What Are Phytoplankton?

Chelsea Technologies’ LabSTAF is a new generation fluorometer that uses single turnover active fluorometry to measure very low levels of photosynthesis, ideal for oligotrophic zones - the world’s leading scientific instrument for studying phytoplankton primary productivity and phytoplankton photosynthesis. A benchtop instrument to measure primary productivity using fluorescence, giving data for over 50 useful parameters within 15 minutes

Phytoplankton are microscopic, plant-like organisms that live throughout Earth’s oceans, lakes, and rivers.

The name Phytoplankton is not a species but a description, and comes from the Greek words φυτόν (phyton), meaning ‘plant’, and πλαγκτός (planktos), meaning ‘wanderer’ or ‘drifter’, which alludes to their nature, insofar that the majority do not swim at all or are very weak swimmers, and simply drift or move with ocean currents.

Phytoplankton types

What are phytoplankton?
Phytoplankton dynamics
Phytoplankton and climate change
Studying phytoplankton
Phytoplankton are not one single plant species but a descriptor for numerous distinct types, with estimates up to one hundred thousand distinct species, the most common of these being diatoms and dinoflagellates. Diatoms are single-celled algae which often join in long chains, the only organism on the planet with cell walls composed of transparent, opaline silica. Dinoflagellates (Greek δῖνος dinos “whirling” and Latin flagellum “whip, scourge”) are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae.


diatom phytoplanktonDiatom cell walls are adorned with intricate and striking patterns of silica. Diatoms are the most diverse protists (a single-celled organism classified as Protista, such as a protozoan or simple alga) on earth: they are one of the Heterokont algae. Estimates of the number of diatom species vary considerably and science discovers new species every year.

A single diatom cell can divide and form two new cells – cells may divide as quickly as once a day up to once every several weeks. The silica cell wall, a defensive trait, is also their biological constraint, because with each cell division diatom cells become progressively smaller. As a result, the older the diatom cell, the smaller it is. Diatoms are usually microscopic – cells range in size from 2 µm to 500 µm (= 0.5 mm) with the biggest diatoms being about the width of a human hair.

The cell walls of diatoms are composed of inorganic silica and do not decompose; when diatom cells die, their silica cells sink to the bottom. Because the cell walls are inorganic, they can be preserved over long periods of time – up to tens of millions of years. This unique feature enables paleolimnologists and geologists to use fossil diatoms to understand past conditions on earth.


Dinoflagellate phytoplanktonDinoflagellates are mostly marine plankton, but they also are common in freshwater habitats, and their populations vary with sea surface temperature, salinity, and depth. Many dinoflagellates are photosynthetic, but a large fraction of these are also mixotrophic, combining photosynthesis with ingestion of prey (phagotrophy, the process of ingesting relatively large particles of food carried out via intracellular digestion, and myzocytosis, also called “cellular vampirism” as the predatory cell pierces the cell wall and/or cell membrane of the prey cell with a feeding tube, the conoid, and sucks out the cellular content and digests it).

In terms of number of species, dinoflagellates are one of the largest groups of marine eukaryotes, although substantially smaller than diatoms. Dinoflagellates come in many shapes and sizes – some have shells, and some do not, some use photosynthesis for all their energy and some wrap themselves around food and absorb it, and some can make light using bioluminescence and some cannot.

Dinoflagellates possess two flagella, microscopic appendages that enables many protozoa, bacteria, spermatozoa, etc. to swim. About 1,555 species of free-living marine dinoflagellates are currently described, although the latest estimates suggest a total of 2,294 living dinoflagellate species, which includes marine, freshwater, and parasitic dinoflagellates. A rapid accumulation of certain dinoflagellates can result in a visible coloration of the water, colloquially known as red tide (a harmful algal bloom), which can cause shellfish poisoning if humans eat contaminated shellfish.

Phytoplankton perform photosynthesis

The vast majority of phytoplankton obtain their energy through photosynthesis, in the same manner that trees and plants do on land. This means phytoplankton require light from the sun, so they live in the well-lit surface layers (euphotic zone) of oceans and lakes. Phytoplankton are estimated to account for about half of all photosynthetic activity on Earth.

Phytoplankton photosynthetic physiology can be investigated through single turnover active fluorometry (STAF) approaches, which carries the unique potential to autonomously collect data at high spatial and temporal resolutions. Chelsea Technologies’ LabSTAF is a new generation fluorometer that uses single turnover active fluorometry to measure very low levels of photosynthesis, ideal for oligotrophic zonesDuring photosynthesis (daytime), phytoplankton take in carbon dioxide and release oxygen, and at night, in common with all other plant life, they respire oxygen. If solar radiation is too high, phytoplankton may fall victim to photodegradation.

