Marine Protists: Diversity & Types of Species

Instructor: Robin Monegue Keeler

Robin has taught college microbiology and environmental science. She has two master's degrees: one in environmental microbiology and the other in public health.

In this lesson, you will learn about the huge variety of marine protists. At the end, you should be able to impress your friends with your knowledge of some pretty remarkable marine protists.

Marine Protists

The ocean is teeming with a fascinating world of organisms - the marine protists.

Protists are a weird collection of organisms. Most are unicellular, although there are a few that are multicellular. Generally, this group has been used as a catch-all classification for organisms that are not categorized as plants, animals, fungi, or bacteria. Protists don't easily fall into a single taxonomic group. In fact, as more is learned about the protists, their classification is constantly changing.

With such a huge variety of organisms, there are some really interesting marine protists. Let's take a look at some of them.

Foraminifera, the Little Architects

Foraminifera ('forams') are one of the largest unicellular organisms. They can be as tiny as 100 micrometers, but some grow as grand as seven inches in length. They are remarkable little architects that build themselves a shell, called a test, most commonly made out of calcium carbonate.

Foram shells are pierced by holes called foramina, and sprouting from the foramina are a number of long, thin extensions called reticulopodia. The forams use their reticulopodia to seek out and catch various types of prey, including bacteria, diatoms, dinoflagellates, and even small animals, such as copepods.

Forams are abundant in the ocean. There are an estimated 4,000 species living today. Most forams are benthic, meaning they live on the sea bottom, although a few are planktonic, meaning they float around in the ocean water. They exist in such impressive numbers that the remnants of their shells become massive limestone deposits. For example, foram shells are what makes the famous White Cliffs of Dover white; they are also a major part of the limestone rocks that were used for the construction of the pyramids of Egypt.

Tests of Foraminifera
Forams

Diatoms, the Tiny Glass Ornaments

The diatoms are the largest group of unicellular algal protists, with over 13,500 species. But the number of species varies, depending on the scientist you talk to. Some say there are 20,000 species, some 200,000, and some say there are greater than 1,000,000 species! Diatoms are found in nearly every aquatic environment. Many, of course, are found in the ocean.

The diatoms are also one of the most beautiful protists - they look like tiny glass ornaments, consisting of an inner box and an outer lid, which are made of silicon dioxide. The intricate designs of these little glass-like boxes are created by the arrangement of tiny holes where gas and water are exchanged. Diatoms can be radially symmetrical (straight lines coming out of the center of a circle) or bilaterally symmetrical (having two sides).

Microscope photo and drawings of some of the beautiful shapes and symmetry of diatoms
Diatoms

Dinoflagellates, the Fire Plants

Dinoflagellates are the second largest group of algal protists, after the diatoms, and include about 3,400 species. Dinoflagellates belong to the classification Pyrrophyta - which literally means fire plants.

Night-time visitors to coastal areas may be mesmerized by tiny shimmering lights in ocean waves. It is not magic, it is the result of bioluminescent dinoflagellates, which bioluminesce, or glow, when the water is agitated. One area in Puerto Rico is known as 'Biobay' because it has such high concentrations of these algae.

These glowing ocean waves are caused by bioluminescent dinoflagellates
Bioluminescent dinoflagellates

While only a small number of dinoflagellates bioluminesce, they are all unicellular, and most have two flagella (whip-like tails) that enable them to swim around. The dinoflagellates are quite diverse, and about half of the known species can photosynthesize. Most dinoflagellates are found in the ocean, although there are a few freshwater species.

Some dinoflagellates make their home within other organisms. These are called endosymbionts, and both the dinoflagellates and their host mutually benefit from the relationship. These types of dinoflagellates are found in many marine invertebrates, including sponges, corals, jellyfish, and forams.

Some corals depend almost entirely upon dinoflagellates for their survival. The dinoflagellates, after being incorporated into the coral, multiply and photosynthesize. This process makes the corals happy, since photosynthesis produces carbohydrates (e.g., food) for the coral. In turn, the coral provides carbon dioxide the dinoflagellates need to photosynthesize.

When a coral becomes stressed (for example, from unusually warm water temperatures or pollution), the dinoflagellates leave, which causes bleaching of the coral and makes the coral vulnerable to disease. It may even lead to the coral's death.

Some dinoflagellates aren't so helpful. There are some that are responsible for red tide, which can injure or even kill marine life due to toxins secreted by the dinoflagellates.

Brown Algae

While most protists are single-celled organisms, there are a few that are multicellular. Brown algae (phaeophytes) are a great example. There are about 2,000 species of brown algae that have been identified. They are also called seaweeds, but they are not plants. They are much simpler than plants in that they lack differentiated cells and tissues.

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