How Do Phytoplankton Reproduce?

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Phytoplankton are microscopic creatures that multiply prolifically through asexual and sexual means. Reproductive rates of phytoplankton directly affect and reflect the balance of the ecosystem.

According to National Geographic, marine plants like phytoplankton, algae and kelp produce 70 percent of atmospheric oxygen, which is even more than the rainforest. However, under certain environmental conditions, populations of phytoplankton can explode, creating malodorous, toxic blooms.

Types of Plankton

The main categories of plankton are phytoplankton and zooplankton. Plankton can be eukaryotic or prokaryotic. Plant-like phytoplankton include algal plankton and microalgae.

Phytoplankton can be unicellular plants, protists (algae) or bacteria:

  • Dinoflagellates: These are distinguished by whiplike tails and a complex shell. About half of all dinoflagellates are non-photosynthetic. Some species are bioluminescent and glow at night.
  • Diatoms: These are immobile, photosynthetic algae that float on the surface of fresh and marine water. Diatoms are also present in moist soil. The unique coat of diatoms is comprised of silicon that is used commercially.
  • Cyanobacteria: These are primitive bacteria that can give rise to toxic blooms.
  • Cocolithophores: These are plankton covered in scales similar to limestone. They are an important source of calcite.

What Are Zooplankton?

Zooplankton, also called animal plankton, include:

  • protozoa
  • larvae
  • copepods
  • flatworms

Zooplankton are among the most ubiquitous marine organisms and include well-known organisms like jellyfish. Zooplankton are consumers in the food chain.

Definition of Phytoplankton in Biology

Phytoplankton produce food and release oxygen as a byproduct, much the same way as terrestrial plants support the Earth.

Phytoplankton get their name from the Greek word planktos, which means wanderer or drifter – an apt description of how phytoplankton float through life. International researchers may also refer to these organisms as “fitoplankton” or “fitoplancton” in other languages.

Importance of Phytoplankton

Phytoplankton rank among the most important organisms on Earth. Besides providing food for the rest of the food web, phytoplankton oxygenate the water and air.

Phytoplankton mitigate the effects of global warming by absorbing 33 percent of carbon dioxide from natural sources and fossil fuels, according to the Kudela Lab of at the University of California, Santa Cruz. Upon death, phytoplankton and other organic waste may sink to the ocean floor and one day turn to fossil fuel – gas, oil and coal.

Environmental Threats to Phytoplankton

Nitrogen-based fertilizer runoff from fields, animal waste from feedlots and untreated sewage enter the waterways and disrupt the ecological balance. Large-scale dead zones in areas like the Gulf of Mexico result from warmer global temperatures and phytoplankton overgrowth that suffocates marine life. Bacterial decomposers use up additional oxygen when consuming decaying matter from the bloom.

Scientists monitor algal population fluctuations to protect clean water – an increasingly scarce natural resource. Samples are taken in the field using plankton nets for collection of specimens. Mesh nets typically work well for capturing phytoplankton, but tiny nanoplankton must be filtered out of a water sample.

The amount and type of plankton indicate overall water conditions and show plankton reproduction rates.

Asexual Phytoplankton Reproduction

Efficient reproductive strategies are a hallmark of phytoplankton. When growing conditions are right, phytoplankton multiply quickly through various means of asexual reproduction.

The simplicity of plankton enables them to reproduce with ease:

  • Fast-growing dinoflagellates typically divide through binary fission. A parent cell divides into two identical cells that will divide again and again. Filaments can form if the cells do not separate completely during cell division. 
  • Protists can reproduce asexually through multiple fission. Cells prepare to divide, replicate their nucleus and then split into multiple cells that are identical to the original cell unless mutations occurred.
  • The rectangular cells of spirogyra (algal phytoplankton) attach end to end, forming very long chains called filaments. When a filament divides, each section floating on the water will grow into a new filament through simple mitosis. This type of reproduction is called fragmentation.
  • Zooplankton like hydra can reproduce through budding. Like yeast, a hydra can grow a bud that will mature and break off, becoming a clone of the parent. 

Green algae and bacteria can produce spores that continue dividing inside the parent cell. Mature endospores are released to form identical offspring.

Sexual Phytoplankton Reproduction

Sexual reproduction involves recombination of genetic material to produce offspring with a unique genome. Biodiversity within a population helps a species adapt to adverse conditions such as heat or drought.

Some phytoplankton can sexually reproduce:

  • Diatoms produce and release diploid male and female gametes – spermatogonia and oogonia – that divide by meiosis to become haploid sperm or an egg. An egg fertilized by sperm develops into a zygote called an auxospore that can enter dormancy. The cell will grow under the right conditions and then releases full-size diatoms. 
  • Hermaphroditic monoecious colonies of volvox (green algae) species produce both sperm packets and eggs. Dioecious colonies produce either sperm or eggs. In female volvox colonies, individual cells grow to become oogametes that enter a resting diploid zygote stage after an egg and sperm fuse (syngamy). 

Where Do Phytoplankton Live?

Phytoplankton are found near the shore, in standing open water, on ice caps and near the surface of lakes where essential nutrients and sunlight are easily accessible for cell growth and division. Phytoplankton living in the ocean are normally in the euphotic zone of the water column that is penetrable by sunlight.

