Dinoflagellate and Diatoms

Diatoms

Dinoflagellate

  • The dinoflagellates (also known variously as Pyrrophyta, or ‘fire algae’) are chiefly marine planktonic types, comprising some 2000 species.
  • This is another unicellular group, but one whose cells are often covered with armoured plates known as thecae (sing:theca).
  • They are generally biflagellate, with the two dissimilar flagella lying in part within the longitudinal and lateral grooves that run around the cell.
  • The beating of the flagella causes the cell to spin like a top as it moves through the water (the group takes its name from the Greek word ‘to whirl’).
  • Although many non-photosynthetic (chemoheterotrophic) types exist, most dinoflagellates are photosynthetic, containing chlorophyll a and c plus certain carotenoids and xanthophylls, which give them a red/golden appearance.

dinoflagellate

  • As a group, they are second only to the diatoms (see below) as the primary photosynthetic producers in the marine environment.
  • Some dinoflagellates form endosymbiotic relationships with marine animals such as corals and sea anemones; these are termed zooxanthellae.
  • An unusual feature of dinoflagellate ultrastructure is that the chromosomes contain little, if any, histone protein, and exist almost permanently in the condensed form.
  • Some tropical species of dinoflagellate emit light, the only algae to do so.
  • Due to an enzyme–substrate (luciferin–luciferase) interaction, this can cause a spectacular glow in the water at night, especially when the water is disturbed, for example by a ship.
  • Bioluminescence of this kind has proved to be a useful ‘tagging’ system for cells in biological research.
  • Other marine dinoflagellates can produce metabolites that act as nerve toxins to higher animals.
  • Shellfish such as mussels and oysters can concentrate these with no harm to themselves, but they can be fatal to humans who consume them.
  • Sometimes, when conditions are highly favourable, an explosion of growth results in the development of huge ‘red tides’ of dinoflagellates in coastal waters.
  • This produces a build-up of toxins, and may lead to the death of massive numbers of fish and other marine life.
  • The greatly increased incidence of these blooms in recent decades is probably due to pollution by fertilizers containing nitrates and phosphates.
  • Reproduction by asexual means involves binary fission.
  • In armoured forms, the theca may be shed before cell division, or split along suture lines; in either case, daughter cells must regenerate the missing sections.
  • Sexual reproduction is known to occur in some dinoflagellates, and is probably more widespread.
  • Gametes produced by mitosis fuse to produce a diploid zygote; this undergoes meiosis to reinstate the haploid condition in the offspring.
  • In some species we see isogamy, the fusion of identical, motile gametes, while in others, anisogamy occurs, in which gametes of dissimilar size fuse.
  • Fusion may occur between genetically identical gametes, or only when the gametes come from genetically distinct populations.

Why would an alga want to glow in the dark?

  • The production of bioluminescence by several dinoflagellate species is thought to have a protective function.
  • Such algae are the natural prey of copepods, tiny crustaceans found in astronomical numbers as part of the zooplankton.
  • The bioluminescence could have an effect directly, by acting as a warning signal to the copepods, or indirectly, by making those crustaceans which had consumed glowing algae much more conspicuous to their own predators.

Diatoms

  • The diatoms, which belong to the division Chrysophyta (the golden-brown algae), make up the majority of phytoplankton in marine food chains, and as such are the most important group of algal protists in terms of photosynthetic production.
  • Over 10 000 species of diatom are recognised, but some experts feel that the real number is many times greater than this. 

Diatoms

  • As with the dinoflagellates, chlorophylls a and c are present, but not chlorophyll b.
  • Their colour is due to carotenoids and xanthophylls (particularly fucoxanthin) masking the chlorophyll.
  • Diatoms have their cells surrounded by a silica-based shell known as a frustule, composed of two overlapping halves (the epitheca and the hypotheca).
  • Microbiologists are rarely able to resist the temptation to liken this structure to that of a petri dish, and with good reason.
  • With the electron microscope it can be seen that the frustule is perforated with numerous tiny pores that connect the protoplast of the cell with the outside environment.
  • Diatom classification is based almost entirely on the shape and pattern of these shells, which are uniform for a particular species, and often have a very striking appearance.
  • When diatoms die, their shells fall to the bottom of the sea, and can accumulate in thick layers where they represent a valuable mineral resource.
  • This fine, light material (diatomaceous earth) has a number of applications, for example in filtration systems, and also as a light abrasive in products such as silver polish or toothpaste.
  • Reproduction is usually asexual by binary fission, but a sexual phase with the production of haploid gametes can occur.
  • Chrysophytes are unusual among the three primitive groups of algae in that they are diploid.
  • In diatoms, asexual reproduction involves mitotic cell division, with each daughter cell receiving one half of the parental frustule, and synthesizing a new one to complement it.

Asexual reproduction in diatoms.

  • The newly formed half, however, always acts as the hypotheca (lower half) of the new cell; consequently, one in two daughter cells will be slightly smaller than the parent, an effect which is heightened over a number of generations.
  • This process continues until a critical size is reached, and the diatoms undergo a phase of sexual reproduction, which re-establishes the normal frustule size.
  • In species whose frustules have a degree of elasticity, the daughter cells are able to expand, and the problem of cell diminution does not arise.
  • In bilaterally symmetrical (long, thin) diatoms, meiosis in parental cells produces identical, non-motile gametes, which fuse to form a zygote.
  • The radially symmetrical (round) forms provide an example of the third pattern of gamete fusion found in the algae: oogamy.
  • Here, there is a clear distinction between the small, motile sperm cell and the larger, immobile egg cell.
  • Both are produced by meiosis in the parental cell, followed, in the case of the male, by several rounds of mitosis, to give a large number of sperm cells.

Dinoflagellate and Diatoms

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