Coralmania by Eric Borneman

The Food of Reefs, Part 3: Phytoplankton


The past few years in the reef aquarium hobby has witnessed an explosion in the use and availability of phytoplankton products. As is my normal mode of operation, I will not be covering the various advantages or differences in the various phytoplankton products available. However, what I will cover are some of the immensely complex and largely unknown aspects of what phytoplankton are: and what they are not. I have witnessed the widespread belief that phytoplankton are unicellular plants that are consumed by corals, and that the addition of phytoplankton comprises a newly discovered beneficial supplement for reef aquariums. Unfortunately, that belief is largely erroneous and highly simplified. I am, by no means, an expert on phytoplankton - either by science, or by experience with their uses in aquaria. I would urge anyone seeking a greater depth of information to seek out review articles and texts on the subject, of which there are thousands, or to contact someone with more expertise in the field. Yet, perhaps something of value may be gained from the words that follow.

What Are Phytoplankton?

Suspended drifting material in the water is either living or dead. In the first case, it is commonly referred to as plankton, and in the latter, detritus. Detritus will be covered in a future article. The word phytoplankton comes from the Greek words, phyton, meaning plant, and planktos, meaning to drift or wander, and combined to mean, "drifting plant." A problem arises from the fact that the original name certainly stemmed from the notion that green things that used sunlight and had cell walls were plants. Unfortunately, this is not the case. Most of the phytoplankton are in the Kingdom Protista, or protists, and may share unique attributes that are neither animal nor plant, or perhaps a bit of both.

Phytoplankton belong to a diverse taxonomical assemblage that had origins nearly 2 billion years ago. Some phytoplankton are among the most evolutionarily ancient organisms on earth, while others, such as the diatoms, have evolved relatively recently. Most current taxonomical sources recognize five Kingdoms: Monera (the bacteria and blue-green algae), Protista (protozoa, algae, and slime molds), Plantae (true plants), Fungi, and Animalia. Phytoplankton are found in Monera and Protista, and none are found in the plant kingdom Plantae. Current classification may not incorporate the five kingdom scheme and more information can be found at:

Kingdom Monera, or the bacteria and bacteria-like organisms, have about 2000 discovered species in marine environments, although it is likely many, many more exist. This kingdom is divided into two primary groups, the Archeobacteria and the Eubacteria, with the latter group having members, the Cyanobacteria and the Chloroxybacteria, that contain chlorophyll a. For many years, it was thought that the blue-green algae, or cyanobacteria, were exclusively freshwater species, until the discovery of marine species in the late 1950's. It is now known that there are large numbers of them in both the marine benthos and in marine plankton, and they play a major role in nitrogen fixation in the marine environment.

Kingdom Protista was a "catch-all" of organisms grouped mainly by being generally simple unicellular species that do not fit in the other Kingdoms. It really has disappeared from recent (last 5-10 years) treatments. Most of the algae are considered to be part of this Kingdom today, having been assigned mainly in the past to the Kingdom Plantae. Even the macroalgae are largely considered to be in Protista, and not Plantae, with a few somewhat disputed exceptions. Phytoplankton are considered to be algae, and are not plants. Algae are typically separated by the types of their photosynthetic pigments, storage products, chloroplast structure, cellular features, cell wall structure and composition, flagella (if present), types of cell division, and life history traits. Protist phytoplankton, with some exceptions, belong predominantly to the divisions Chrysophyta, Pyrrhophyta, Euglenophyta and Cryptophyta. There are none from the red or brown algae divisions (Rhodophyta and Phaeophyta).

           From Dawes (1998)

Most phytoplankton are unicellular, either solitary or colonial, although some may be multicellular and filamentous. About 5000 species are currently described, but estimates of 100,000 species of diatoms (based mainly on freshwater forms) would indicate that the marine environment is, to put it mildly, hugely unexplored. Phytoplankton are comprised primarily of diatoms, dinoflagellates, coccolithophorids, cyanobacteria, and other flagellates. Phytoplankton are found in bodies of water ranging in size from small puddles, formed after a rain, to the oceans. It may seem confusing that many of these same organisms may normally live on sediments or surfaces and become suspended by water turbulence; however, these sedentary forms are not usually properly considered true phytoplankton. Furthermore, some species have life cycles that include a dormant or encysted stage that can last for extended periods of time (up to years), and only part of their life is planktonic. Most phytoplanktors are motile, with the most motile being dinoflagellates that have swimming speeds of 50-500 µm/sec. They are grouped according to cell size and are often counted by measuring the amount of chlorophyll in the water, rather than by actually sorting and counting the cells, although the latter may be done for some studies.. The majority of phytoplankton, in numbers and biomass, can be considered an autotrophic part of the microbial community since they are in the smallest planktonic size classes; the picoplankton and nanoplankton, also known as ultraplankton.

