Cyanobacteria developed a metabolic process, the photosynthesis, which exploits the energy of visible light to oxidize water and simultaneously reduces CO2 to organic carbon represented by (CH2O)n using light energy as a substrate and chlorophyll a as a requisite catalytic agent. Formally oxygenic photosynthesis can be summarized as:
All other oxygen producing algae are eukaryotic, that is, they contain internal organelles, including a nucleus, one or more chloroplasts, one or more mitochondria, and, most importantly, in many cases they contain a membrane-bound storage compartment or vacuole. The three major algal lineages of primary plastids are the Glaucophyta lineage, the Chlorophyta lineage, and the Rhodopyta lineage (Figure 1.48).
Glaucophyta lineage occupies a key position in the evolution of plastids. Unlike other plastids, the plastids of glaucophytes retain the remnant of a Gram-negative bacterial cell wall of the type found in cyanobacteria, with a thin peptidoglycan cell wall and cyanobacterium-like pigmentation that clearly indicate its cyanobacterial ancestry. In fact, the Cyanophora paradoxa plastid genome shows the same reduction as other plastids when compared with free-living cyanobacteria (it is 136 kb and contains 191 genes). The peptidoglycan cell wall of the plastid is thus a feature retained from their free-living cyanobacterial ancestor. In this context, the Glaucophyta are remarkable only for their retention of an ancestral character present in neither green nor red plastids. No certain case of a secondary plastid derived from Glaucophyta is known.
Since that time, however, a second group of eukaryotes has risen to ecological prominence; that group is commonly called the “red lineage.” The plastids of the red algae (Rhodophyta) constitute the third primary plastid lineage. Like the green algae, the red algae are an ancient group in the fossil record, and some of the oldest fossils interpreted as being of eukaryotic origin are often referred to the red algae, although clearly these organisms were very different from any extant alga. Like those of green algae, the plastids of red algae are surrounded by two membranes. However, they are pigmented with chlorophyll a and phycobiliproteins, which are organized into phycobilisomes. Phycobilisomes are relatively large light-harvesting pigment/protein complexes that are water-soluble and attached to the surface of the thylakoid membrane. Thylakoids with phycobilisomes do not form stacks like those in other plastids, and consequently the plastids of red algae (and glaucophytes) bear an obvious ultrastructural resemblance to cyanobacteria.
A number of algal groups have secondary plastids derived from those of red algae, including several with distinctive pigmentation. The cryptomonads (Cryptophyta) were the first group in which secondary plastids were recognized on the basis of their complex four membrane structure. Like red algae, they have chlorophyll a and phycobiliproteins, but these are distributed in the intrathylakoidal space rather than in the phycobilisomes found in red algae, Glaucophyta, and Cyanophyta. In addition, cryptomonads possess a second type of chlorophyll, chlorophyll c, which is found in the remaining red lineage plastids. These groups, which include the Heterokontophyta (including kelps, diatoms, chrysophytes, and related groups), Haptophyta (the coccolithophorids), and probably those dinoflagellates (Dinophyta) pigmented with peridinin, have chlorophylls a and c, along with a variety of carotenoids, for pigmentation. Stacked thylakoids are found in those lineages (including the cryptomonads) that lack phycobilisomes. The derivation of chlorophyll c containing plastids from the red algal lineage is still somewhat conjectural, but recent analyses of both gene sequences and gene content are consistent with this conclusion.
A few groups of dinoflagellates have plastids now recognized to be derived from serial secondary endosymbiosis (the uptake of a new primary plastid-containing endosymbiont) such as Lepidodinium spp. or tertiary endosymbiosis (the uptake of the secondary plastid-containing endosymbiont), such as Dinophysis, Karenia, and Kryptoperidinium.
All these groups are comparatively modern organisms; indeed, the rise of dinoflagellates and coccolithophorids approximately parallels the rise of dinosaurs, while the rise of diatoms approximates the rise of mammals in the Cenozoic. The burial and subsequent diagenesis of organic carbon produced primarily by members of the red lineage in shallow seas in the Jurassic period provide the source rocks for most of the petroleum reservoirs that have been exploited for the past century by humans.
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