A liter of seawater can hold millions of phytoplankton cells. Most are only a few micrometers across, so they are invisible without a microscope. A single drop of pond water may hold a dozen different kinds.
Phytoplankton are the drifting cells that photosynthesize in lakes, rivers and seas. They sit at the base of most aquatic food webs. They also produce about half of the oxygen in the air and carry out about half of all photosynthesis on Earth.
Identification is the step that turns a bucket of water into information. It answers three questions. Which group is present? Which species? And how many?
Those answers decide whether a lake is safe for swimming, whether a bloom is toxic, and how much carbon is moving through a food web. They also underpin every study of ocean productivity.
This article follows the work in the order it happens. It starts with the sample, moves to the microscope, then to pigments, DNA and automation. Each step narrows the answer, and each has limits worth knowing.
What Counts as Phytoplankton
“Phytoplankton” is a job description, not a family tree. The word gathers any microscopic cell that photosynthesizes and drifts in the water. That includes bacteria and several unrelated groups of protists.
The name comes from Greek words for plant and drifter. The organisms behind it share little else. A diatom builds a glass shell, and a cyanobacterium has no nucleus at all.
The category is also looser than it looks. Many dinoflagellates and coccolithophores can eat other cells as well as photosynthesize. Researchers now call these mixoplankton, because neither plant nor animal fits.
About 5,000 species of marine phytoplankton have been described, and new ones are still found. Three groups dominate most counts: diatoms, dinoflagellates and cyanobacteria. Coccolithophores matter most in the open ocean.
Diatoms
A diatom lives inside a shell called a frustule, made of silica. The shell has two halves that fit together like a box and its lid. The pores and ribs on that shell are the main identification character.
There are two broad shapes. Centric diatoms are radially symmetric, like a wheel or a drum. Pennate diatoms are long and bilaterally symmetric, like a boat or a needle. Most species are pennate.
Most diatoms look golden brown under the microscope. The pigment fucoxanthin hides the green of chlorophyll. Many pennate diatoms glide along a surface, while centric diatoms drift with the water.
About 12,000 diatom species have been described. Their silica shells survive after the cell dies. That preserves them in the fossil record and creates a trap for counters, because an empty shell is not a living cell.
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| An assortment of diatoms seen through a microscope. These cells were living between crystals of annual sea ice in McMurdo Sound, Antarctica, and each is enclosed in a silicate cell wall. Image: Prof. Gordon T. Taylor, public domain, via Wikimedia Commons |
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| Three chains of Chaetoceros diatoms under phase contrast. Long setae, the bristle-like extensions, link the cells into chains and are a genus character visible at low magnification. Image: Atsukada, CC BY 4.0, via Wikimedia Commons |
_Ehrenberg_1840_diatom_shell._Colored_SEM_image.jpg) |
| A false-color scanning electron micrograph of a Coscinodiscus oculus-iridis shell. The field of view is 40 micrometers, which shows the porous structure of the silica frustule that identifies a centric diatom. Image: Pavel.Somov, CC BY-SA 4.0, via Wikimedia Commons |
Dinoflagellates
A dinoflagellate has two flagella, and both sit in grooves. One lies in a transverse groove called the cingulum and spins the cell. The other trails in a longitudinal groove called the sulcus and steers.
Most species are covered by cellulose plates, called theca. The number, shape and arrangement of those plates identify the genus. Some species have no plates, which makes them much harder to name.
The nucleus is unusual. Its chromosomes stay condensed through the whole cell cycle, a state called a dinokaryon. That detail helps in a stained preparation, where the nucleus looks unlike any other.
Dinoflagellates cause many harmful blooms. Alexandrium species make saxitoxin, which causes paralytic shellfish poisoning. Dinophysis species make okadaic acid, which causes diarrhetic poisoning. Karenia brevis makes brevetoxins.
Not every bloom is toxic. Some species turn the sea red or brown simply by their numbers. Others, such as Noctiluca, glow blue when the water is disturbed at night.
