Heterokontophyta IV — Diatoms


  1. Introduction
    1. The Bacillariophyceae are more commonly called diatoms
    2. A moderately large, but extremely important group
    3. The cell walls are made of silica, and consequently fossilize extremely well. Diatoms are not, however, an ancient group. They first appear in the fossil record in the Cretaceous (a little over 100 ma).
      1. The cell walls are elaborately ornamented, and can be quite beautiful. The patterning is very precise, and this is used to calibrate high-precision instruments including microscopes.
      2. Because the cell walls preserve well and are elaborate in structure, their structure is important in diatom taxonomy, and there is an extensive terminology associated with them.
      3. Diatomacous earth is a geological deposit composed of nearly pure diatom frustules. This is mined commercially for a number of purposes.
    4. Extremely important marine phototrophs; marine planktonic diatoms contribute roughly 25% of total global primary productivity.
    5. Widely distributed in both marine and freshwaters, as well as in damp soil, mud, sand, etc.
  2. Structure & metabolism
    1. Cell wall is silica, composed of two overlapping frustules that fit together like a box and lid.
      1. The upper (larger, overlapping) piece is called the epitheca, while the lower (smaller, internal) piece is called the hypotheca.
      2. Two parts can be identified in both the epitheca and hypotheca; the valve, which is the flat part, and the cingulum, which is the curved part encircling the cell; thus one can refer to the epivalve, epicingulum, hypovalve, or hypocingulum.
      3. The valve and cingulum are connected by a complex suture.
      4. The cingula themselves are divided into bands called copulae, and the cingula together are referred to as girdle bands.
      5. The silica cell walls are deposited in special silica deposition vessicles similar to those found in the Chrysophyceae
      6. Because silica is needed to build the cell walls, it is an essential nutrient for diatoms, and excluding silica from a culture medium can help reduce unwanted diatom growth. Diatoms are, however, extremely efficient at scouring silica from the environment, and can grow with only trace quantities of silica available.
      7. Similarly GeO2 -- germanium dioxide -- is toxic to diatoms because it disrupts silica deposition. Addition of low concentrations of GeO2 to a culture medium can inhibit diatom growth.
    2. Rigid, overlapping cell walls and internal formation of the new cell walls during cell division mean that as diatom cultures age, the cells become progressively smaller
      1. This can proceed to the point that the cells become too small to survive, and the culture dies
      2. Full cell size is restored by sexual reproduction
    3. The diatoms are separated into two categories on the basis of their valve symmetry
      1. In centric diatoms the cells are radially symmetrical
      2. In pennate diatoms they are (approximately) bilaterally symmetrical
    4. Many (but not all) pennate diatoms have a raphe, which is a longitudinal groove involved with gliding motility. Pennate diatoms without raphes are said to be araphid. In some araphid diatoms there is a pseudoraphe, a line running down the longitudinal axis of the cell which can easily be confused with a true raphe.
      1. The mechanism for gliding mobility is still under investigation.
      2. The raphe is also involved in mucilage secretion in some species, and can anchor the cell to the substrate.
    5. The raphe is probably homologous to the labiate process of centric diatoms
      1. In at least some diatoms, labiate processes are involved in mucilage secretion
      2. The biological role of labiate processes remains poorly understood
      3. Some slow mobility has been observed
      4. Possible roles for mucilage secretion (in both centric and pennate diatoms, as well as in other organisms) would include serving to hold the alga in place, reducing dessiccation, consolidating sediments, reducing settling rate, and interfering with herbivory.
    6. When diatoms divide, the new thecae formed are always hypothecae; as a consequence the average size of cells in a growing culture will gradually become smaller and smaller. The full size of the cell is restored only by sexual reproduction.
    7. Mitosis is open, with a persistent telophase spindle.
    8. Flagella are found only in reproductive cells (spermatozoids) of centric diatoms.
      1. The spermatozoid has a single, apically inserted pleuronematic flagellum.
      2. The flagellum is 9+0, lacking the two central microtubules, rather than the usual eukaryotic 9+2 arrangement, and there is no transitional helix.
      3. The spermatozoid is does not have a chloroplast.
    9. Chloroplasts are secondary, and are associated with a CER, but there is no nucleomorph.
    10. Pigmentation is Chlorophylls a and c, with fucoxanthin as the primary accessory pigment. This gives the plastids a golden-brown appearance.
  3. Reproduction
    1. Life cycle is diplontic (gametic meiosis), i.e., the vegetative cells are diploid,and meiosis gives rise to gametes. The only haploid cells are the gametes.
