Mireille Consalvey PhD Thesis - University of St Andrews

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1. Introduction
I. I. General
Diatoms are a unicellular algal class (Bacillariophyceae) within the
kingdom Protista and there are estimated to be at least 200 000 species (Mann
and Droop 1996). Diatoms are characterised by pectin cell walls heavily
impregnated with ornamented silica. They consist of two halves, one of which
fits inside the other. Each half is tenned a valve and the two valves are
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linked by
a series of silica structures called girdle elements. The whole protoplast encasing
structure is called the frustule. Diatoms can be divided into two major groups:
centric and pennatc. The majority of free-living motile forms belong to the
Biraphidineae (possess raphes systems on both valves) arc always pennate and
have bilateral symmetry (Round 1971). The raphe structure is synonymous with
the ability of these cells to adhere and glide whilst leaving behind a train of
mucilage (Poulsen et al. 1999). The evolution of motility has contnbuted to the
ubiquitous success of pennate diatoms.
Diatoms first became abundant in the fossil record 100 million years ago,
in the Cretaceous period. Round (1981), Round and Crawford (1981,1984) and
Mann and Marchaant (1989) propose that diatoms evolved from a scaly ancestor.
Their scheme suggests that two major scales evolved into dome shaped valves
and others became the girdle bands. The radiation from this point is unknown
(Medlin et al. 1993) and many of the species present in the late Cretaceous are
morphologically similar to modem counterparts. The centric diatoms evolved in
the early Cretaceous period (Gersonde and Harwood 1990) with pennate diatoms
following in the late Cretaceous (Harwood 1988). The first pennate diatoms were
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