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| A light micrograph of Ceratocorys horrida. The spines and the grooved outline are the characters a taxonomist uses at this magnification, before any plate detail is visible. Image: Soupremacy, CC BY-SA 4.0, via Wikimedia Commons |
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| A scanning electron micrograph of a dinoflagellate. The surface plates and the groove that holds the transverse flagellum are clear here, while a light microscope shows them only in outline. Image: CSIRO, CC BY 3.0, via Wikimedia Commons |
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| A cell of Dinophysis tripos under an epifluorescence microscope at 640x. Cells of this genus make okadaic acid, so a bloom of it closes shellfish beds. Image: Phycoplankton, CC BY-SA 4.0, via Wikimedia Commons |
Cyanobacteria
Cyanobacteria are bacteria that photosynthesize. They have no nucleus, no chloroplasts and no membrane-bound organelles. Their pigments sit on folded membranes inside the cell.
Their accessory pigments are phycobilins, which give many species a blue-green color. That color gave the group its old name, blue-green algae. The name is wrong, because they are bacteria and not algae.
Two genera matter for water quality. Microcystis forms colonies of cells inside mucus and floats on gas vesicles. It can make microcystins, which damage the liver. Trichodesmium is marine and forms visible colonies shaped like puffs or tufts.
Trichodesmium also fixes nitrogen. It turns dissolved nitrogen gas into a form other plankton can use. That makes it important in nutrient-poor seas, where nitrogen is often the limiting nutrient.
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| Colonies of Microcystis aeruginosa, with one needle-like colony of Aphanizomenon flos-aquae, photographed under monochromatic blue light. In summer these colonies form green clots that float to the surface and look like paint. Image: T.Voekler, CC BY-SA 3.0, via Wikimedia Commons |
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| Trichodesmium colonies sorted into their morphological classes: radial puffs, non-radial puffs and tufts. Colony shape is one of the characters used to describe a surface bloom of this genus. Image: Mary R. Gradoville et al., CC BY-SA 4.0, via Wikimedia Commons |
Coccolithophores and Other Groups
A coccolithophore is a single cell covered in plates of calcium carbonate, called coccoliths. The plates form a shell called a coccosphere. The group belongs to the haptophytes.
Coccolithophores are among the most productive calcifying organisms on the planet. When they bloom, shed plates scatter light and turn the water turquoise. Emiliania huxleyi is the most abundant species.
Fresh water adds other groups. Green algae have chlorophyll b and a clear grass-green color. Euglenoids have a red eyespot and one visible flagellum. Cryptophytes are small, brownish and easy to overlook.
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| Scanning electron micrographs of common coccolithophores from an Indian Ocean transect: Rhabdosphaera, Gephyrocapsa, Umbellosphaera, Umbilicosphaera, Oolithotus, Emiliania huxleyi and Michaelsarsia. The shape of the individual coccoliths separates the genera. Image: Jeremy R. Young et al., CC BY-SA 4.0, via Wikimedia Commons |
| Group | What stands out | Identification clue |
| Diatoms | Glassy shell, golden-brown color | Silica frustule with pores; centric or pennate symmetry |
| Dinoflagellates | Two flagella, one in each groove | Cellulose plates; cingulum and sulcus |
| Cyanobacteria | No nucleus; often blue-green | Phycobilin pigments; colonies and gas vesicles |
| Coccolithophores | Covered in chalk plates | Coccosphere built from calcite coccoliths |
| Green algae | Grass-green chloroplasts | Chlorophyll b; varied cell shapes |
The five groups a beginner is most likely to meet, and the character that separates each one at the microscope.
Collecting a Sample That Can Be Identified
The sample decides what you can identify. A net catches large cells and misses small ones. A bottle catches everything and needs concentration. Match the gear to the cells you care about.
Nets, Bottles and Pumps
A plankton net is a cone of fine mesh, towed through water. A 20-micrometer mesh keeps most diatoms and dinoflagellates and lets smaller cells through. Nets are fast and cheap.