      Plankton type                Size in microns (mm)
0.2 - 2.0
2.0 - 20
20 - 200
200- 2000
> 2000

The picophytoplankton, perhaps the most important component of the phytoplankton in terms of their abundance and global ecological role, were only discovered about 25 years ago (Johnson and Sieburth 1979). In fact, the larger sizes, such as the microplankton seem only able to develop when nutrient levels are above those required by the pico- and nanoplankton. Picophytoplankton, comprised mainly of prokaryotes including bacteria, cyanobacteria, and prochlorophytes, as well as protist forms, can account for 50% or more of the primary production of oceanic waters. A recent study has shown that a single species of prochlorophyte, Prochlorococcus marinus that occurs to a depth of 250m in the subtropical and tropical oceans, can account for more than 50% of the total chlorophyll-a in the central Pacific Ocean! (Suzuki et al. 1995). Nannochloropsis, a genus of marine picophytoplankton having cells smaller than 0.2 µm, is commonly employed in "phytoplankton" products for marine aquariums. This genus also is capable of forming dormant stages resulting in forms where chilling can allow darkness tolerance for up to 24 weeks (Antia and Cheng 1970). This is a relatively rare trait that makes it an excellent candidate for live culture products such as DT's phytoplankton.

There is a great diversity of shape and form in phytoplankton, and dinoflagellates and diatoms are quite "famous" for these attributes. It is thought that the various shapes are predominantly adaptations related to their suspension in water. Generally, phytoplankton are denser than water, having silica, cellulose and/or carbonate components, and tend to sink. Various modes exist whereby these tiny cells may remain as drifters in currents, rather than sinking to the bottom. Some remain easily suspended because of their diminutive size. Others have shapes that alter the hydrodynamic forces and tend to give them "lift" or "resistance." Some have gliding, flexing, or active swimming behaviors, while still others have utilized cellular components filled with gas or positively buoyant material.

It is generally true that phytoplankton use sunlight, carbon dioxide, nutrients and trace minerals as requirements for their existence. However, a large component of them, notably flagellates, can have pronounced heterotrophic qualities; they can engulf, build up and store dissolved or particulate organic material, and in many cases may be predatory. It is important for phytoplankton to remain suspended in order that they are able to obtain adequate light to supplement scarce oceanic nutrients. Picophytoplankton, for instance, have a negligible sinking rate, and their tiny size allows for the most efficient diffusion of nutrients into and out of the cell. Diatoms, in contrast, may depend on sinking resulting from their density and size to force conduction and lower the diffusive boundary layer surrounding them. Swimming behavior also accomplishes this, especially when coupled with positive phototaxis, or swimming toward light, such as seen in some of the flagellates. Positive buoyancy, as found in many cyanobacteria, also accomplishes higher nutrient uptake, but movement occurs upwards instead of downwards.

Phytoplankton are largely limited to the photic zone; an area from the water surface down to the point, called the compensation depth (or critical depth), where the energy production of phytoplankton by photosynthesis matches their energy destruction by respiration. Even so, some phytoplankton are able to exist short to rather long durations without light, and can exist far below the photic zone. As might be expected, since the zooxanthellae of corals are dinoflagellate algae acting as "captive phytoplankton," the responses of phytoplankton to light irradiance follow some similar patterns. In general, they can modify their photosynthetic and associated pigment concentrations, plastid size, thylakoid density, number of photosynthetic antennae, or production of light shielding compounds (mycosporine-like amino acids, or MAA's).

Division class:

a   b   c1  c2
ß-carotene Major xanthophylls
Cyanophyta    Cyanophyceae   (Cyanobacteria)  +        
 +              +

 +         +   +

 +         +   +
 +         +   +



 +              +
   (and higher      plants)
 +   +
 +   +

Table 3 . The distribution of photosynthetic pigments in the most common phytoplankton taxa (from Richardson et al. 1983)

Phytoplankton can use swimming and/or positive and negative buoyancy attributes to moderate their light environment. In general, phytoplankton are species that prefer a reduced light environment; both photosystem damage and photoinhibition are common at higher irradiance levels, such as those found in shallow oligotrophic waters like coral reefs. Generally, dinoflagellates and cyanobacteria grow best at very low irradiance levels and may be photoinhibited even at low levels. Diatoms can tolerate much higher irradiance levels, but may not prefer to be exposed to it. The green algae typically have the highest photosynthetic compensation points of the phytoplankton and can tolerate very high irradiance levels (Richardson et al. 1983).

