Are Diatoms Protists? 15 Facts You Should Know

In this post, you will find the five facts about, are protists grouped and are diatoms protists, their detailed explanation of their groups and characteristics.

Diatoms are protists grouped under photosynthetic protistan algae. The golden protists algae having cell wall made up of silica and and cell wall consists of reserve food divided into two halves of oil and leucosin.

Kingdom Protista

Protists were the first eukaryotes to evolve on this earth some 1600 million years ago. All other eukaryotes developed from them.


Protists were the first acellular eukaryotic microorganism that thrive in aquatic as well in  moist habitats. Some are free floating and some are benthic. They constitute 95% of the plankton on surface water biota. Phytoplankton are the photosynthetic members of this kingdom.

Are Diatoms Protists
A sampling of protists, composed of images from Wikimedia Commons. Clockwise from top left: red algae (Chondrus crispus); brown algae (Giant Kelp); ciliate (Frontonia); golden algae (Dinobryon); Foraminifera (Radiolaria); parasitic flagellate (Giardia muris); pathogenic amoeba (Acanthamoeba); amoebozoan slime mold (Fuligo septica) from Wikipedia


Protists are generally microscopic. Shape varies in different groups of protists. They can solitary or colonial. They are unicellular but have eukaryotic structure with double envelope system. They have a nucleus which is equivalent to two nucleoids of prokaryotes.


Many protists show locomotion but only few are sedentary. Organism can move from one place to another by five different types of locomotion.

These are –

1.Pseudopodial locomotion :

This type of locomotion is done by using pseudopodia which are false feet. Organism shows creepy and slow movement over the solid surfaces that appear them to be irregular in shape, e.g., Amoeba, Euglypha.

2.Flagellar locomotion :

Organisms perform locomotion with the help of flagella. Flagella is attach at one end of the organism. Flagella are generally 1-4 in number e.g., Giardia and Euglena.

3.Ciliary locomotion :

Locomotion is done with the help of cilia. Cilia are quite similar to flagella but they are quite numerous and short. They are generally occur all over the surface of protists, e.g., Paramecium.

4.Wriggling locomotion :

Movement is done by contraction and expansion by using myoneme fibrils.e.g.,Monocystis.

5.Mucilage Propulsion :

This type of locomotion is generally occurs in diatoms. They secrete mucilage in one direction which propelled their body in the opposite direction.


Mode of nutrition is diverse, they are autotrophic, heterotrophic, saprotrophic and mixotrophic.


It occurs by both means-  Physical and Non-physical methods. Non-physical reproduction is done by various methods depending upon the species. The methods include binary fission, plasmotomy, multiple fission, cyst formation and sporulation. Physical reproduction occurs by haploid gametes formation and fusion to form zygote and develops into new individual.

Are Diatoms Protists and their Characteristics?

  • Diatoms are golden protists algae having cell wall made up of silica and and cell wall consists of reserve food divided into two halves of oil and leucosin.
  • Diatoms belongs to the phylum bacillariophta which contains about 5500 species.
Are Diatoms Protists
Light microscopy of several species of living freshwater diatoms from Wikipedia
  • They are found in abundance in aquatic and moist habitats. 50% of phytoplankton constitutes of diatoms. They give colour to the substratum. A blue wale have 2 tonnes of diatoms.
  • Their form and size varies. They are found in various shapes like circle, semi-circle, rectangular, triangular, boat shaped etc. Diatoms can form zig-zag, stellate and fan-shaped colonies.
  • They possess two types of symmetry- radial and bilateral symmetry. Diatoms showing radial symmetry are called centric diatoms (Melosira, Triceratium, Biddulphia) while those who possess bilateral symmetry are called pennate diatoms (Synedra, Actinella, Pinnularia).
Are Diatoms Protists
Selections from Ernst Haeckel‘s 1904 Kunstformen der Natur (Art Forms of Nature), showing pennate (left) and centric (right) frustules from Wikipedia
  • Protoplast is peripheral around a central vacuole. Central vacuole is large but traversed by a number of cytoplasmic strands. A cytoplasmic bridge may also occur in the middle.
  • Diatoms are uninucleate. Nucleus is generally present in central vacuole but in some cases it lies on one side of the cell.
  • They possess photosynthetic pigments like chlorophyll a, chlorophyll b and carotenoids. A pigment called Fucoxanthin (found in brown algae) occurs in some diatoms giving them a brownish tinge. There are other rare pigments also which are found in diatoms- beta carotene, diadinoxanthin and diatoxanthin.
  • Nutrition is photosynthetic while some diatoms lacks chromatophores and are saprotrophic in nutrition,e.g.,Nitzschia alba.
  • They store their reserve food material in the form of fat drops and leucosin (chrysolaminarin). An these fat drops help the planktonic diatoms to float well on water surfaces.
  • Locomotion takes place by secreting mucilage while oil globules help them to remain afloat over the water surface.
  • Life cycle of diatoms are diploid. Gametic meiosis occurs in life cycle.
  • They can also produce perennating or resting spores, when there is deficiency or decrease in the phosphate and moisture level. These spores are also called statospores.
  • The skeletons of diatoms are made up of silica that do not decay beacuse of low organic content. They usually settle down at bottom to form a heap called  diatomaceous earth. With passage of time, some natural eruptions at the bottom pushes them to the surface.
  • Diatomite earth posses fine pores which is used for filtration in many industries. It can adsorb many coloured impurities. It is also used as cleaning agent in many metal polishes and toothpaste.
  • Diatomite is a good insulator because of low thermal conductivity.

