Diatoms: Introduction, Structure & Life Cycle

Diatoms: Introduction, Structure & Life Cycle

Diatoms are microscopic algae widely distributed in aquatic environments. They are important primary producers and play a crucial role in ecological processes such as oxygen production, carbon fixation, and nutrient cycling. Their unique silica cell walls and diversity make them significant both biologically and in forensic investigations.

Introduction to Diatoms

• Diatoms are single-celled photosynthetic organisms belonging to the class Bacillariophyceae.

• Ehrenberg was the first scientist to study diatoms in India.

• Diatoms are unique microscopic algae having intricate cell walls made up of silica.

• They are unicellular, eukaryotic, and photosynthetic organisms.

• Their cell size ranges between 5 µm – 0.5 mm.

• Diatoms exist either as planktonic (free-floating) or benthic (attached to a substratum) in nature.

Classification of Diatoms

Taxonomists classify diatoms into two major groups based on symmetry:

1. Centrics

• Circular in shape

• Exhibit radial symmetry

2. Pennates

• Elongated in shape

• Exhibit bilateral symmetry

Importance and Diversity of Diatoms

• Diatoms represent a major group of organisms in terms of diversity, abundance, and productivity in both marine and freshwater ecosystems.

• They are responsible for approximately 20–25% of global oxygen production, meaning about every fourth breath of oxygen we inhale comes from diatoms.

• Diatoms are extremely diverse, with around 200 genera and approximately 10,000–12,000 known species.

• Some studies estimate that the actual number of species may range from 0.2 to 10 million.

• They play an important role in the silica cycle and other biogeochemical cycles, particularly the carbon cycle, by converting CO₂ into biomass.

• Most diatoms are non-motile, but some benthic diatoms possess a specialized raphe system that secretes mucilage, allowing them to attach or glide along surfaces.

• Diatoms contain chlorophyll a, c1, and c2, along with a carotenoid pigment called fucoxanthin (Lee 2018).

• Fucoxanthin gives diatoms their golden-yellow color, which is why they are often referred to as golden-brown algae.

• Diatoms can grow in various habitats including:

  • Freshwater

  • Saltwater

  • Terrestrial environments

  • Damp places

  • Ice

  • Moist soil

Structure of Diatoms

Frustule (Siliceous Cell Wall)

• The diatom cell wall, known as the frustule, is composed of hydrated silica (SiO₂).

• The frustule is divided into two overlapping halves:

1. Epitheca

• The larger, upper half.

2. Hypotheca

• The smaller, lower half.

• A single frustule consists of:

  • A larger valve (epivalve)

  • A smaller valve (hypovalve)

  • Girdle bands that hold both valves together

• The epivalve fits over the hypovalve like the lid of a box, often referred to as the “soap box” structure.

• The frustule is highly ornamented with pores, ridges, and spines, which vary among species and assist in nutrient exchange.

Plasma Membrane and Cytoplasm

The plasma membrane lies beneath the frustule and contains typical cellular organelles such as:

• Nucleus

• Mitochondria

• Chloroplasts

• Vacuoles

Raphe System (in Pennate Diatoms)

• The raphe is a slit-like structure present in some pennate diatoms.

• It allows gliding movement on surfaces and facilitates locomotion in benthic environments.

Note

Very few living organisms can utilize silica. Since the silica present in these transparent frustules is similar to glass, diatoms are often referred to as “algae in glass houses.”

Life Cycle of Diatoms

1. Asexual Reproduction (Binary Fission)

• The primary mode of reproduction in diatoms.

• Each diatom cell divides into two daughter cells.

• Each daughter cell inherits one half of the parent frustule:

  • One receives the epitheca (larger half)

  • The other receives the hypotheca (smaller half)

• A new half is synthesized to complete the frustule.

• Result: With successive divisions, the average cell size gradually decreases.

2. Size Reduction and Threshold

• Due to the rigid silica frustule, diatom cells cannot increase in size after division.

• Over time, cells become too small to divide further.

• This triggers sexual reproduction to restore the original size.

3. Sexual Reproduction

Sexual reproduction occurs in both centric and pennate diatoms, although the mechanisms differ.

• Centric diatoms produce:

  • Oogonia (female gametes)

  • Spermatozoids (male gametes)

• Pennate diatoms usually undergo isogamy, which involves the fusion of similar gametes.

4. Auxospore Formation

• The fusion of gametes forms a zygote, which develops into an auxospore.

• The auxospore is a non-siliceous and flexible cell.

• It grows to the maximum size characteristic of the species.

• After reaching full size, the auxospore forms a new frustule and resumes asexual reproduction.

Diatoms: Introduction, Structure & Life Cycle