What is Phytoplankton?

Phytoplankton are microscopic marine algae.

Foundational elements of the aquatic food chain, marine phytoplankton are a category of plankton, microscopic organisms which are unable to swim against the current and float around in the upper region of water bodies.

Inhabiting either salt or freshwater, plankton are and are defined more by their ecological niche rather than any shared characteristics. Plankton might include bacteria, archaea, algae, protozoa, and even drifting creatures like jellyfish. Some creatures (known as meroplankton) are categorised as plankton for the early part of their lifecycle (larval forms of sea urchins, sea stars etc.) but then, as adults, take up residence on the seafloor.

Phytoplankton also produces about half of the world’s oxygen through photosynthesis, making them an important player in the global carbon cycle.

In this article, we will explore what phytoplankton are, their characteristics, classification, distribution, and, most importantly, their significance in the marine ecosystem and beyond. Additionally, we will look at the threats facing phytoplankton, the importance of conservation efforts and further research to protect these vital organisms.

What are Marine Phytoplankton

What are the Characteristics of Phytoplankton?

Here are some of their main characteristics:

  1. Size and Shape: Phytoplankton range in size from less than one micrometre to several millimetres, with most species being less than 0.1 millimetres. They come in various shapes, including spherical, cylindrical, and disc-shaped, and some have appendages like flagella that allow them to move.
  2. Diversity: Phytoplankton are incredibly diverse, with over 5,000 known species. They come in various colours, including green, brown, and red; some are even bioluminescent.
  3. Photosynthesis: Like plants, phytoplankton use photosynthesis to convert sunlight, carbon dioxide, and nutrients into energy. This process produces oxygen, which is released into the atmosphere, making phytoplankton absolutely critical for the survival of life on Earth.
  4. Role in the Food Web: Phytoplankton are the base of the entire marine food chain, providing a vital source of energy for other organisms. They are consumed by a variety of animals, including zooplankton, fish, and whales, and are essential for the survival of many marine species.

Classification of Phytoplankton

  1. Diatoms: Diatoms are single-celled organisms with a unique cell wall made of silica. They are one of the most abundant types of phytoplankton and are an important food source for many marine animals.
  2. Dinoflagellates: Dinoflagellates are single-celled organisms with two flagella that allow them to move through the water. They are a diverse group of phytoplankton that can be either autotrophic (able to produce their own food through photosynthesis) or heterotrophic (require organic matter for food).
  3. Coccolithophores: Coccolithophores are single-celled organisms with a unique calcium carbonate shell made of small plates called coccoliths. They are an important ocean carbon source and play a role in regulating the Earth’s climate.
  4. Haptophytes: Haptophytes are single-celled organisms with a variety of shapes and sizes. They are an important source of food for zooplankton and are also known for producing a group of pigments called carotenoids, which give them their characteristic colour.

Cyanobacteria, also known as blue-green algae, are sometimes considered to be phytoplankton, but their classification is a matter of debate among scientists.

Distribution of Phytoplankton

Phytoplankton are found in aquatic environments all around the world, from the surface of the open ocean to freshwater lakes and rivers. The distribution of phytoplankton is influenced by a variety of factors, including:

  1. Nutrient Availability: Phytoplankton require nutrients like nitrogen, phosphorus, and iron to grow and reproduce. Areas with high nutrient availability, such as upwelling zones and river estuaries, often have high phytoplankton concentrations.
  2. Sunlight: Phytoplankton require sunlight to carry out photosynthesis, so they are most abundant in surface waters where sunlight can penetrate.
  3. Water Temperature: Different types of phytoplankton have different temperature preferences, so their distribution can be influenced by water temperature.
  4. Ocean Currents: Ocean currents can transport phytoplankton over large distances and affect their distribution patterns.
  5. Salinity: Phytoplankton have different salinity tolerances, so changes can influence their distribution in water salinity.

Phytoplankton blooms occur when there is a sudden increase in the phytoplankton population, can be seen in many areas of the ocean and can significantly impact the marine ecosystem. Some phytoplankton species, like the harmful algal bloom species, can produce toxins that can harm marine life and humans.