The euphotic zone is no deeper than 900 feet; average ocean depth is around 13,000 feet, as estimated by the Woods Hole Oceanographic Institution.

Phytoplankton Life Cycle

The typical life cycle of phytoplankton includes growth, reproduction and death. The life cycle can also include a period of dormancy that happens regularly or only when conditions are not conducive to growth.

For example, chrysophytes can form cysts or spores that remain dormant for months or decades. Some diatoms and dinoflagellates form cysts from winter to spring.

Phytoplankton life cycles vary by species. For instance, marine flagellates (Phaeocystis pouchetii) produce tiny motile cells that keep multiplying until nutrient levels decline. Next, they form colonies surrounded by a sticky mucous coat containing nutrients that allow for continued reproduction.

If nutrients drop off altogether, the membrane disintegrates and washes up on shore as smelly, gooey white foam.

Helpful Phytoplankton Reproduction

Phytoplankton growth fluctuates with the seasons. Reproduction explodes in polar regions each spring when receding ice deposits rich nutrients on the surface of the water. Cool water is ideal for phytoplankton reproduction. In late summer, increased sunlight excites the pigments in floating phytoplankton, resulting in another growth spurt.

Phytoplankton are consumed by fish and krill, which subsequently provide a hearty meal for Adélie penguins, seabirds and seals. Penguins have adapted their breeding cycle to coincide with peak times of phytoplankton reproduction.

According to the National Snow and Ice Data Center, some of the largest fisheries in the world are located in the Bering Sea where plankton bloom profusely and sustain fish populations.

Harmful Phytoplankton Reproduction

An abundance of phytoplankton attracts birds, insects, fish and animals, and enhances biodiversity in an aquatic biome. However, excessive reproduction of nontoxic phytoplankton can still be harmful because of resulting oxygen depletion and clogging of fish gills.

Some species of cyanobacteria produce toxins such as microcystin. Cyanobacteria are commonly called “blue-green algae" and turn the water green.

Toxin producing harmful algal blooms (HAB) have occurred in every coastal state, according to the National Ocean Service. HABs can sicken or kill humans in addition to marine life. HABs in places like the Florida Gulf Coast are commonly called “red tides” because the bloom turns the water red.

Drinking water can be contaminated and beaches closed due to noxious odors and risk of infection. HABs occur seasonally in late summer when temperatures and nitrogen pollution spur phytoplankton growth.

What Do Phytoplankton Eat?

Lakes and oceans rich in nitrogen, iron and phosphate provide a smorgasbord for countless species of phytoplankton. Blooms often follow in the wake of hurricanes because nutrients get churned up from the bottom. Growth rate slows when nutrients are in short supply.

Other factors influencing reproduction include temperature, depth, light variability and saltwater concentration (salinity). Plankton is not found in many parts of the ocean due to a scarcity of iron in those regions.

How Do Phytoplankton Get Food?

Depending on the species, phytoplankton meet all their energy needs through photosynthesis, or they can supplement their diet by consuming other living or decaying organisms. The two major types of phytoplankton employ different strategies for acquiring food.

For instance, dinoflagellates hunt and move through the water by swishing their tails; however, they are weak swimmers and cannot go against the current. Diatoms do not use flagella (tails) and absorb nutrients needed for metabolism and reproduction as they ride the currents.

What Eats Phytoplankton?

Phytoplankton serve as the food bank of the aquatic world because of their plant-like ability to absorb sunlight and produce food energy through photosynthesis. A plethora of sea creatures, from snails to whales, owe their existence to a steady diet of phytoplankton. Direct consumers of phytoplankton include zooplankton, anemones, shrimp and clams.

In turn, smaller plants and animals are consumed by omnivores, which are then eaten by tertiary consumers or apex predators. Foodstuffs in the human diet can be traced back to a primary producer like phytoplankton.

Plankton Reproduction and Clouds

According to NASA satellite imagery, brighter clouds form over certain places like the Southern Ocean during periods of high phytoplankton reproduction. Quickly multiplying phytoplankton such as coccolithophores release gases and organic substances into the air, which seeds clouds.

Clouds reflect more sunlight and appear brighter when plankton blooms occur because reflection depends on the amount of suspended water in the cloud and the particle size of cloud droplets.

Phytoplankton Reproduction and Biofuels

Researchers have found that phytoplankton can use photosynthesis to convert carbon dioxide into biomass and oils for biofuel production. Algal farms could benefit the planet because phytoplankton absorb (sink) more carbon than they release back into the environment.

Another benefit is rapid crop production. According to the Environmental and Energy Study Institute, microalgae double in mass every day and grow up to 100 times faster than plants on land.

Further, many algal species grow in salt water, which is more readily available than fresh water. Algal farms could be located in areas where other crops cannot grow. Algal biofuel could reduce dependence on domestic and imported fossil fuel. Algae has already been used in the manufacture of skincare products, pharmaceuticals and cosmetics.


About the Author

Dr. Mary Dowd studied biology in college where she worked as a lab assistant and tutored grateful students who didn't share her love of science. Her work history includes working as a naturalist in Minnesota and Wisconsin and presenting interactive science programs to groups of all ages. She enjoys writing online articles sharing information about science and education. Currently, Dr. Dowd is a dean of students at a mid-sized university.