Phytoplankton require a source of nitrogen that they can utilize after direct uptake. Nitrogen is usually taken up as ammonium-N, with nitrate and nitrite uptake also possible. Phosphorous is preferentially taken up as phosphate, although they can also utilize polyphosphate and organic phosphate sources. In general, microphytoplankton populations increase significantly in response to nitrogen and phosphorus enrichment, but the smaller nano- and picophytoplankton populations do not (Takahashi et al. 1982). Silicon is required and acquired by diatoms and silicoflagellates. Additionally, organic nutrients such as vitamins are required. Although phytoplankton are autotrophic with respect to carbon production, many are auxotrophic (requiring a substance beyond levels normally found in the environment) with respect to vitamins. In a study of 400 clones of phytoplankton, 44% required vitamin B12, 21% required thiamin and 4% needed biotin (Swift 1980). Vitamin requirements were found to vary according to species, with certain groups, such as diatoms, requiring different vitamins than dinoflagellates. Nanomolar concentrations of trace elements required usually include zinc, iron, copper and manganese. Higher levels may be toxic (Huntsman and Sunda 1980). The vitamins and trace metals are thought to be acquired by their association with bacteria, through the death of other microalgae and zooplankton, and in areas where such organic and inorganic components are found in greater abundance, such as in coastal zones or areas of higher sediment loading.

Phytoplankton distribution, while quite consistent when considered on a global scale, tends to occur in patches on smaller scales. This is believed to be mostly a function of water motion, patchy distribution of nutrients, and the presence of herbivory by zooplankton or other sessile phytoplanktivores. Blooms also occur, usually in spring, and often occur successionally. The successions are usually initially diatoms, followed by coccolithophorids and then dinoflagellates. This is thought to result from both increased metabolism in warmer temperatures, a response to rising irradiance levels coupled with reduced zooplanktivory. While bacterioplankton outnumber phytoplankton by several orders of magnitude on coral reefs, diatoms and naked dinoflagellates tend to be the predominant larger forms, along with picophytoplankton and nanoplankton such as Platymonas spp. Other studies found seasonal variations on coral reefs with 70% of winter phytoplankton composed of protist nano- and picophytoplankton, while Synechococcus sp. cyanobacteria formed the majority during the summer (Yahel et al. 1998).

Rapid responses by the phytoplankton create a nutrient deficit during the summer and result in reduced phytoplankton growth. Most notable are the "red tide" blooms of cyanobacteria and dinoflagellates that can produce seriously harmful toxins. In the tropics, these are usually caused by Trichodesmium spp. cyanobacteria. However, not all cyanobacterial or dinoflagellate species that produce blooms produce toxins, and not all harmful algae blooms are red. Here are some links for supplemental information on red tides:

Ecologically, phytoplankton are the major source of primary production in the ocean, and one of the most important driving forces of global ecology. In fact, phytoplankton production influences all life by being at the lowest rings of the food chain, and even plays a role in global climate. In terms of their growth and ecology, they are in many cases most similar to bacteria. In fact, only bacteria share such similarities in size, growth rate ecological tolerance, and rapid response to nutrient enrichment.

As an aside, I am reminded of an advertisement in the aquarium literature that uses the promotional headline, "Nature uses algae, not bacteria, to filter water." In fact, this may be more the case than we realize. For many years, we have operated under an old assumption that nitrifying bacteria are responsible for converting ammonia into nitrite, and then nitrate. However, the rapid proliferation of algal "blooms" and diatoms in newly established tanks begs the question of whether the phytoplankton are not equally, or perhaps even more, involved in tank "cycling." The single celled algae have doubling times from a few doublings per day (in the smaller, faster growing species) to once every week or so for slower species. During red tides in the Puget Sound region, dinoflagellates such as Gonyaulax can double every 30 minutes or so. They are also able to directly take up ammonia from the water column, thus "cycling the water prior to the nitrification "cycle."