Nets also damage fragile cells. A dinoflagellate can lose its flagella or burst in the cod end. A net sample is best for a quick look at what is large and abundant.
A water bottle closes at a chosen depth and brings back a known volume. That volume matters, because counts are reported per liter. Bottles catch everything, but the sample is dilute.
Pumps and hoses do the same job over a depth profile. They are the tool for thin layers, where cells concentrate in a band a few centimeters thick.
Fixatives and What They Cost You
Lugol's iodine is the standard fixative for phytoplankton. It stains starch and preserves flagellates well. It keeps samples for months in the dark and turns the water brown.
Formalin lasts longer and is better for hard shells. It is toxic, so it needs careful handling. Neither fixative preserves soft cells perfectly, and both make some cells shrink.
Glutaraldehyde is used when samples will go through a flow cytometer. It holds cell shape better than Lugol's and keeps fluorescence measurable.
Some identifications need living cells. Swimming, flagellar movement and bioluminescence vanish within minutes of fixation. If those characters matter, look at the sample fresh.
Concentrating the Sample
Most bottle samples hold too few cells to count directly. Settling is the gentlest way to concentrate them. The sample stands still in a chamber for a day, and the cells sink to the bottom.
The Utermöhl chamber is the standard settling chamber. It has a known base area, so a count on the bottom converts to cells per liter. The method is slow, but it keeps small and fragile cells intact.
Filtration is faster, but it crushes soft cells. Centrifugation is faster still, and it damages fragile flagellates. Both are acceptable for tough cells and poor choices for delicate ones.
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| Utermöhl counting chambers. A fixed volume is settled in the cylinder, the cells sink onto the base, and the base is counted under an inverted microscope. Image: Panek, CC BY-SA 4.0, via Wikimedia Commons |
Identifying Cells Under the Microscope
Microscopy is still the reference method. It is the only method that shows shape, color, arrangement and behavior at once. Every molecular result is eventually checked against a named, photographed cell.
Choosing Optics
Bright-field microscopy works for large, pigmented cells. Phase contrast adds edge detail and shows flagella that would otherwise be invisible. Differential interference contrast gives a three-dimensional look and suits silica shells.
Magnification depends on the cell. A total of 100x is enough for a large dinoflagellate. Small cells need 400x, and the smallest need oil immersion at 1000x.
A camera helps more than it seems. Photographing a cell forces you to record the characters you used. It also lets a second person check the identification later.
Characters That Separate the Groups
Start with size and shape. Then look at the outline: round, needle-like, star-shaped or chained. Then look at the wall.
A glassy, patterned shell means a diatom. A smooth cell with visible plates means a dinoflagellate. A tiny cell with no nucleus means a cyanobacterium, though you need high magnification to be sure.
Flagella and grooves are decisive when you can see them. So is the number and shape of chloroplasts. One large chloroplast looks very different from many small ones.
Colony form matters too. Chains, ribbons, stars and gelatinous blobs are typical of particular genera. Record the form before the sample is disturbed.
Counting and Measuring
A count is not a biomass. A sample can hold many small cells and little carbon, or fewer large cells and much more. Report abundance and biovolume when the question is about production.
Biovolume comes from measured dimensions, using a geometric shape for each taxon. Carbon is then estimated from biovolume with a conversion that depends on the group. Those conversions are approximations, not measurements.
Counting rules matter as much as identification. Count a fixed number of fields or a fixed area. Set a minimum count, and record cells that are empty, dead or too small to identify.
Where Morphology Runs Out
Some species look identical under a light microscope. They differ in DNA, in mating behavior or in toxin production. No amount of careful looking separates them. These are cryptic species.
Empty shells are a second problem. A diatom frustule can persist long after the cell inside has died. Counting empty shells as living cells inflates the biomass estimate.
The smallest cells are a third problem. Picoplankton sit near the resolution limit of light microscopy. They are usually counted by flow cytometry or by DNA, not by eye.
Pigments, DNA and Automation
Microscopy runs out of resolution in two directions. Very small cells blur together, and very similar cells look the same. The next methods read chemistry and code instead of shape.
