Diatoms Structure And Nutrition // Detailed notes by Rohit Jirobe//

 Diatoms

Diatoms are unicellular, microscopic golden brown' algae. The word 'diatoms" literally means "cut into two". This refers to the characteristic double nature of the silica wall. Diatoms are considered as "jewels of nature" and are famous for their wonderful symmetry. They are commonly known as "grass of the sea". It includes 200 genera and 16,000 species. It is included under the class Bacillariophyceae.

Occurrence

Diatoms occur in a variety of habitat. They live both in freshwater and sea water. Some diatoms are terrestrial. Many species grow as epiphytes on other algae or water plants. Many diatoms occur as fossils forming large deposits. Deposits of fossil diatoms are known as diatomite or diatomaceous earth or kiesulguhr.

Some of the common Diatoms are -

Pinnularia, Melosira, Cymbella, Biddulphia, Navicula, Trabellaria ,Fragillaria

Structure of Diatoms
Diatoms are golden brown algae.
They are considered as 'jewels of nature.
They are also known as grass of the sea.
They are included in the class Bacillariophyceae.
They are unicellular and microscopic.
In some cases, the cells are united by their gelatinous sheath forming colonies. Eg. Melosira.
The diatoms are diploid cells.
They are of different sizes and shapes.
They may be cylindrical, rectangular, ovoid and triangular or boat shaped.
The diatom cell consists of central protoplasm surrounded by a cell wall.
The cell wall is made up of silica.
The cell wall of diatom is called a frustule.

Each frustule is made up of two equal overlapping halves, called valves. These two valves are closely fitted together as the two halves of a petridish.

Each valve has a flat surface called valve and incurved sides called connecting bands. The outer valve that overlaps the other is called epitheca or epivalve or upper valve.

The inner valve that is overlapped by the epitheca is called hypotheca or hypovalve or lower valve.

The connecting bands of the two valves are firmly united in the overlapping region. This united portion of the connecting bands is called girdle.

The overlapping region of epitheca is known as epicingulum.

The overlapping region of hypotheca is known as hypocingulum. The epicingulum and hypocingulum are united to form the girdle. The bent over lateral portion of the valve is known 25 valve jacket or mantle.

Diatoms can be seen in two views under a microscope.

1. Valve view (or) top view

2. Girdle view (or) side view

If the surface of the valve is visible, it is said to be the valve view or top view. If the girdle is visible, it is said to be the girdle view or side view.

The cell wall is composed of pectin and silica.

The deposition of silica produce typical markings on the surface of cell wall. There is a median fissure running along the apical axis. This fissure is known as axial

strip or raphe. The raphe may be found on one or both valves.

It has a thickened area in the middle and the area is called central nodule.

The thickened areas at both ends of the raphe are known as polar nodules. In some cases, the raphe remains an unthickened line called pseudoraphe.

The wall markings are called striations and are arranged on either side of the raphe. The striations may be small pores or cavities or channels or furrows depending upon the species.

On the basis of arrangement of striations (or) markings, the diatoms are classified into two types.

1. Pennate diatoms (E.g) Pinnularia

2. Centric diatoms (E.g) Cyclotella

In pennate diatoms, the striations or wall markings are arranged in two rows one on either side of the axial strip. But in centric diatoms the striations are radially disposed.

Inner to the cell wall is a dense protoplasm delimited by plasma membrane.
The protoplasm has a large nucleus at its centre.
The nucleus is eukaryotic. It is connected with small cytoplasmic strands called cytoplasmic bridges.
The nucleus has one or more nucleoli in it.
A centriole is attached to the nuclear membrane.
In pennate diatoms, the nucleus is large and it is located in the centre of the protoplasm.
In centric diatoms, the nucleus is found close to the inner surface of plasma membrane.
The protoplasm contains one or two large chromatophores.
The chromatophores are yellow brown or olive green in colour and are lobed.

In centrales, the chromatophores are small and numerous in each cell.
They may be discold or granular in shape.
There is a single pyrenold inside the chromatophore.
The chromatophores contain pigments such as chlorophyll-a and c, ẞ-carotene, fucoxanthin, neofucoxanthin-a and -b, diatoxanthin and diadinoxanthin. The reserve food of diatoms are oil, volutin and leucocin. The reserve foods are
distributed in the cytoplasm.
Nutrition
Diatoms are autotrophic but can utilize organic substances.
Locomotion
The centric diatoms do not move independently.
Some of pennate diatoms show spontaneous locomotion.
These pennate diatoms have raphe.
The movement is by gliding or by a series of jerks.
According to Muller, the cytoplasm of the diatom shows cyclosis, which is the main cause for the jerks.
During cyclosis, the cytoplasm moves from a polar nodule to central nodule and bifurcates at the central nodule.
Gliding is absent in centric diatoms.