Despite their rapid response to elevated nutrients, phytoplankton are not particularly important primary producers, by themselves, in terms of efficiency. The reason they are so important on a regional or global scale is simply by virtue of the fact that the upper 200m of oceanic waters is filled with phytoplankton and covers over 70% of the earth's surface. One source (Dawes 1998) compares primary producers in terms of carbon production (kg C m-2 y-1) as follows, from most to least efficient: corals and large seaweeds (0.5 - 2.5), benthic microalgae (0.2 - 2.0), salt marsh grasses and seagrasses (0.4 - 1.5), mangroves (0.5 - 1.0), coastal phytoplankton (0.1 - 0.5) and oceanic phytoplankton (0.2).

Grazing - The Big Question.

What eats phytoplankton? In the water column, zooplankton are without question the primary consumers of phytoplankton. Zooplankton grazers vary according the area and the time of year, but include primarily ciliates, copepods, amphipods, and tintinnids. Protozoans and some invertebrate larvae are the primary consumers of nanoplankton with copepods and amphipods consuming the larger phytoplankton size classes. Seasonal cycles with zooplankton increasing following phytoplankton blooms are well known (Harvey et al 1935, Frost 1980). However, the efficiency of zooplankton feeding on phytoplankton can be extended to encompass a diurnal cycle. Evidence for zooplankton as influencing daily cycles of phytoplankton abundance comes from Yahel et al. (1998), where it was found that phytoplankton abundance was greatly reduced over the course of the night when zooplankton are at their highest abundance.

Of perhaps the most interest to those keeping reef aquaria is to know what non-planktonic grazers of phytoplankton exist on coral reefs. Benthic grazers are mainly bivalves, ascidians (tunicates), sponges and polychaetes. Other major consumers are gastropods, crinoids, foraminiferans and soft corals. Because more carbon is contained in the biomass of phytoplankton than zooplankton (usually by at least an order of magnitude), and the nutrients are often limited around coral reefs, phytoplankton are an obviously important food resource to many organisms. The community of phytoplankton consumers on coral reefs can drop phytoplankton levels 15-65% below adjacent open ocean waters (Yahel et al 1998). Further support for their consumption comes from a concomitant increase in the levels of phaeopigments, breakdown products of phytoplankton and substances released by zooplankton.

While some studies have indicated that some stony corals are capable of clearing phytoplankton from the water, these experiments have not been rigorous (Wilkinson et al. 1988, Szmant-Froelich and Pilson 1984, Sorokin 1981, 1995). Ingestion does not equate to digestion. The extent to which phytoplankton contribute to stony coral nutrition is unknown, but it is probably unlikely that phytoplankton are an important food source for most stony corals. Among those reported or suggested to clear or ingest phytoplankton are: Acropora, Siderastrea, Montipora, Porites, Astrangia and Tubastraea. Other studies tend to directly refute these suggestions for all but Astrangia and Porites. More directly, Goniopora and Alveopora may have more herbivorous tendencies (Peach unpublished thesis). Stony corals are generally not well adapted to the sieve or filter type feeding that characterizes the soft corals (Fabricius et al. 1995, 1998). They are, however, well suited to the capture of zooplankton prey. I am sure that future studies will examine potential roles of phytoplanktivory in the Scleractinia in more detail. However, I think it safe to assume that the number of stony corals that depend on phytoplankton as a food source will be minimal, or that the relative contribution of phytoplankton to their energy needs will be slight.

Big Question Number 2 - How Much Should I Feed?

As I said, I am not fond of giving recommendations for aquarium protocol. Rather, I would prefer simply to communicate some information and let each individual work out the "rules" that apply to their own circumstances. It is of paramount importance to recognize that the biomass of potential grazers in an aquarium is many times what it would be in the same volume of water or surface area as the bottom of oceans or on reefs, and also that the availability of water column borne food is many times greater in the ocean than in an aquarium. This creates an immediate and perhaps irresolvable dilemma. It is also important to consider the types, sizes, and numbers of potential consumers of phytoplankton in a given aquarium. For example, a clam and soft coral tank with a large sponge community and a refugium with many small crustaceans and a deep sand bed with many polychaetes will have a vastly greater uptake/consumption of phytoplankton than a bare bottom, no refugium stony coral garden-type tank.

However, if one were to attempt to recreate natural levels of phytoplankton in the aquarium, the following chart may be of some use, especially if the density of cells per phytoplankton product is known.