Chlorophyll Fluorescence
Chlorophyll a fluoresces red when blue or ultraviolet light hits it. Under an epifluorescence microscope, photosynthetic cells glow against a dark background. Detritus and mineral particles stay dark.
Fluorescence is a fast way to separate living cells from everything else. It does not identify species. It tells you which cells are photosynthetic, and roughly how many.
Fluorescence also reports physiology. A stressed cell emits less light. Instruments that pulse light can estimate photosynthetic efficiency, which shows how well the cells are doing.
Marker Pigments by HPLC
High-performance liquid chromatography separates pigments extracted from a filtered sample. Each pigment appears at a known time and wavelength. The pattern is a fingerprint of the community.
A few marker pigments are especially useful. Fucoxanthin points to diatoms. Peridinin points to dinoflagellates. Zeaxanthin points to cyanobacteria. Chlorophyll b points to green algae.
The method is quantitative, fast and works on small samples. Its weakness is resolution. A marker pigment can belong to more than one group, so the result is a proportion, not a species list.
| Marker pigment | Group it suggests | Caution |
| Fucoxanthin | Diatoms | Also found in some other brown-gold groups |
| Peridinin | Dinoflagellates | Only in the species that make it |
| Zeaxanthin | Cyanobacteria | Also found in green algae |
| Chlorophyll b | Green algae | Also found in euglenoids and others |
| 19'-hexanoyloxyfucoxanthin | Coccolithophores and other haptophytes | Shared with a few other groups |
Marker pigments used to describe a community, and the caveat that keeps each one from being treated as a species name.
Flow Cytometry
A flow cytometer passes cells one at a time through a laser. Each cell scatters light and emits fluorescence. Forward scatter relates to size, and side scatter relates to internal complexity.
Thousands of cells are measured each second. The output is a map of populations, and each cluster is a group of similar cells. Sorting instruments can physically separate those populations for later microscopy or DNA work.
Flow cytometry is excellent for picoplankton and for counting. It is weaker at naming species. Two species of similar size and pigment land in the same cluster.
The Standard Barcodes
DNA gives the finest resolution available. A short, standard piece of DNA is sequenced from a single cell or from a whole community. The sequence is then compared with a reference library.
The 18S ribosomal RNA gene is the workhorse for eukaryotes. It has conserved stretches for priming and variable stretches for identification. The V4 and V9 regions are the usual targets.
Diatoms are often barcoded with rbcL, a chloroplast gene. Dinoflagellates are often identified with the ITS region. Cyanobacteria use 16S or 23S ribosomal markers.
No single barcode fits every group. A study that mixes groups must use several markers or accept that some cells will go unnamed.
Metabarcoding and Quantitative PCR
Metabarcoding sequences a marker from all the DNA in a sample at once. The result is a list of taxa and their relative abundance. It can find rare species that a microscope would miss.
Quantitative PCR targets one species with specific primers. It is fast, cheap and sensitive, and it gives a number rather than a list. It is the usual tool for monitoring a known toxic species.
Fluorescence in situ hybridization sits between the two. A labeled probe binds a target sequence inside an intact cell. You can see the cell and know its identity at the same time.
What Sequences Add and What They Miss
Sequences add resolution and speed. They separate cryptic species and detect cells too small or too rare to see. They also work on preserved samples, which morphology handles poorly.
They miss morphology, so they cannot give the size of a cell or the shape of a colony. They also pick up DNA from dead cells that have drifted away from where they lived.
Reference libraries are the other limit. Many groups are poorly represented, so a sequence may match nothing known. A match to the nearest genus is a result, but it is not a species name.
Imaging Flow Cytometry
An imaging flow cytometer photographs every cell as it passes. Software then groups the images by shape. Instruments such as the Imaging FlowCytobot run unattended for months and report every hour.
The method depends on training data. A classifier is only as good as the images it learned from, and it struggles with cells it has never seen. Most programs still check unusual images by eye.
