Algae

:This article is about an organism. See algae programming language for a programming language in computing.

Relationships among algal groups

Prokaryotic algae

Traditionally the cyanobacteria have been included among the algae, referred to as the cyanophytes or Blue-green Algae, though some recent treatises on algae specifically exclude them. Cyanobacteria is one of the first groups of living things to appear in the fossil record, dating back some 3800 million years ago (Precambrian) when they may have played a major role in creating Earth's oxygen atmosphere. They have a prokaryotic cell structure typical of bacteria and conduct photosynthesis directly within the cytoplasm, rather than in specialized organelles.

Related Topics:
Cyanobacteria - Fossil record - Precambrian - Oxygen - Prokaryotic - Cytoplasm

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Eukaryotic algae

All other algae are eukaryotes and conduct photosynthesis within membrane-bound structures (organelles) called chloroplasts. Chloroplasts contain DNA and are similar in structure to cyanobacteria, presumably representing reduced cyanobacterial endosymbionts. The exact nature of the chloroplasts is different among the different lines of algae, possibly reflecting different endosymbiotic events. There are three groups that have primary chloroplasts:

Related Topics:
Eukaryote - Chloroplast - Endosymbionts

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Green algae, together with higher plants
• Red algae
• Glaucophytes

In these groups the chloroplast is surrounded by two membranes, both now thought to come from the chloroplast. The chloroplasts of red algae have a more or less typical cyanobacterial pigmentation, while the green algae and higher plants have chloroplasts with chlorophyll a and b, the latter found in some cyanobacteria but not most. There is reasonably solid evidence that these three groups originated from a common pigmented ancestor; i.e., chloroplasts developed in a single endosymbiotic event.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Two other groups have green chloroplasts containing chlorophyll b, the euglenids and chlorarachniophytes. These are surrounded by three and four membranes, respectively, and were probably retained from an ingested green alga. Those of the chlorarchniophytes contain a small nucleomorph, which is the remnant of the alga's nucleus. It has been suggested that the euglenid chloroplasts only have three membranes because they were acquired through myzocytosis rather than phagocytosis.

Related Topics:
Euglenid - Chlorarachniophyte - Nucleus - Myzocytosis - Phagocytosis

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The remaining algae all have chloroplasts containing chlorophylls a and c. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with the red algae suggest a relationship there. These groups include:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Heterokonts (e.g., golden algae, diatoms, brown algae)
• Haptophytes (e.g., coccolithophores)
• Cryptomonads
• Dinoflagellates

In the first three of these groups (Chromista), the chloroplast has four membranes, retaining a nucleomorph in cryptomonads, and it now appears that they share a common pigmented ancestor. The typical dinoflagellate chloroplast has three membranes, but there is considerable diversity in chloroplasts among the group, some members presumably having acquired theirs from other sources. The Apicomplexa, a group of closely related parasites, also have plastids though not actual chloroplasts, which appear to have a common origin with those of the dinoflagellates.

Related Topics:
Chromista - Apicomplexa - Plastid

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Note many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost them entirely.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~