Organism type
Density of cells/ml water
Phototropic picoplankton and nanoplanktonn
Nanoplanktonic flagellates
Viral particles

I mention bacteria and viruses in the chart for two reasons. First, to call attention to an upcoming part in this series on the food value of bacteria. Second, because viral infection accounts for between 30-60% of mortality in bacteria and cyanobacteria in the ocean (Proctor and Fuhrman 1990), and it is suggested that similar mortality figures for phytoplankton are likely, especially given the abundance of viral particles in the water. Third, and perhaps most importantly, is the almost ubiquitous interaction between bacteria and phytoplankton. Phytoplankton release dissolved organic substances and bacteria utilize them as nutrient sources. Most phytoplankton cells, especially large ones, are coupled nearly continuously with coatings of bacteria. The bacteria are apparently both beneficial and detrimental in that they provide breakdown products able to be utilized by the phytoplankton, but also increase the diffusive boundary layer whereby the phytoplankton would ordinarily be able to directly uptake nutrients from the water.

In conclusion, phytoplankton are hugely abundant in numbers and biomass and are present at levels that make them among the most ecologically important groups on the planet. Diversity and biomass is likely to be higher near coastal environments and in temperate environments rather than in oceanic or oligotrophic tropical waters like coral reefs. They constitute an important trophic resource to zooplankton and many sessile benthic invertebrates, perhaps ironically not those to which a majority of aquarists apparently expect to see direct benefits of phytoplankton additions. In regards to aquariums, I offer the following bulleted list of suggestions and comments regarding phytoplankton in reef aquariums:

  • The amounts of phytoplankton present in reef aquariums are not known but are probably considerable. However, they are also probably rapidly removed by grazing and export devices.

  • Similarly, the amounts required to produce an equivalent level to sustained natural seawater levels is not known, and is probably highly dependent on individual differences in tank stocking, but is likely a considerable amount.

  • Algae "blooms" in aquariums are normal, and follow seasonal patterns that are apparently similar to those found in the wild. As in the wild, grazing or nutrient conditions probably change rapidly to limit blooms to rather short durations.

  • Many of the phytoplankton are species not normally considered or deemed desirable, including diatoms, dinoflagellates, and cyanobacteria. Yet, just as they are important and functional constituents of wild phytoplankton populations, so they should be in aquariums.

  • Not all phytoplankton will likely be beneficial in aquariums. Many species produce toxins that can be harmful to tank inhabitants. Common evidence of this is found in coral deaths involving cyanobacterial blooms and snail and echinoderm deaths during dinoflagellate blooms. However, not all dinoflagellates or cyanobacteria necessarily have these toxins or produce these results in aquariums.

  • "Psuedo-phytoplankton" is probably available in tanks in significant amounts when substrate associated algae are put into circulation by strong water flow or during tank glass scraping.

  • In order to maximally benefit the largest number of potential tank inhabitants that are phytoplankton consumers, it is probably wise to allow for the occurrence of similar ratios of various size classes as found in the wild. To my knowledge, despite the numbers of available products, relatively few variations in size classes are available, and thus supplementation with a certain size class will probably not benefit large groups or organisms incapable of feeding on them.

  • Supplements using living cultures or products containing living cells are probably far more beneficial than those made of dead cells. Dead cells are, for all practical purposes, decomposing particulate material that, while potentially useful as a food source, do not offer the benefits of living cells as described in the article.

To end this article in the continuing series on coral reef food, I offer the following list of "famous" phytoplankton:

     Symbiodinium spp. - the symbiotic algae of corals, clams, and other organisms are dinoflagellate protist phytoplankton.

     Pfiesteria piscicida - dinoflagellate ambush predator responsible for fish kills by micropredation.

     Trichodesmium spp. - one of several cyanobacterial phytoplankton causing "red tides".

     Gambierdiscus toxicus - one of several dinoflagellates responsible for ciguatera poisoning.

     Ostreobium quecketii - the microalgae found living in the extremely low light of coral skeletons below the living tissue.

     Nannochloropsis oculata - the primary phytoplankton of aquarium phytoplankton products.

     Pseudo-nitzschia australis - newly discovered toxic diatom that accumulates in fish and shellfish and can cause brain damage in low amounts.

Links to Part 1, Part 2 , Part 4, Part 5, Part 6, Part 7

If you have any questions about this article, please visit my author forum on Reef Central.


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Fabricius K, Behayahu Y, Genin A. 1995. Herbivory in asymbiotic soft corals. Science 268(5207): 90-92.

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The Food of Reefs, Part 3: Phytoplankton by Eric Borneman -