Satellites and Ocean Color
Satellites measure the color of the surface ocean. Chlorophyll changes that color, so a satellite can map where phytoplankton are abundant. A bloom appears as a patch of green, turquoise or red.
Coccolithophore blooms are especially visible. Their chalk plates reflect light and turn the water milky turquoise. The color is a signature of the group, not of a species.
Satellites cannot identify species. They see only the surface layer, and clouds block the view. Their value is scale: they show where to sample and how a bloom is moving.
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| A coccolithophore bloom off the southern coast of Newfoundland, seen by the SeaWiFS sensor. The chalk plates color normally dark North Atlantic water a bright turquoise, which is how a bloom of this group is recognized from orbit. Image: NASA SeaWiFS Project, public domain, via Wikimedia Commons |
Putting the Methods Together
No single method answers every question. A practical program combines a few, each covering the weakness of another. The combination is chosen for the question, not for the instrument.
A Workflow That Works
A lake survey might start with a net tow for a quick look and a bottle sample for counts. The bottle sample is fixed in Lugol's, settled and counted under an inverted microscope.
Chlorophyll and marker pigments then give a fast biomass estimate. They also warn when a group is present that the count did not capture, such as picocyanobacteria.
When a bloom appears, the program adds a targeted method. Quantitative PCR or a probe confirms a toxic species within hours. Metabarcoding then describes the rest of the community. Satellite images place the bloom in the wider water body.
| Method | What it answers | Strength | Limit |
| Light microscopy | Which group and genus; how many | Shows shape, color and behavior | Slow; needs expertise |
| Pigment analysis | Which groups are present, and in what proportion | Fast and quantitative | Low taxonomic resolution |
| Flow cytometry | How many cells of each optical type | Very fast; counts small cells | Weak at naming species |
| Metabarcoding | Which taxa are present | Finds rare and cryptic taxa | No morphology; needs libraries |
| Quantitative PCR | Whether a target species is present, and how much | Sensitive and specific | Answers for one target only |
| Satellite ocean color | Where biomass is high | Covers whole seas | Surface only; no species |
What each identification method actually answers, and the limit that makes a second method necessary.
Where Identifications Go Wrong
Four mistakes account for most bad identifications. Each is easy to make and easy to avoid.
The first is counting abundance as biomass. A count treats a 5-micrometer flagellate and a 200-micrometer diatom as equals. They are not. Report size or biovolume with the count.
The second is trusting an old name. Genera are split, species are moved and synonyms accumulate. Check names against a current register such as AlgaeBase or the World Register of Marine Species, and record the date you checked.
The third is mistaking a surface scum for a bloom. Duckweed, filamentous algae and cyanobacterial scum all look green from the shore. They are different things. A scum holds cells concentrated at the surface, not a measure of the whole lake.
The fourth is ignoring the empty shells. A count that includes dead frustules overstates the living community. Look for chloroplasts before you count a cell.
