In freshwater habitats, diatoms often comprise the dominant algal flora in thermal waters between 30 and 40 °C. Fairchild and Sheriden (1974) showed that Achnanthes exigua (Fig. 17.33(f )) isolated from a hot spring showed optimum photosynthesis at 42 °C, with maximum and minimum temperatures for growth at 44 and 10 °C, respectively – characteristics of a thermophilic organism. This particular diatom is usually associated with an

algal mat covered by a shallow layer of water in these hot springs.

Fossil diatoms

The fossil record of diatoms begins in the Jurassic Period (185 million years ago) with diatoms becoming more common in the mid-Cretaceous

(Medlin et al., 1997). The fossil record indicates that centric diatoms evolved before pinnate diatoms (Round et al., 1990). Another way of dating diatoms is by molecular fossils. Some diatom genera (Rhizosolenia, Fig. 17.33(c); Haslea, Navicula, Figs. 17.2(d), 17.16, 17.26(a), 17.33(b), 17.35; and Pleurosigma) contain unique isoprenoids called highly branched isoprenoid (HBI) alkenes that have a T-branch formed by attachment of a C10 isoprenoid unit to a C15 isoprenoid unit (Fig. 17.37) (Damste et al., 2004). The occurrence of these alkenes in sediments beginning 90 million years ago parallels the widespread explosion of diatoms in the marine environment at this time.

After death of diatom cells, the frustule usually dissolves after bacteria degrade the organic casing around the frustule (Bidle and Azam, 1999). However, under certain circumstances, the frustules remain intact and accumulate at the bottom of the water (Fig. 17.38). Where conditions are exceptionally favorable and long continued, such accumulations may reach considerable thickness. Deposits of fossil diatoms, known as diatomaceous earth or kieselguhr, are found in various parts of the world. None of these deposits originated earlier than the Cretaceous; some diatomaceous earths originated in freshwaters, others in the ocean. However, known deposits of marine species are found inland and above the ocean as a result of geological changes. The best-known and most extensive deposits of marine species are those at Lompoc, California, where the beds are several kilometers in extent and 200 m in thickness (Fig. 17.39). Most of the commercially available diatomaceous earth comes from California, where the deposits are worked as open quar-

ries. In quarrying, the overburden of soil is removed, and the diatomaceous earth is mined. Diatomaceous earth is also obtained from lakes in Florida by dredging with a suction pump and carrying the material through sluiceways to settling tanks. The material from some deposits can be used directly; that from other deposits must be incinerated to remove organic substances.

The industrial uses of diatomaceous earth are varied. One of the first uses was as a mild abrasive in toothpaste and metal polishes. Diatomaceous earth was also used as an absorbent for liquid nitroglycerin to make dynamite that could be transported with comparative safety. The inert medium used in the present-day manufacture of dynamite is wood meal. Probably the most extensive industrial use of diatomaceous earth is in the filtration of liquids, especially those of sugar refineries. Another major use is in the insulation of boilers, blast furnaces, and other places where a high temperature is maintained.

Analysis of sediments containing remains of diatoms can provide information on past environmental conditions of lakes (Reavic et al., 1998; Clavero et al., 2000). This technique has been applied to late glacial and postglacial diatom deposits in the English Lake District (Round, 1957, 1961). During the end of the last glacial period, there were few diatoms present, with the sediments being mostly of inorganic origin, indicating that the lakes initially were very oligotrophic and lacked planktonic diatoms. This period was followed by diatoms characteristic of melt waters under semi-arctic conditions (Melosira arenaria, Fig. 17.40(a). Cyclotella antique, Fig. 17.40(b); Gyrosigma, Fig. 17.40(d); Cymatopleura, Fig. 17.40(e);