HETEROKONTOPHYTA, BACILLARIOPHYCEAE	377

Fig. 17.12 Pinnularia viridis. Left: Scanning electron micrograph of a cross section of a cleaned valve. Right: Atomic force micrograph of the surface of a cleaved area of a valve showing the 40–45 nm silica spheres that compose the valve. (From Crawford et al., 2001.)

Fig. 17.13 Schematic structure of silaffin and the polyamine side chains of silaffin, two molecules that control the precipitation of silica spheres in diatoms. (Adapted from Kroger et al., 1999.)

living diatoms appears to be protected by the layer of organic material that surrounded the frustule. After death, the silica usually dissolves (Bidle and Azam, 1999). In certain environments, however, the walls of planktonic diatoms may settle and accumulate on the bottom faster than they dissolve, thereby forming diatomaceous ooze.

Extracellular mucilage, biolfouling, and gliding

Diatoms produce five types of mucilaginous aggregation: (1) tubes, (2) pads, (3) stalks, (4) fibrils,

and (5) adhering films (Figs. 17.16, 17.17, 17.18) (Hoagland et al., 1993). Stalks, pads, and tubes are more common in freshwater diatoms than in marine diatoms (Staats et al., 1999).

A substantial part of the carbon fixed by benthic diatoms is secreted as extracellular mucilages (de Brouwer and Stal, 2002). The biofilms produced by the diatoms play an important part in stabilizing mud flats where the diatoms grow. In the open ocean, the mucilage produced by planktonic diatoms is the triggering mechanism in the formation of “marine snow”, which is sticky gelatinous aggregations of bacteria and algae (up to 100 mm in diameter) that create serious problems for fisheries (Alcoverro et al., 2000). The wind drives the marine snow onto the beaches,