Diatoms

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Diatoms Kingdom Protista DivisionChrysophyta Cla
ss Bacillariophyceae (Unicellular algae)

• It has two valves composed of silica (SiO2) with
  average size 10 to 100 µm in length.

Images for diatoms
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Introduction

• Diatoms are one of the most important and
  abundant components of marine phytoplankton with
  two valves (frustules) composed mainly of of
  silica (SiO2).
• Very important for palaeoclimatic reconstruction
  (environmental conditions), marine biology and
  ecology, and industry.
• Inhabit all aquatic environments. They occur in
  the body of the water as plankton or on the
  bottom where they may be attached to plants or
  rocks or sand particles, or may be free living
  and able to move between particles of the
  substratum. In the open ocean, they live only in
  upper 200 m of the surface water.

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LIVING DIATOM
They occur living in a single cell or colonial
attached together by gelatinous filaments. When
observed as larger and larger masses, the color
takes on a darker color, often deep brown or
black. Each cell possesses yellow, olive or
golden-brown photosynthetic granules, a central
vacuole and a nucleus, although it lacks flagella
and pseudopodia.
Colony of pennate diatoms (Asterionella sp.)
Diatoms colonies, long chains
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• Cell contents of living diatom
• Protoplasm occurs within the silicified
  frustule, containing a nucleus, mitochondria,
  plastids, and other various organelles.
• Cytoplasm a colorless plasma on the inner side
  of the frustule and cell wall.
• Nucleus it is at the center of the diatom. It is
  often spherical or lenticular and encloses the
  chromosomes and nucleoli.
• Plastids they are organelles similar to
  chlorophyll in their photosynthetic abilities.
• Volutin Volutin molecules are nitrogen reserves.
  
• Oil Drops they are actually sugar and starch in
  globular form. They are located in the cytoplasm.

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Structure and morphology of a diatom
Diatom frustule is similar to pillbox or a petri
dish. It has two interlocking halves , one larger
than the other (called "epitheca" and
"hypotheca", respectively). It can vary greatly
in shape, ranging from box-shaped to cylindrical,
symmetrical as well as asymmetrical.
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• Diatom frustule consists of
• Epitheca Larger older valve of the frustule
  with the girdle elements (epicingulum) connected
  with it.
• Hypotheca Smaller younger valve of the
  frustule with the girdle elements (hypocingulum)
  connected with it.
• Connective Zone This zone is composed of the
  overlapping girdle elements of the diatom (the
  epicingulum and hypocingulum). It acts to connect
  the valves, forming sutures that still allow the
  two valves to move apart or towards one another.
  
• Septa Partitions that are formed within the
  valves. It can be used to characterize diatoms.
• Raphe occurs in pennate diatoms as an elongated
  fissure in a valve. The raphe is used by diatom
  for movement. It is important for identification.

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Raphes in the Pennate diatoms
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DIATOM SYMMETRY (PLANES)

• Bilaterally symmetrical around the apical and
  transapical axes (Fig. 3, Fig. 4A).
• Bilaterally symmetrical, around the transapical
  (Fig. 4B) or the apical (Fig. 4C) axis.
• Rotational symmetry occurs in some bipolar genera
  that have no planes of symmetry (Fig. 4D).
• Circular symmetry occurs in large group of genera
  (Fig. 4E).

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DIATOM ORNAMENTATION

• Ornamentation is important for diatom
  identification.
• Valve surface is usually covered with striations,
  pores, spines, punctations or raphes (an
  elongated fissure through the valve wall,
  involved in cell motility).

Centric diatoms a) Melosira, b) Coscinodiscus,
c) Actinoptychus and Thalassiosira.
Some ornamented diatoms
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TAXONOMY
Diatoms divided into two main orders

• Order Pennales
• (pennate diatoms)
• bilaterally symmetric
• benthic marine and fresh water communities
• Example, Genus Pinnularia

• Order Centrales
• (centric diatoms)
• Radially symmetric
• dominate marine planktonic communities
• Example, Genus Melosira

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• Members of both classes may be, in general, found
  in either fresh or salt water, though centrate
  forms tend to predominate in marine habitats,
  while pennate diatoms are more typical of
  freshwater environments.
• The Pennate diatoms are a monophyletic group
  derived from centric diatoms. Pennate diatoms
  have long been viewed as a derived group, and
  molecular studies agree with this interpretation.
  Centric diatoms appear in the fossil record about
  120 ma, while pennate diatoms do not appear until
  about 70 ma.