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Marine phytoplankton

marine phytoplankton

Phytoplancton Marin

Marine phytoplankton consist of the microscopic algae that live in suspension in the surface waters of oceans. Most of them are single-celled organisms, but some form chains of several or many cells. Like terrestrial plants, phytoplankton organisms have the green pigment chlorophyll a, which is essential to photosynthesis. Phytoplankton can only be observed by means of a microscope; most of them are just a few micrometers (thousands of millimeters) in size.

Question 1: What are phytoplankton organisms?

Phytoplankton organisms are not all identical. They include numerous groups that are characterized by a variety of shapes and sizes, and play different roles in marine ecosystems. For example:

Diatoms

These organisms are protected by a very thin, transparent glass cell wall. Thousands of species can be identified based on the shape and ornamentation of their glass cell wall. Diatoms need much nutrients to grow. When the conditions are favorable for growth, they respond quickly! One may then observe what is called a diatom bloom.

Dinoflagellates

Dinoflagellé Ceratium massiliense var protuberans (Photo : Sophie Marro)

These organisms possess two flagella that enable them to move like animals. Several species are characterized by both plant-like traits (they carry out photosynthesis) and animal-like traits (they also feed on organic matter). Dinoflagellates often possess collar-like structures ("cingular lists"), wing-like structures ("sulcal lists"), or horns. Researchers use these characteristics to identify species. Thanks to their flagella, dinoflagellates are capable of vertical migrations to make the most of their environment and utilize both sunlight (near surface) and nutrients (at depth).

Coccolithophores

These organisms possess flagella and, more importantly, they are covered with microscopic plates made of limestone (calcite). When coccolithophores die, they shed their small calcite plates, which sink into the deep ocean when incorporated into heavier particles. They accumulate on the ocean floor for millions of years and form limestone, which is actually chalk!

+ Experiment ...

Experiment with a piece of chalk ...

Take a piece of chalk and put it in an acidic solution (lemon juice or vinegar): you should observe effervescence when chalk dissociates into carbon dioxide (CO2) and calcium (Ca). It is limestone!

Cyanobacteria

These organisms are also named blue-green algae. Because they are so small, some species have been discovered lately, in the 1980s. Yet these tiny cyanobacteria might be the most abundant organisms in the ocean! Cyanobacteria are usually found in relatively warm and nutrient-poor waters.

Question 2: What is the role of phytoplankton in oceans?

photo-autotrophic organisms

As for terrestrial plants, phytoplankton organisms synthesize their own organic matter by utilizing sunlight, mineral substances (nutrients), carbon dioxide (CO2) dissolved in water, and water itself. This process is known as photosynthesis, and phytoplankton organisms are said to be "photo-autotrophic". On the opposite, animals are "heterotrophic". They use the organic matter of other organisms to make their own organic matter.

On land, plant growth is often water-limited. In the oceans, phytoplankton growth is more frequently light-limited. For this reason phytoplankton organisms develop preferentially in the surface layer of the oceans, where light is available. Nutrients are abundant in deep waters, from where they must be brought up to the surface by different physical mechanisms before being consumed by phytoplankton. In brief, phytoplankton find optimal growth conditions in surface waters when these are sufficiently sunlit and nutrient-rich.

the basis of the marine food web

Phytoplankton organisms form the basis of the food web (or trophic web) in the ocean, like plants in meadows and forests on land. The (photo)synthesis of organic matter by phytoplankton is called "primary production". This organic matter is consumed by zooplankton, which serve in turn as food for fish or marine mammals or shellfishes.

Question 3: What is the significance of phytoplankton to humans?

the air we breathe

Through photosynthesis, phytoplankton produce large amounts of oxygen (O₂), which dissolves in seawater. As the ocean continuously exchange gases with the atmosphere, part of the oxygen dissolved in seawater is released into the atmosphere. Hence, at least 50% of the oxygen we breathe come from phytoplankton organisms!

the carbon cycle

To build their own organic matter via photosynthesis, phytoplankton use atmospheric CO₂ that is dissolved in seawater. This contributes to the "sequestration" of CO₂ in the deep ocean, a process called by oceanographers "biological carbon pump". Globally on earth, marine phytoplankton organisms fix the same amount of CO₂ as terrestrial plants. Hence, marine phytoplankton are as important to Planet Earth as meadows and forests. Without phytoplankton, the increase in temperature caused by human activities (greenhouse effect) would be much larger than it is today and the functioning of ecosystems, including human societies, would be strongly affected!

toxic phytoplankton

Some phytoplankton species are toxic, and may develop into large numbers under special circumstances. As shellfishes filter seawater, they may retain cells of toxic phytoplankton. For this reason, the marketing of mussels or oysters is sometimes forbidden, to prevents us from food poisoning caused by the accumulation of toxins in shellfishes.