Class Bacillariophycea (Diatoms):

The diatoms are important components of planktonic and benthic communities in a wide range of marine and fresh water habitats. The cells are normally protected by a wall called a frustule, composed of silica (SiO₂) and surrounded by mucilage. The frustula is composed by two valves epitheca or epivalve (larger) and hypotheca or hypovalve (smaller). The loops inserted between the valves are the Girdle bands or cingulum.

Figure 1. (Diatom frustule )

Figure 2. Detailed frustula description, girdle view side and valve view face

The diatoms may be derived from an ancestral cell in which siliceous scales on opposite sides of the cell enlarged as the valves, while other scales become the Girdle bands or cingulum. Cingulum can be one piece or several. Can be closed, open or overlapping pieces.

Diatoms Taxonomy.

Taxonomically, diatoms are divided into two groups based on the symmetry of the cells. Centric diatoms (order Centrales) have radially arranged markings. Pennate diatoms (order Pennales) have markings bilaterally arranged about the valve’s long axis.

Figure 3. Diatoms spine extensions (centric)

Figure 4. Diatoms raphe slit (pennates) can be held in colonies (short chains)

Centric diatoms are more primitive than pennates (diploid nucleus). Many pennales diatoms have a raphe, which is used for secreting mucilage, which is also used for diatom mobility (benthic)

Figure 5. Diagram showing the diatom mobility using raphe.

Diatoms reproduce asexually by cell division to produce two daugther cells by mitosis; each daughter cell receives one valve and it is reproduced by furrowing. The cell has vesicles that increase the deposition of silica creating a new valve girdle band. When the diatom cell size is less than half of the original size, normally its size can be recovered by sexual reproduction "max, sexual reproduction stimulated".

Figure 6. Diatom in mitosis division .

Vegetative cells are diploid, and gametes are the only haploid stages. Sexual reproduction differs in centric and pennates diatoms. Centric diatoms undergo meiosis to form eggs and sperm. Oogonial cells usually produces one egg, which may be released into the water or retained. Size decreases as mitosis proceeds. This provides a timing mechanism between sexual reproduction.

Figure 7. Diatoms sexual division

Centric, antheridial cells meiosis produces 4 sperms or more per cell. Oogonial cells "eggs", 1 per cell can be released from cell or retained in the frustule.

Figure 8. Sperm with single flagellum and auxospore

Penate are cells not flagellate, gametangial cells pair in muscilage, and each cell undergoes meiosis.

Figure 9. Diagram of sexual reproduction in pennates diatoms

In some diatom, vegetative enlargement occurs by release of the protoplast from its surrounding frustule, enlargement, and formation of a new frustule.

SiO₂ derived from rock and sand can be a limiting factor in the cells grows. Diatoms are mostly part of the plankton (Nearly all centric and some pennates). However, there are some as part of the benthos attached on substrates.

Euplanktonic, permanent in the plankton (centric and few pennates).

Benthic- pennates

Meroplanktonic, found temporally in the plankton.

Diatoms effect on earth and ocean.

Diatoms removes silica from water availability control blooms need silicic acid Si(OH)₄, In the Gulf of California

Figure 9. Gulf of California Si cycle due to the Diatoms concentration in the ocean.

200+ genera, 12,000 + species