Glaucophyta

As already described in previous sections these algae possess inclusion termed cyanelles that are probably symbiotic cyanobacteria functioning as chloroplasts. Each cyanelle, surrounded by a reduced peptoglycan cell wall (except in Glaucosphaera sp.), is enclosed in a vesicle of the host cytoplasm. Cyanelles do not fix molecular nitrogen in contrast with cyanobacteria; they contain polyphospate granules and a conspicuous central carboxisome similar to the pyrenoids of other algae. The thylakoid are not stacked but they are single and equidistant with a concentric arrangement. Cyanelle pigments are chlorophyll a and β-carotene, which represents the main carotenoid. Interthylakoidal phycobilisomes contain allophycocyanin and phycocianin. Phycoerytrin is absent from Glaucophyta but phycoerythrocyanin can be found in some species.


Rhodophyta
Ultrastructurally, red algal chloroplasts are composed of a double-membrane envelope inside of which are one or more parallel, thylakoidal photosynthetic lamellae. These chloroplasts are not associated with the endoplasmic reticulum, a feature shared with Glaucophyta and Chlorophyta. Encircling thylakoids are present in all Florideophyceae and in some taxa of Bangiophyceae while the thylakoids in the other red algae lie equidistant and are single, that is, not stacked, unlike any other group of eukaryotic algae (except Glaucophyta), and typically oriented parallel to each other. All thylakoids have phycobilisomes attached to their stromal surface, which contain the accessory phycobiliprotein pigments, that is, allophycocianin, phycocyanin, and five forms of phycoerythrin (Figure 2.77). Chlorophyll a is the only chlorophyll present in the thylakoid membrane, together with carotenoids such as β-carotene and lutein. Plastid number, shape, and position (many, discoid, and parietal) is rather uniform throughout the Florideophyceae, and pyrenoids may or may not be present. A single stellate plastid with a central pyrenoid is commonly associated with bangiophycidean red algae, such as Phorphyridium. DNA is organized into numerous nucleoids scattered throughout the chloroplast.

Heterokontophyta

Some ultrastructural features of the chloroplast compartments of these algae are common to all the seven classes of the division, with few exceptions. Four membranes surround the chloroplasts, the outer two being the chloroplast endoplasmic reticulum and the inner two being the chloroplast envelope. When the chloroplasts are located close to the nucleus, the chloroplast endoplasmic reticulum is continuous with the nuclear envelope. In the Xanthophyceae, this connection is not the rule. Thylakoids are grouped into lamellae of three, which are two in some Raphidophyceae, with varying degrees of coherence depending on the species. Thylakoids from adjacent lamellae frequently interconnect across the stroma. In all the classes, with the exception of Eustigmatophyceae and Chattonella (Raphidophyceae), one lamella runs around the periphery of the chloroplast beneath the chloroplast envelope, enclosing all the other lamellae. The lamella is called girdle lamella.

One or more plate-like (Chrysophyceae, Figure 2.78a and 2.78b; Eustigmatophyceae, Figure 2.79a and b), discoid (Xanthophyceae, Raphidophyceae, and Dictyochophyceae) or ribbon-like (Phaeophyceae), plastids are typically present, often lobed, parietal or located in close connection with the nucleus. In the Bacillariophyceae, chloroplasts are the most conspicuous feature, and their number and shape are consistent features of taxonomic importance. They may be rounded or lobed discs or large plate-like with or without lobed margins, and may range from one to two, four or more. A typical centric diatom has many disc-shaped plastids, arranged close to the periphery surrounding a large central vacuole or scattered throughout the cell. The raphid diatoms tend to have large chloroplasts (one to four) lying along the girdle with central nucleus flanked by two large vacuoles.


The chloroplast DNA is ring-shaped and located in the region between the girdle lamella and the others in all the classes, with the exception of the Eustigmatophyceae, where it is organized into many dot-like nucleoids, which may be united in a sort of reticulum. The main photosynthetic pigment is chlorophyll a, which is the only chlorophyll present in the Eustigmatophyceae. In addition, chlorophylls of the c group occur, both c₁ and c₂ (Chrysophyceae, Rhaphidophyceae, Phaeophyceae, Dictyocophyceae, and in only extremely low concentrations in Xanthophyceae) or only c₂ (Bacillariophyceae). The most important accessory pigment is fucoxanthin in Chrysophyceae, Bacillariophyceae, Dictyocophyceae, and Phaeophyceae, violaxanthin in Eustigmatophyceae, and vaucheriaxanthin in Xanthophyceae. Other accessory pigments are β-carotene and xanthophylls such as diadinoxanthin, heteroxanthin, vaucheriaxanthin, antheraxanthin, and lutein. In the Raphidophyceae marine and freshwater species differ in their accessory pigments, marine species possessing mainly β-carotene, fucoxanthin, and violaxanthin, and freshwater species having β-carotene, diadinoxanthin, heteroxanthin, and vaucheriaxanthin.

Pyrenoids are present in all the classes, except in the zoospores of the Eustigmatophyceae and in the freshwater species of Raphidophyceae. They are of a semi-immersed type, attached to the inner face of the chloroplast, pear-shaped in the Phaeophyceae, or stalked in the Eustigmatophyceae. They can be one or many (Bacillariophyceae and Phaeophyceae). No storage material or capping vesicles have been found to be associated with pyrenoids, but lipid or oil droplets normally distributed randomly in the chloroplast matrix are often concentrated at the periphery of the pyrenoid.