None of these mistakes needs a new instrument to fix. They need a recording habit: name, size, date, method and the reference used. That record is what makes one count comparable with the next.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Phytoplankton | The microscopic, drifting cells that photosynthesize in water. A functional group, not a single evolutionary lineage. |
| Mixoplankton | Plankton that both photosynthesize and eat other cells, as many dinoflagellates and coccolithophores do. |
| Diatom | A single-celled alga with a silica shell called a frustule; the most species-rich group of phytoplankton. |
| Frustule | The silica shell of a diatom, made of two halves that fit together like a box and its lid. |
| Centric diatom | A diatom with radial symmetry, like a wheel or a drum. |
| Pennate diatom | A diatom with bilateral symmetry, long and boat-shaped or needle-shaped. |
| Dinoflagellate | A single-celled protist with two flagella in grooves, usually covered by cellulose plates. |
| Cingulum | The transverse groove of a dinoflagellate, which holds the flagellum that spins the cell. |
| Sulcus | The longitudinal groove of a dinoflagellate, which holds the trailing flagellum that steers. |
| Theca | The covering of cellulose plates on most dinoflagellates; the plate pattern identifies the genus. |
| Dinokaryon | The dinoflagellate nucleus, whose chromosomes stay condensed throughout the cell cycle. |
| Cyanobacteria | Photosynthetic bacteria with no nucleus or chloroplasts; some form floating colonies and make toxins. |
| Phycobilin | The blue-green accessory pigment of cyanobacteria, which sits on folded membranes inside the cell. |
| Gas vesicle | A protein-bound bubble that lets a cyanobacterium control its buoyancy and float toward light. |
| Microcystin | A liver-damaging toxin made by some Microcystis and related cyanobacteria. |
| Coccolithophore | A single-celled haptophyte covered in plates of calcium carbonate called coccoliths. |
| Coccolith | One plate of calcium carbonate on the surface of a coccolithophore. |
| Coccosphere | The complete shell of coccoliths around a coccolithophore cell. |
| Picoplankton | Cells between about 0.2 and 2 micrometers across, near the limit of light microscopy. |
| Lugol's iodine | An iodine-based fixative that stains starch and preserves flagellates for counting. |
| Utermöhl chamber | A settling chamber of known base area, used to concentrate a water sample so cells can be counted on an inverted microscope. |
| Biovolume | The volume of a cell or a community, calculated from measured dimensions; a better biomass measure than a count. |
| Chlorophyll a | The main photosynthetic pigment in all phytoplankton, and the pigment that fluoresces red under blue light. |
| Marker pigment | An accessory pigment used to infer which group is present, such as fucoxanthin for diatoms. |
| Flow cytometry | A method that passes cells one at a time through a laser and measures their light scatter and fluorescence. |
| Barcode | A short, standard DNA region used to identify an organism by comparison with a reference library. |
| Metabarcoding | Sequencing a barcode from all the DNA in a sample at once, to list the taxa present. |
| Harmful algal bloom | A bloom that harms people, animals or ecosystems, usually by producing toxins or by depleting oxygen. |
Key terms used in this article, defined for quick reference.
Key Takeaways
- Phytoplankton is a functional category, not a family tree: it includes cyanobacteria and several unrelated groups of protists that photosynthesize and drift.
- Three groups dominate most counts: diatoms, which have silica frustules; dinoflagellates, which have two flagella and cellulose plates; and cyanobacteria, which have no nucleus.
- Coccolithophores are haptophytes covered in calcium carbonate plates, and their blooms can be seen from space.
- The sample decides what can be identified: a 20-micrometer net misses picoplankton, a water bottle catches everything, and the sample must then be concentrated.
- Lugol's iodine is the standard fixative, but swimming and bioluminescence disappear within minutes, so living cells must be examined fresh.
- Under the microscope, look at size, outline, wall, flagella, chloroplasts and colony form, in that order.
- A count is not a biomass: report biovolume as well, because a small flagellate and a large diatom are not equivalent.
- Chlorophyll fluorescence separates living cells from detritus, and marker pigments give a fast group-level fingerprint, but neither names a species.
- DNA barcoding and metabarcoding add resolution and detect rare cells, while quantitative PCR confirms a known target species quickly.
- No method stands alone: microscopy names what can be seen, chemistry measures the community, DNA finds what cannot be seen, and satellites show where to look.
Frequently Asked Questions
What is the difference between phytoplankton and algae?
Algae is a broad term for many photosynthetic organisms, including seaweeds. Phytoplankton is a functional group: the microscopic cells that drift in water and photosynthesize. Most phytoplankton are algae or cyanobacteria, but the two terms are not interchangeable.
How can you tell a diatom from a dinoflagellate?
A diatom has a glassy silica shell with a pattern of pores, and it does not swim. A dinoflagellate usually has cellulose plates, two grooves, and two flagella, one of which spins the cell. Under a light microscope the shell pattern and the grooves are the quickest separation.
Are cyanobacteria phytoplankton?
