What are diatoms, really?

For students-of-the-diatoms who are interested in learning more, these are some of the biological features that distinguish diatoms from other organisms (van den Hoek et al. 1995). Diatoms share characteristics with the heterokont algae, including:

• Plastids are enclosed by four membranes. The inner two membranes are homologous with those of the Rhodophyta, Chlorophyta, and Glaucophyta. The outer two membranes, or 'chloroplast endoplasmic reticulum', reflect the origin of the heterokontophyte plastid as a secondary endosymbiont.
  
• Between the outer and inner chloroplast membranes, there is often a network of anastomosing tubules called the 'periplastidial reticulum'.
• Within the plastid, thylakoids are grouped into stacks of mostly three, sometimes two or four stacks (these stacks are called lamellae).
  
• Lamellae are surrounded by a girdle lamella, beneath the plastid membranes.
  
• Chlorophyll a, chlorophyll c, and fucoxanthin are the major light-harvesting pigments for photosynthesis.
• Chloroplast DNA is usually concentrated within a ring-shaped nucleoid at the periphery of the plastid (but there are exceptions in some diatoms!)
• β-1,3-linked glucan is the primary polysaccharide and serves as storage.
• Flagella within gametes 'mastigonemes', which are tripartite, stiff hairs.
  
• Inner membrane of the mitochondrion has tubular invaginations.
• Diatoms and bolidophytes share a unique 2 amino-acid insertion in the large subunit of Rubisco.

More details about the diatoms:

• The diatom life cycle is strictly diplontic. Vegetative cells are, so far, known to be diploid. Mitosis occurs in the diploid phase. Haploid cells, however, have occasionally occurred in culture.
• Most diatom cells have a large central vacuole, or pair of vacuoles.
• During the stationary-phase of the cell cycle, cells often accumulate large quantities of lipids and fatty acids.
• Polyphosphate bodies are present, which may be apparent as spherical or complex 'volutin' granules. One polyphosphate body (only one) is located in a vacuole.
• Secretion of extracellular polymeric substances (EPS) is common in diatoms. These polysaccharides form stalks, pads, capsules, tubes, and "trail" material produced in the sliding motion of pennate diatoms.
• All cells (except gametes and endosymbiotic diatoms) possess a bipartite silica cell wall, comprising two overlapping halves called the epitheca and hypotheca. Each half includes the 'valve' and may include a few, or numerous, bands or segments, which together form the 'girdle'.
• The cell wall is almost always heavily silicified, although there are a number of species with very fine silicification.
• The parts of the cell wall (valves, girdle bands, auxospore scales, and girdle bands) are formed within the cell wall.
  
• The silica deposition vesicle (SDV) is a organelle that is closely associated with the cell membrane. The SDV holds proteins (silaffins) that function to precipitate SiO2 from dissolved SiO4.
  
• New parts of the cell wall are formed within an existing cell. The silica cell wall elements are not secreted from within the cell until they are completely formed. Consequently, cell size usually decreases with successive mitotic divisions.
  
• Diatom cell size is restored via the formation and expansion of a special cell, the auxospore. The auxospore is usually a zygote. The basic shape of each diatom species is established during the expansion of the auxospore, but may be modified in subsequent mitotic cell divisions.
• During vegetative mitoses, the nucleus lies to one side of the cell immediately beneath the girdle, at the edge of the hypotheca.
• Mitosis is open and the nuclear envelope breaks down before metaphase. The spindle is a narrow cylinder, which persists at telophase. The spindle is composed of two interdigitating half-spindles and each half-spindle is associated with a polar plate.
• During metaphase, chromosomes gather closely around the cylindrical spindle. During this part of the cell cycle the chromosomes are too dense to distinguish.
  
• Cytokinesis occurs through cleavage.
• Diatoms are amazingly diverse, with hundreds of genera. There are perhaps 200,000 species, or even far more.
  
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