BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.6. Kingdom Protoctista

From the Greek protos — first; ktistos — to establish

2.6.1. Taxonomy and Characteristics of Protoctists

As already noted in Section 2.2, within the taxonomic scheme of Margulis and Schwartz (Fig. 2.4), Protoctista is perhaps the most contentious group, as the evolutionary affinities among the organisms it comprises are very weak. Essentially, this group serves as a repository for all eukaryotic organisms that fail to fit the strict criteria of any other eukaryotic kingdom. Many protoctists are unicellular organisms.

The kingdom Protoctista includes eukaryotes that are generally considered identical or similar to the ancestors of modern plants, animals, and Fungi (Fig. 2.28). It also encompasses organisms resembling early plants (Algae), early animals (Protozoans), and early fungi (Oomycota). In addition, the kingdom includes a group known as slime Molds, which, like fungi, produce spores, yet are capable of creeping across surfaces, exhibiting animal-like motility. The earliest eukaryotes were likely unicellular organisms that locomotion via flagella.

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Fig. 2.28. Main groups of Protoctista and some Examples of phyla and genera belonging to these groups. Not all groups or phyla are shown.

Protoctists are a fascinating group for anyone interested in evolution, as these organisms act as a missing link between prokaryotes and more advanced eukaryotes such as plants and animals. In the 1960s, for instance, it was discovered that Cell/35.html">Mitochondria — the cellular powerhouses that generate energy through aerobic Respiration — contain their own DNA and Ribosomes, which closely resemble those of prokaryotes. Subsequent studies of base sequences in Mitochondrial DNA have provided compelling evidence that mitochondria were once aerobic Bacteria (prokaryotes) that took up residence within an ancestral Introduction/5.html">Eukaryotic Cell and established a symbiotic relationship with it. Today, mitochondria are present in all Eukaryotic Cells and are no longer capable of independent existence.

Much like mitochondria, METABOLISM/14.html">Chloroplasts — chlorophyll-containing Organelles responsible for Photosynthesis — also possess their own prokaryotic DNA and ribosomes. Chloroplasts appear to have evolved from photosynthetic bacteria that once colonized animal-like heterotrophic cells, thereby converting them into autotrophic algae. Similarly, it is quite possible that red algae originated from blue-green bacteria (cyanobacteria), while green algae descended from green bacteria known as prochlorophytes.

The theory that Mitochondria and chloroplasts are descendants of symbiotic bacteria is known as the endosymbiotic theory. An endosymbiont is an Organism that lives symbiotically within (endo-) another organism.