Phytoplankton species feature a large variety of photosynthetic pigments which enables them to absorb different wavelengths of the variable underwater light. This implies different species can use the wavelength of light different efficiently and the light is not a single ecological resource but a multitude of resources depending on its spectral composition. it has been discovered that changes in the spectrum of light alone can alter natural phytoplankton communities even if the same intensity is available.

For growth, phytoplankton cells additionally depend on nutrients, which enter the ocean by rivers, continental weathering, and glacial ice meltwater on the poles. Phytoplankton release dissolved organic carbon (DOC) into the ocean. Since phytoplankton are the basis of marine food webs, they serve as prey for zooplankton, fish larvae and other heterotrophic organisms.

They can also be degraded by bacteria or by viral lysis (breakdown of the membrane of a cell). Although some phytoplankton cells, such as dinoflagellates, can migrate vertically, they are still incapable of actively moving against currents, so they slowly sink and fertilize the seafloor with dead cells and detritus.

Different phytoplankton require different conditions – larger phytoplankton require more nutrients and have a greater need for the vertical mixing of the water column that restocks depleted nutrients. As the ocean has warmed since the 1950s, it has become increasingly stratified, which cuts off such nutrient recycling. In comparison with terrestrial plants, phytoplankton are spread over a larger surface area, are exposed to less seasonal variation and have markedly faster turnover rates than trees (days versus decades).

As a result, phytoplankton respond rapidly on a global scale to climate variations. Continued warming due to the build-up of Carbon Dioxide (CO₂) is predicted to reduce the amounts of larger phytoplankton such as Diatoms, compared to smaller types, such as Cyanobacteria. Shifts in the relative abundance of larger versus smaller species of phytoplankton have been observed already in places around the world, but whether it will change overall productivity remains uncertain.

Phytoplankton primary productivity

Primary productivity is the synthesis of organic compounds from atmospheric or aqueous CO₂. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through chemosynthesis, which uses the oxidation or reduction of inorganic chemical compounds as its source of energy. Almost all life on Earth relies directly or indirectly on primary productivity. The organisms responsible for primary production are known as primary producers and form the base of the food chain. In terrestrial ecoregions, these are mainly plants, while in aquatic ecoregions algae predominate in this role. Phytoplankton in the ocean contributes to roughly half of the planetary net primary production – through sinking of the fixed organic matter, primary production acts as a biological pump that removes carbon from the surface ocean, thereby playing a global role in climate change. Phytoplankton’s importance to global climate studies and science cannot be understated.

A common feature to all phytoplankton is that they contain chlorophyll-a; but there are other accessory pigments such as chlorophyll-b and chlorophyll-c, as well as photosynthetic carotenoids. These pigments absorb solar energy and convert carbon dioxide and water into high-energy organic carbon compounds that fuel growth by synthesizing vital required components such as amino acids, lipids, protein, polysaccharides, pigments and nucleic acids, but they are also detectable using UV fluorescence which is why Chelsea Technologies’ STAF range is the tool of choice for scientists examining the role of phytoplankton in global events such as climate change.

Phytoplankton productivity monitoring options from Chelsea Technologies


labstaf phytoplankton
LabSTAF is the world’s leading portable instrumentation option for Phytoplankton primary productivity.

  • Benchtop instrument to measure primary productivity using fluorescence, giving data for over 50 useful parameters within 15 minutes
  • LabSTAF includes a peristaltic pump, solenoid unit and flow-through stirrer unit to provide for mixing, sample exchange and a periodic cleaning cycle.
  • Data from LabSTAF is interpreted and analysed internally in the included Surface Go


Detecting bioluminescent dinoflaggelate phytoplankton


GlowTracka is a detector primarily used for the assessment of bioluminescent algae.

GlowTracka’s precision flow meter stimulates bioluminescent organisms – principally dinoflagellates, and measures the light flashes as the organisms pass the detector, giving photon level sensitivity.

Applications include:

  • Biomass studies
  • Bioluminescent species abundance
  • Harmful algal bloom detection
  • Toxic algae bloom tracking

Detecting algae

TriLux Algae

trilux algaeWith three different parameter combinations and depth ratings to 2000m, TriLux Algae is suitable for a wide variety of water environments:

  • Chlorophyll a, Phycocyanin, Turbidity for freshwater deployments
  • Chlorophyll a, Phycoerythrin, Phycocyanin for coastal deployments
  • Chlorophyll a, Phycoerythrin, Turbidity for marine deployments

TriLux is compact and easy to integrate in wider systems and platforms including CTDs, data buoys, monitoring stations, AUVs and gliders.



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