390 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

nuclei and chloroplasts from the gametes. Most of the auxospore volume is occupied by a single vacuole. The auxospores germinate to produce vegetative cells of maximal cell size. Physiologically sexual reproduction is isogamous because the cells of one clone are passive (female) remaining within the parent frustule, while the cells of the other clone are active (male), moving to the passive gametes of the other clone.

Chemical defense against predation

Diatoms are preferred food for invertebrates such as copepods. Some diatoms (e.g., Phaeodactylum tricornutum, Skeletonema pseudocostatum) have evolved a mechanism to reduce predation by releasing chemicals that reduces the fecundity of the next generations of invertebrates. Diatoms cells contain large quantities of highly unsaturated fatty acids such as eicosanoic acid (Fig. 17.30(a)) in vesicles in the cytoplasm. Death of cells during feeding by invertebrates results in the release of the unsaturated fatty acids into the seawater. Upon release, phospholipases convert the unsaturated fatty acids eicosatetraenoic acid and eicosapentaenoic acid into the unsaturated short-chain aldehydes 2,4- decadienal and 2,4,7-decatrienal (Fig. 17.30(b))

(Miralto et al., 1999; Pohnert, 2002; Pohnert and Boland, 2002; Pohnert et al., 2002). These shortchain fatty-acid aldehydes are toxic to developmental stages of a range of invertebrates including copepods, sea urchins, polychaetes, and ascidians (although similar saturated fatty acids with a terminal aldehyde group, such as tridecanal (Fig. 17.30(b)), are not toxic to copepod eggs) (Caldwell et al., 2004). They appear to work by affecting cell membrane characteristics and microtubule and microfilament stability (Casotti et al., 2005). Future generations of grazers are sabotaged, encouraging the survivability of diatom populations.

Eicosanoic acid in the diatom cell is not toxic to the diatom cell; the chemical is only toxic after the unsaturated fatty acid is released into the environment and converted into an aldehyde. Thus, the diatom cells are able to invest their metabolic energy into the formation of unsaturated fatty acids utilized in normal cell activities. When the cells are damaged, the unsaturated fatty acids are released and converted into toxic compounds that reduce the numbers of the next generation of grazers (Pohnert, 2002). Interestingly, the released aldehydes also are toxic to diatoms (Casotti et al., 2005). However, the diatoms cells are being destroyed by the grazing and are already out of the gene pool.

Fig. 17.30 (a) The reaction by which a non-toxic highly unsaturated fatty acid is converted by a phospholipase into a reactive unsaturated fatty-acid aldehyde that is toxic to invertebrates. The reaction is initiated by wounding of the diatom cell. (b) Two unsaturated fatty-acid aldehydes, decatrienal and decadienal, that are toxic to invertebrates and a saturated fatty-acid aldehyde, tridecanal, that is not toxic.