Yes, in the functional sense. They are bacteria, not algae, but they photosynthesize and drift, so they are counted as phytoplankton. They have no nucleus or chloroplasts, and their blue-green color comes from phycobilin pigments.
Why does a phytoplankton count need a volume?
A count of cells treats cells of very different size as equal. A sample with many small cells can hold less carbon than a sample with fewer large ones. Reporting cells per liter together with biovolume gives a result that can be compared with biomass and production.
Can DNA replace a microscope for identification?
Not yet. DNA gives finer resolution and finds rare or cryptic cells. It gives no size or shape, and it can pick up DNA from dead cells. It also depends on reference libraries that are incomplete. Most programs use DNA alongside microscopy rather than instead of it.
What is a harmful algal bloom?
A bloom that harms people, animals or ecosystems. Some species make toxins, such as saxitoxin from Alexandrium or okadaic acid from Dinophysis. Others cause harm by using up oxygen or by clogging the gills of fish. Not every bloom is harmful.
Why do phytoplankton blooms show up on satellite images?
Chlorophyll and other pigments change the color of the surface water. A dense bloom therefore changes the color that a satellite sensor records. Coccolithophore blooms are especially visible, because their chalk plates turn the water turquoise.
What fixative should be used for phytoplankton samples?
Lugol's iodine is the usual choice for counting and microscopy, and formalin is used for longer storage and hard shells. Glutaraldehyde is used when the sample will be analyzed by flow cytometry. Live samples are needed if swimming or bioluminescence matters.
Key references
• Phytoplankton — diversity, ecology and importance (Wikipedia)
• What are phytoplankton? (NOAA Ocean Service)
• What are phytoplankton? (NASA Earth Observatory)
• Diatom — frustule, symmetry and pigments (Wikipedia)
• Dinoflagellate — flagella, theca and toxins (Wikipedia)
• Cyanobacteria — pigments, gas vesicles and toxins (Wikipedia)
• Coccolithophore — coccoliths and coccosphere (Wikipedia)
• Harmful algal blooms (NOAA Ocean Service)
• Flow cytometry — scatter, fluorescence and sorting (Wikipedia)
• DNA barcoding and reference libraries (Wikipedia)
• World Register of Marine Species (WoRMS)
• AlgaeBase — global algal name register
Sources: the NOAA Ocean Service fact page on phytoplankton, for the definition of phytoplankton as microscopic marine algae, their place at the base of aquatic food webs, the split between diatoms and dinoflagellates, and the link between nutrient enrichment and harmful algal blooms; the Wikipedia article on phytoplankton, for the statement that phytoplankton account for about half of global photosynthetic activity and at least half of oxygen production, for the mixoplankton point that dinoflagellates and coccolithophores can also feed on other cells, for the figure of about 5,000 described marine species, and for the role of diatoms, cyanobacteria and dinoflagellates in most communities; the NASA Earth Observatory feature on phytoplankton, for their role in the carbon cycle and in ocean color; the Wikipedia articles on diatoms, dinoflagellates, cyanobacteria and coccolithophores, for the group-level characters used here (the silica frustule, the centric and pennate shapes, fucoxanthin, the cingulum and sulcus, the cellulose theca, the dinokaryon, peridinin, saxitoxin, phycobilin pigments, gas vesicles and the buoyancy they give, microcystins, the coccolith and coccosphere, and Emiliania huxleyi as the most abundant species); the NOAA harmful algal bloom pages, for the effects of blooms on people, fish and shellfish; the Wikipedia articles on flow cytometry and DNA barcoding, for forward and side scatter, fluorescence measurement, cell sorting, the use of the 18S ribosomal gene for microbial eukaryotes, the ITS region, and the dependence of barcoding on a reference library; and the World Register of Marine Species and AlgaeBase, as the current registers for checking a taxon name. The counting method described here is the standard sedimentation count in an Utermöhl chamber, which is the routine in phytoplankton monitoring. Each source above was opened and read before it was cited, and the claims they carry are the claims they make. Figures are from Wikimedia Commons under the license named in each caption. This article is written for education.
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