Evolutionary relationships among cyanobacteria, algae and plants: Revisited in the light of Darwinism

Abstract

Algae represent an ancient group of plant like organisms, comprising a heterogeneous, polyphyletic assemblage of prokaryotes and eukaryotes, which have been primarily classified on the basis of their pigments, storage food material and cell wall characteristics. They have been playing a critical role in shaping the Earth’s atmosphere, recycling major elements such as C and N and mediating key biogeochemical processes and were the first to colonize land. Algae have been often conceptually and systematically linked to the land plants because they include some forms — such as seaweeds/kelps, which look much like plants, and as with plants — they are commonly sessile (attached/rooted/occurring in mixed communities) and oxygenic photosynthesizers. DNA evidence suggests that the first eukaryotes (green plants) evolved from prokaryotes (through endosymbiotic events) between 2500 and 1000 million years ago. Among algae, blue green algae or cyanobacteria represent an ancient group of photosynthetic prokaryotes, whose ubiquity, metabolic flexibility and adaptive abilities have made them a subject of research worldwide. Their biological significance, especially as the prokaryotic interface between the primeval photosynthetic cell and present day plants is also well recognized. Darwin’s Theory of Evolution and Natural Selection have shaped much of our understanding of the evolution of this cosmopolitan group of algae and land plants. However, in the current scenario, with the advent of molecular phylogenetics and the expanding knowledge regarding the evolution of life through the use of bioinformatics, a thorough re-evaluation of algae and interrelationships with plants is needed. This compilation attempts to link Darwinism, Neo-Darwinism and Systemic Darwinism, with the rapidly generated information through modern molecular tools for a better understanding of evolution of land plants, as mediated through algae and endosymbiotic events combined with horizontal and lateral gene transfer processes.