    2. Centric diatoms are oogamous, while pennate diatoms are isogamous.
    3. Reproduction in centric diatoms - Oogamy
      1. Stephanopyxis turris
      2. Receptivity to sexuality is a function of cell size. The larger cells (which have undergone relatively few divisions since thier last sexual cycle) cannot generally be induced to sexual reproduction. Apparently all cells can function as either male or female, but typically the larger cells function as female.
      3. Female cells undergo meiosis I, and undergo asymmetric cytokinesis; the smaller cell degenerates. Meiosis II is similarly asymmetrical, and the result is a single egg.
      4. The egg expands and separates the frustules, so that a fertilization papilla is exposed.
      5. Male cells undergo a series of rapid mitotic divisions, producing eight small cells with incomplete cell walls, all of which lie within slightly expanded parental cell walls. These cells expand and force apart the parental cell walls, and then undergo meiosis, with each cell giving rise to four spermatozoids.
      6. The spermatozoids are tiny, with a single tinsellate flagellum, and do not carry a chloroplast.
      7. Following fertilization, the zygote swells and forms a silicified auxospore.
    4. Reproduction in pennate diatoms - Isogamy
      1. Eunotia arcus
      2. As with centric diatoms, pennate diatoms become sexually compentent when they are below a critical size.
      3. Reproductively competent cells settle close to each other, and secrete a common mucilaginous sheath
      4. Asymmetric meiosis occurs, giving rise to a single isogamete.
      5. The cell swells, cracks open the parental cell wall, and extends a copulation papilla
      6. The two copulation papillae grow toward each other and fuse to form a conjugation canal
      7. When the conguation canal is complete, the protoplasts migrate together and fuse, but the haploid nuclei remain distinct, hence is formally a dikaryon rather than a zygote.
      8. The dikaryon swells and deposits a silica cell wall, forming an auxospore.
      9. Karyogamy occurs only when the auxospore is mature.
  4. Classification
    1. Traditional classification divides diatoms into two orders on the basis ofsymmetry.
      1. Order Centrales - centric diatoms
        1. Cells are radially symetrical in valve view
      2. Order Pennales - pennate diatoms
        1. Cells are elongate (typically bilaterally symmetrical) in valve view
      3. This distinction is useful for remembering the general properties of the group, but is somewhat simplistic. Current evidence suggests that the pennate diatoms are a monophyletic group derived from centric diatoms.
        1. Pennate diatoms have long been viewed as a derived group, and molecular studies agree with this interpretation
        2. Centric diatoms appear in the fossil record about 120 ma, while pennate diatomsdo not appear until about 70 ma
    2. Organisms to know
      1. Melosira
      2. Stephanopyxis
      3. Coscinodiscus
      4. Chaetoceros
      5. Sceletonema
      6. Fragilaria
      7. Odontella
      8. Bacillaria
      9. Navicula
      10. Nitzschia
      11. Cylindrotheca
      12. Gyrosigma
      13. Surirella
  5. Ecology
    1. Diatoms -- particularly centric diatoms -- constitute a major component of the marine phytoplankton.
      1. Per acre productivitity is not especially high in the open ocean, but because of the vast area covered by oceans, diatoms turn out out be major players in the global carbon balance.
      2. Also important in fresh water primary productivity, although they have more competition in the freshwater environment.
      3. Can achieve very high cell densities in nature.
      4. To maintain bouyancy, many species have elaborate spines that increase the cell's effective surface area, and consequently decrease the settling rate
        1. Cell density can also be decreased by reducing the concentration of heavy ions (K+, Na+, Cl-)
        2. It has also been argued, however, that spines promote turbulence, thus reducing the width of the boundary layer, and promoting nutrient exchange
      5. They are a good source of food for zooplankton; the silica cell walls do not seem to prevent herbivory
    2. Benthic diatoms are generally pennate diatoms, and are often capable of gliding mobility
    3. Symbiosis
      1. A diatom is the permanent endosymbiont of the dinoflagellate Peridinium foliaceum. This diatom retains it eukaryotic nucleus, and little is known about the degree of genetic interdependence between the endosymbiont and host.
  6. Economic importance
    1. Their major role in primary productivity makes diatoms important in the global carbon cycle and in aquatic food chains
    2. Diatomacous earth is widely used in commerce

Required Reading: VdH Chapter 9

Supplementary Reading:


Medlin, L.K, W.H.C.F. Kooistra, R. Gersonde and U. Wellbrock. 1996. Evolution of the diatoms (Bacillariophyta). II. Nuclear-encoded small subunit rRNA sequence comparisons confirm a paraphyletic origin for the centric diatoms. Mol. Biol. Evol. 13:67-75.

Bourrelly, P. 1968. Les Algues D'eau Douce, Tome II: Les Algues Jaunes et Brunes. Editions N. Boubee & Cie, Paris.