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• 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 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

• 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)

• 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)