The Vital Importance of Phytoplankton

Phytoplankton are essential to the health of the marine ecosystem and the planet as a whole. Here are some of the key reasons why:

  1. The base of the Marine Food Chain: Phytoplankton are the primary producers in the marine food chain, providing a vital source of food for zooplankton, fish, and other marine animals. Without phytoplankton, many marine species would not be able to survive.
  2. Oxygen Production: Phytoplankton produce about half of the world’s oxygen through photosynthesis. This makes them critical for the survival of life on Earth.
  3. Carbon Cycling: Phytoplankton play a crucial role in the global carbon cycle, absorbing carbon dioxide from the atmosphere through photosynthesis and storing it in their bodies. When they die, their bodies sink to the ocean floor, where the carbon can be stored for thousands of years.
  4. Biotechnology Applications: Phytoplankton have unique properties that make them attractive for use in biotechnology, including their ability to produce a wide range of bioactive compounds with potential therapeutic applications.

Overall, the importance of phytoplankton cannot be overstated. They play a critical role in the marine ecosystem and are essential for the survival of many marine species and the health of the planet as a whole.

What’s the Difference between Phytoplankton and Zooplankton?

Phtyplankton might be classed as the plants of the sea, while zooplankton the animals. While phytoplankton has the capacity to manufacture it’s own food from sunlight and carbon dioxide through photsynthesis, zooplankton are tiny animals which must feed to survive, usually on phytoplankton. Both are key members of the marine ecosystem, and provide primary food sources for larger species like whales. The crucial thing from our perspective is that phytoplankton:

  • Are single celled
  • Can photosynthesize
  • Are entirely vegan/vegetarian

Phytoplankton and the Carbon Cycle

Although scientists are still learning about marine carbon cycling, it is clear that phytoplankton play an integral role in the sequestering of carbon from our atmosphere, likely significantly higher than that of terrestrial forests. In a process called ‘the biological pump’, phytoplankton are responsible for approximately half of the carbon fixation on earth. From an ecological perspective, however, we can note than over 99.0% of all the carbon dioxide incorporated into living things over geologic time is buried in marine sediments, suggesting that the role of phytoplankton is absolutely primary to all of life on this planet.

Risks to Phytoplankton

Here are some of the main threats to phytoplankton:

  1. Climate Change and Ocean Acidification: Rising global temperatures and increased carbon dioxide emissions are causing changes in ocean temperature, chemistry, and circulation that can have negative impacts on phytoplankton growth and survival.
  2. Pollution: Chemical pollutants such as oil spills, sewage, and agricultural runoff can harm phytoplankton and disrupt their growth and reproduction. Plastic pollution is also a growing problem, with microplastics accumulating in the ocean and being ingested by phytoplankton.
  3. Overfishing: Overfishing can disrupt the marine food chain and remove important predators of phytoplankton-eating zooplankton, which can lead to increases in phytoplankton populations and harmful algal blooms.
  4. Habitat Destruction: Coastal development, such as dredging and construction, can destroy important habitats for phytoplankton, while physical factors like water circulation changes or sedimentation can limit the growth and reproduction of phytoplankton.
  5. Harmful Algal Blooms: Some species of phytoplankton can form harmful algal blooms, producing toxins that can harm marine life and humans.

Climate Change

Many scientists now question whether phytoplankton offers our planet its biggest opportunity to reduce carbon in the atmosphere and combat climate change. As weather patterns change, scientists are also becoming aware that changes in light, and nutrients in the pelagic region of the oceans could have a major impact on plankton. The biggest threat would be that oxygen levels fall below the level required for phytoplankton to live which would rapidly kill of all oceanic life as we know it.

What eats Phytoplankton?

The most common predators of phytoplankton would be things like krill and larger plankton like jellyfish. Big mammals like whales are massive plankton eaters. Blue whales can eat  2,000-9,000 pounds (900-4100 kg) daily during the summer feeding season.

As this site demonstrates, humans are also consuming marine phytoplankton for its health-giving properties.

Phytoplankton for Human Consumption

In terms of its consumption as a superfood, phytoplankton is relatively new to the human food chain. The last twenty years have seen a huge amount of research focusing on it, however, firstly because of its extraordinary nutrient profile, which attracted it to the cosmetic industry, and secondly, its ability as a vegetarian oil source which has piqued the interest of biofuel pioneers. As our oceans become more and more polluted, the power of this humble organism to sustain the human species may prove to be most suitable, however, as the whales have always used it: simple nutrition.

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Cara Hayes is an experienced natural health practitioner with a Masters in Clinical Nutrition & Dietetics from the University of Sydney. She has been writing for Plankton for Health since 2019 and contributes widely to many well-known health publications.