view comes from a partial picture of nutrient dynamics. Those who claim phytoplankton is absent rely on the extreme clarity of tropical water. But clarity does not mean sterility: it means efficiency. In the tropics plankton turnover is very fast, cells are eaten as soon as they are produced and the water stays poor in dissolved nutrients, but full of biological energy on the move.

In the aquarium many people have had failures, probably linked to low-quality or non-living products that lead to harmful bacterial blooms and pollution. Hence the scepticism. On the contrary, good-quality live strains (Nannochloropsis, Isochrysis, Tetraselmis) have given outstanding results in keeping "impossible" species and in preventing Old Tank Syndrome.

Which phytoplankton to choose

The strain should be chosen according to the goal.

Nannochloropsis oculata: ideal for nutrient control and basic stability. A very small (2-4 µm) and hardy cell, excellent for rotifers and as a "sponge" for nitrates.

Isochrysis galbana (T-Iso): brown in colour, it is the richest source of DHA. Important for coral colour (especially red and purple pigments) and for the health of clams.

Tetraselmis chui: a larger, mobile cell, much loved by copepods; it improves the feeding response of LPS.

Rhodomonas salina: known for the pigment phycoerythrin, it is a powerful antioxidant and helps reduce phosphates and nitrates.

Nannochloropsis

• Very small · 2-4 µm
• Basic stability, a "sponge" for nitrates
• Food for rotifers

Isochrysis (T-Iso)

• The richest in DHA
• Colour, especially reds and purples
• Excellent for clams

Tetraselmis

• Large and mobile
• Much loved by copepods
• Boosts the LPS response

Rhodomonas

• Rich in phycoerythrin
• Antioxidant
• Helps reduce nitrates and phosphates

Live, not preserved: live cells stay suspended until something eats them; dead ones sink and break down.

Which phytoplankton to choose. Start with minimal doses and watch the water and the copepods.

Summary and practical conclusions

Adding live phytoplankton and zooplankton to a reef aquarium is not a superfluous practice or a passing fad, but an ecological necessity for anyone who wants to reproduce the complexity of a natural ecosystem. Sorokin's evidence, confirmed by the 2025 publications, shows that plankton is the link between water chemistry and coral physiology.

Is it absent in nature? No: it is abundant, but it is eaten very quickly by the reefs themselves (the "wall of mouths") and regenerated by the Island Mass Effect.

Is it worth dosing in the tank? Yes, especially live phytoplankton: it feeds the microfauna, stabilises nutrients and supplies fatty acids the polyps do not get from the zooxanthellae alone.

What do the 2025 discoveries say? Heterotrophic feeding is the deciding factor for coral survival during heatwaves and bleaching. Technologies such as UZELA confirm that more plankton can help reefs survive.

The secret is balance: start with minimal doses, watch the water clarity and the copepod population, and temporarily switch off the most aggressive filtration to let microscopic nature do its work. The aquarium is not a collection of animals in clean water, but a controlled biological "soup", where every microorganism plays a part in the health of the holobiont.

Sources cited by the author

• NASA Science, Phytoplankton Enlivens Swaths of Barren Ocean.
• Sorokin, Y. I. Coral Reef Ecology.
• Coral Heterotrophy in an Era of Climate Change.
• Ohio State News, Feeding coral reefs can aid their recovery from bleaching events.
• Microbial dynamics in coral reef waters: Diel cycles in contrasting seasons (PMC, NIH).
• Critical factors driving spatiotemporal variability in the phytoplankton community structure of the coral habitat in Dongshan Bay, China (Frontiers).
• Inter-seasonal phytoplankton dynamics in the UAE coastal waters (Frontiers).
• PBS News, This key phytoplankton species that fuels the food web is threatened by warming oceans.
• UltraMarine Magazine, Direct Coral Feeding Found To Be More Important Than Assumed.
• Nutritional and Microbial Responses of Pocillopora verrucosa to Co-Culture With Chromis viridis Damselfish (PMC).
• Skomal, G. Saltwater Aquariums For Dummies, 3rd Edition.
• Dosing zoo or phyto plankton for your corals.
• AlgaeBarn, Do Corals Really Eat Plankton?
• Heterotrophy of Particulate Organic Matter Subsidies Contributes to Divergent Bleaching Responses in Tropical Scleractinian Coral (ODU Digital Commons).
• Salty Underground, Live food.

Who wrote this article

Sources

1. Gove J.M., McManus M.A., Neuheimer A.B. et al. (2016). Near-island biological hotspots in barren ocean basins. Nature Communications 7: 10581.
2. Leibniz Centre for Tropical Marine Research (ZMT) (2025). How do corals feed? Energy supply from directly ingested food previously underestimated (study: Love C.R., Stuhr M., Fox M.D. et al., Communications Biology 8: 1285). ZMT Bremen.
3. The Ohio State University, Ohio State News (2025). New technology lights way for accelerating coral reef restoration (study: Grottoli A. et al., Limnology and Oceanography: Methods). The Ohio State University.
4. The Ohio State University, Ohio State News (2024). Feeding coral reefs can aid their recovery from bleaching events (study: Dobson K. et al., Communications Earth & Environment). The Ohio State University.
5. Hoeksema B.W., Cairns S., Samimi-Namin K., World List of Scleractinia (2026). Desmophyllum pertusum (Linnaeus, 1758). World Register of Marine Species (WoRMS).
6. Farmer J.R., Hönisch B., Robinson L.F., Hill T.M. (2015). Effects of seawater-pH and biomineralization on the boron isotopic composition of deep-sea bamboo corals. Geochimica et Cosmochimica Acta 155: 86-106.