origin and evolution of algae

Most algologists believe that eukaryotic chloroplasts are monophyletic, and that they arose once from a cyanobacterial ancestor. In this hypothesis, the ancient ancestral eukaryote took up a cyanobacterium and transformed it into a chloroplast. This fact is supported by eukaryotic algae, Euglena and Chlamydomonas. Both of these are very distantly related on the basis of their cytoplasmic 18S rRNA sequence but closely related on the basis of chloroplast 16S rRNA sequences.

Hoek and co-worker(1992) based on nucleotide sequences of both cytoplasmic and chloroplast rRNAs summarized the phylogenetic reflections on the algae. The following are important main evolutionary events:

Event 1: The main prey of the primitive phagotrophic flagellates of the Precambrian were probably Cyanobacteria. Initially, preys were always digested but in one of the last major lineages, the cyanobacteria were taken up but not digested and being transformed instead into photosynthetic organelles. These organelles had a reduced genome containing inverted repeats. In this way, the first primaeval photosynthetic flagellate arose. The present day Glaucophyte may perhaps be the direct, little changed descendants of the primaeval flagellates. The Glaucophyte chloroplasts are in several ways intermediate between cyanobacteria and the chloroplasts of other algae. The primitive nature of the glaucophyte chloroplast is confirmed by the 16S rRNA phylogenetic tree of cyanobacteria and chloroplasts. More recent 16S rRNA data on the chloroplasts of Rhodophyta, Cryptophyte and Heterokontophyte also support the idea that the glaucophyte chloroplast is primitive and its position is at the divergence of a vestigial peptidoglycan wall around the chloroplast, which of the Glaucophyte is the presence of a vestigial peptidoglycan wall around the chloroplast, which is considered as a relic  or vestige of its origin as a cyanobacterium.

Event 2: The Rhodophyta arose from a primaeval glaucophytes like alga, through the loss of flagella. They retained several primitive features of cyanobacteria and Glaucophyte, such as equidistant thylakoids and the presence of phycobilisomes. Thus, red algae are considered to have arisen through the evolutionary transformation of the photosynthetic eukaryote, and not through an independent symbiotic event.

Event 3: The Chlorophyta arose from the primaeval glaucophytes by the transformation of the photosynthetic apparatus from one based on chlorophyll a and phycobilin's, to one based on chlorophyll a and chlorophyll b. Phycobilisomes were lost and stacking of the thylakoids became possible. Flagella were retained, except in cases where these arose secondarily. Chloroplast genes coding for RuBP carboxylase/oxygenase were transferred to nuclear genes.

Thus, green algae are thought to have arisen through evolutionary transformation of pre-existing eukaryotic alga and not through separate endosymbiosis.

Event 4: The higher plants evolved from the green algae between 400 and 500 million years ago as highly specialized lines adapted to terrestrial life.

Event 5: The photosynthetic members of Heterokontophyte may have evolved when a primaeval heterotrophic  heterokontophyte ingested one of the early glaucophyte-like algae and incorporated it into its cell, first as endosymbiont and later as a chloroplast. The presence of four membrane surrounding the chloroplast is consistent with this idea. The photosynthetic apparatus of the glaucophytes, which is based on chlorophyll a and phycobilin's, has been replaced by one based on chlorophylls a and c. The inverted repeats of the chloroplast genome have been retained but in extremely  compact form.

Event 6: The Dinophyte with their triple chloroplast envelope evolved from another endosymbiosis, involving the independent capture and transformation of another primaeval glycophytic by an ancient heterotrophic dinophyte. The inner most two membranes of the chloroplast represents the standard double membrane of the chloroplast while the outer membrane may represent the host's food vacuole membrane.

Event 7: The evolution of Haptophyte might have involved a third independent incorporation of a primaeval glaucophyte. The chloroplasts are surrounded by four membranes. Similarities between chloroplast pigments, between Haptophytes and Heterokontophytes reveal that transformation of photosynthetic apparatus may have taken place before their divergence into two lines.

Event 8: The ancient heterotrophic cryptophytes by independent engulfment of a primaeval glaucophyte like alga with the B-purple bacterium type of rubisco genes may have been involved in the origin of the photoautotrophic cryptophyte. The chloroplasts of cryptophyte are each surrounded by four membranes, of which inner most two membranes are interpreted as the standard double chloroplast membrane while the outer two are thought to represent the plasmalemma of the ingested glaucophyte and the food vacuole membrane of the host cryptophyte. A recent phylogenetic analysis of the 18S rRNA genes in the nucleus nucleomorph have confirmed that each cryptophyte cell is chimaera, containing two unrelated eukaryotes.

Event 9: Incorporation and transformation of an ancient chlorophyte by heterotrophic euglenophyte may have given rise to photoautotrophic euglenophyte. The chloroplasts are surrounded by three membranes, may be interpreted as two standard membranes of the chloroplast and one vacuolar membrane of the host. The 16S rRNA studies supports the idea that euglenophyte chloroplasts have been derived from chlorophyte chloroplasts.

Event 10: The dinophyte with heterokontophyte photosynthetic pigments nay have been evolved by the ingestion of a heterokontophyte by a heterotrophic dinophyte. The endosymbiont, although fully integrated with a dinophyte cell, is still recognizable as a heterokontophyte; as it has its own complete nucleus and even mitochondria. The chloroplasts are surrounded by five membranes being interpreted as the standard double chloroplast membrane, the inner and outer membrane of chloroplast ER and the host food vacuole membrane.

Hoek and co-worker believe that the Rhodophyta, Glaucophyte and Chlorophyta probably evolved from a common eukaryotic ancestor which was photoautotrophic flagellate. The remaining algal phyla arose from different heterotrophic flagellate ancestors, through the ingestion and incorporation of a primaeval eukaryotic alga.

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