ABSTRACT
Eurkaryotic algae with secondary plastids can be found in the clade of photobiotic heterokonts, which consists of about 18 algal classes differing in cell structure and pigmentation. The major groups in this clade are the fucoxanthin-containing diatoms, the brown algae, chrysophytes (golden algae), the greenish Xanthophytes and Eustigmatophytes, and finally the phycobilin-containing Cryptophytes. Their plastids derived from a primitive red alga, which was engulfed by a nonphotosynthetic, animal-like host cell. During evolution, large parts of the genome of the autotrophic partner were transferred to the nucleus of the host by lateral gene transfer, which resulted in a sophisticated regulation of transcription and protein translocation apparatus in these cells. Since the host nucleus is phylogenetically far from that of green algae or higher plants, the regulation of the metabolic pathways is quite different from green cells. This chapter reviews the differences in metabolic regulation of the photosynthetic machinery and surprising modifications in the cellular compartmentalization of the major metabolic pathways in these cells. In this chapter, we concentrate on diatoms because, in this taxon, the present state of the art is much more advanced compared to the other groups. However, it has to be emphasized that, up to now, it is not clear if the metabolic situation found in diatom plastids can be extended to the other groups of the heterokontic clade.
