Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 36

Algae are a large group of eukaryotic organisms (Graham et al., 2008). Most of them are pigmented and carry out oxygenic photosynthesis. They are either unicellular, and therefore usually microscopic in size, or colonial, containing many cells. The colonial algae are macroscopic in size and can sometimes form huge structures that may look like plants but are quite distinct. There are many different groups of algae, which are usually distinguished by their pigment compositions and morphological features. In aquatic habitats, algae are the dominant photosynthetic life forms, although they are also found on land, including habitats as seemingly unlikely as the surface of snowfields and the hairs of polar bears. Many competing systems of algal classification are in use. We will not attempt to enumerate all the myriad types, but will instead just list some of the most commonly studied types in terms of their photosynthetic properties. In general, the algae all have rather similar electron transport chains, but widely variable antenna complexes from one group to another.

The green algae (chlorophytes) are the most widely studied, because their properties are the closest to higher plants. They contain both chlorophyll a and chlorophyll b as photopigments. They are certainly the evolutionary precursors to plants. The red algae (rhodophytes) are mostly marine organisms that contain chlorophyll a and phycobilisomes, antenna complexes similar to those found in most cyanobacteria. They often have a complex life cycle. The green and red algae, plus one other group (the glaucophytes), are thought to be primary endosymbionts, in that they arose from a single endosymbiotic event. All other algal groups are the result of additional endosymbiotic events, in which a eukaryotic alga was itself incorporated into an organism to form a new type of chimeric cell that in many cases retained the photosynthetic capability of the endosymbiont. Most of these secondary and in some cases tertiary endosymbiotic events involved the red algal line and the complex history of this group includes a dizzying array of gain, loss, and regain of photosynthesis. Many of these organisms contain chlorophyll c as an accessory pigment. The chromoalveolate hypothesis proposes that most of the non‐green eukaryotic algae have been derived from secondary endosymbiosis of red algae and subsequent events (Cavalier‐Smith, 1999).

Figure 2.7 Electron microscopic tomographic surface representation of the thylakoid network within a ruptured chloroplast. The different views are of the same thylakoid network from different angles.

Source: Daum and Kühlbrandt (2011). Reproduced with permission of Oxford University Press.