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Diatoms (Bacillariophyceae)
  • Most diverse and abundant aquatic algae, dominate phytoplankton of cold, nutrient-rich water (upwelling)
  • Silica cell walls (frustules) provide important silica deposition and good fossil record
  • Non-flagellated cells, but sperm of diatoms exhibit tripartite flagella typical for ochrophytes
  • Pigments: chl. a, c, fucoxanthin
  • Two major groups: Centric diatoms have discoid or cylindrical cells, pennate diatoms exhibit bilateral symmetry (elongated)

  • Frustule is made of two valves, epitheca and hypotheca; connected by epicingulum and hypocingulum, both forming the girdle 
  • Girdle can possess closed cingula rings or open rings, may have a protrusion opposite to the cingulum‘s opening (ligula)
  • Raphe present in some pennate diatoms enable gliding movement
  • Cell growth (cell expansion) can only occur at the cingulum
  • Pores penetrate valves and cingula, entry of gas and nutrients into the cell
  • Areolae possess a „bottom“ of thin silica with smaller pores = velum; 
  • Loculate areolae are chamber-like with a silicate layer on the outside
  • Poroid areolae lack outher silicate layer (open)
  • Striae: rows of areolae, visible in light microscope
  • Costae are rib-like, parallel sturctures in pennate diatoms
  • Central nodule interrupt raphe in some pennates
Cell Division and Frustule Development
  • Cell division always occurs in the plane of the valve, involving production of a new valve
  • Chains are formed by some centric diatoms; each cell has its own frustule, no plasma connections
  • Open mitosis: nuclear envelope dissolved, division spindle with microtubuli, but no centrioles
  • Silica deposit vesicles from the Golgi apparatus transport silica between daughter nuclei and produce new valve
  • Areolae and costae production is not understood
  • Reproduction is dependent on sufficient silica in the envrionment (water); silica depletion stops cell division
  • New valve is always a hypotheca
  • Daughter cells are of the same size (1) and smaller size (1) of the mother cell; mean cell size in the population will decrease over time

Sexual Reproduction: 
  • Centric Diatoms
    • Vegetative cells are diploid, and gamete formation involves meiosis
    • Trigger for sexual reproduction largely unknown
    • Oogamy, vegetative cells producing 1-2 egg cells or 4-128 single- flagellated sperm cells per vegetative cell as a result of meitotic cell division after meiosis
    • Gametes possess plastids, but no frustule
    • Gamete fusion forms a large zygote, the auxospore; auxospores can possess no, one, or two parental valves

  • Pennate Diatoms
    • Isogamy: similar sized gametes lacking flagella
    • Pairing of vegetative cells within a common mucilage starts sexual reproduction
    • Unequal cytokinesis follows meiosis, and the smaller daughter cell dies
    • Frustules then gape open and protoplasts begin to emerge prior to second meiotic cell division, one haploid nucelus dies, leaving one haploid nucleus
    • Protoplasts fuse, producing a bi-nucleate perizonium
    • Nuclear fusion then restores diploidy
    • Autogamy: auxospore formation by fusion of two haploid nuclei within the same mother cell (exception)

Diatom Spores and Resting Cells
  • Spores and resting cells serve a perennation fuction, i.e. to  survive periods of unfavorable environmental conditions
  • Development of spores and resting cells usually does not involve sexual reproduction
  • Resting cells remain morphologically similar to vegetative cells
  • Resting spores possess a thick frustule and may become rounded and less ornamented; spores cannot be produced under silica limitation (due to increased thickness of frustule)