How are protists grouped?

Kingdom protista  is divided into three groups:

  • Photosynthetic protistan algae
  • Slime moulds or Protistan fungi
  • Protozoan protist

Photosynthetic protistan algae

Protista have eukaryotic acellular groups of algae that includes dinoflagellates, diatoms, euglenoids and yellow-green algae. They constitute about 95% of phytoplankton and performs 80% of total photosynthesis around the globe.

Slime moulds or Protistan fungi

They are simple microorganism and have naked naked plasmodium or pseudoplasmodium, cellulose covered spores inside sporangia and myxamoebae from spores.

Protozoan protists

They are microscopic acellular, non-photosynthetic organism previously grouped under the phylum protozoan. They are found in aquatic as well as moist habitats. Most of them are parasites and commensals.

Why are protists grouped?

Protista kingdom consists of all acellular and colonial eukaryotes except some members of fungi and algae. This kingdom was developed by Earnst Haekel  in 1866 that includes all the microorganisms (bacteria, algae, fungi and protozoa) which do not show tissue differentiation. Later on, he reserved this kingdom only for acellular eukaryotes. They are the link between the eukaryotes, all other eukaryotes develop from them.

Why are protista and bacteria grouped into different domains?

The reason is clear, bacteria is a prokaryote while protists are unicellular eukaryote. There are so many reasons to grouped bacteria and protists in different domains. Bacteria posses cell wall (peptidoglycan) which is unique to every other microorganisms. Protists have well defined nucleus and higher DNA content than bacteria.

What is Mac Donald-Pfitzer Law?

This law states that the diatoms size gets reduced over generations due to the fact that one of the daughter cell receives parent’s small sized hypotheca during binary fission. During fission, each daughter gets one valve from the frustule of the parent. The daughter cell getting the epitheca of the mother cell remains in same size but hypotheca receiving daughter cell will be slightly shorter in size. The size of diatoms shows progressive decrease over generations.


To wrap up this post, we conclude that Diatoms are protists which comes under the group of Photosynthetic protistan algae. They possess photosynthetic pigments like chlorophyll a, chlorophyll b and carotenoids. Nutrition is photosynthetic while some diatoms lacks chromatophores. They constitute about 95% of phytoplankton and performs 80% of total photosynthesis around the globe.

Saif Ali

Hi, I am Saif Ali. I obtained my Master's degree in Microbiology and have one year of research experience in water microbiology from  National Institute of Hydrology, Roorkee. Antibiotic resistant microorganisms and soil bacteria, particularly PGPR, are my areas of interest and expertise. Currently, I'm focused on developing antibiotic alternatives. I'm always trying to discover new things from my surroundings.  My goal is to provide readers with easy-to-understand microbiology articles. If you have a bug, treat it with caution and avoid using antibiotics to combat SUPERBUGS. Let's connect via LinkedIn.

Recent Posts