Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Setting
Simple Living Organisms
Photosynthetic and Nitrogen-Fixing Prokaryotes

It is believed that Earth's primitive atmosphere was completely anaerobic, containing methane, formaldehyde, and more complex Organic compounds. Under such conditions, the earliest living organisms must have resembled modern Bacteria of the genus Clostridium [8].

At the next stage of evolution, organisms related to modern photosynthetic bacteria (Purple and green bacteria) presumably emerged, capable of utilizing solar energy. Interestingly, the majority of these Gram-negative photosynthetic bacteria are strict anaerobes. Unlike higher plants, none of these microorganisms release oxygen. Instead, to reduce carbon dioxide during Photosynthesis, they require hydrogen, which they obtain either by splitting Inorganic Compounds such as H₂S, thiosulfate, or H₂, or from organic substances.

In all photosynthetic organisms, including higher plants, photosynthesis takes place within membrane structures. In purple bacteria, light-absorbing pigments (bacterial chlorophylls and carotenes) are embedded in membranes formed by infoldings of the outer Cell membrane. These specialized regions are known as chromatophores. They consist of interconnected hollow vesicles, parallel tubules, or parallel lamellae, with an overall Structure diameter of 50–100 nm. In green bacteria, pigments line intracellular vesicles. Today, photosynthetic bacteria inhabit only sulfur springs and deep lakes, but at one time they were likely much more widespread and served as the sole photosynthetic organisms on Earth.

The evolution of a different photosynthetic system (Chap. 13) was required within Cells to release oxygen by splitting H₂O into O₂. The simplest oxygen-evolving organisms existing today are blue-green Algae (cyanobacteria) [9]. Some of these primitive plants are unicellular, with a structure similar to that of colorless bacteria. Others, such as Oscillatoria—a slimy Organism that frequently coats the inner walls of home aquariums—form long filaments about 6 µm in diameter (Fig. 1-9). These filaments exhibit slow gliding movements, allowing them to migrate to clean areas of the Glass. All blue-green algae contain two pigments not found in other prokaryotes: chlorophyll a and β-carotene. These exact pigments are also present in true algae and higher plants. The filaments of certain blue-green algae, notably Nostoc, contain weakly pigmented cells called heterocysts alongside pigmented cells; the former apparently perform a specialized function by fixing molecular nitrogen. The Development of The ability to utilize N₂ to synthesize organic nitrogen compounds represents another major milestone in evolution. Thanks to their capacity for Nitrogen Fixation and photosynthesis, blue-green algae are the least nutritionally demanding organisms known. For growth, they require only N₂, CO₂, Water, light, and minerals.

The generation of molecular oxygen from water via photosynthesis was undoubtedly a pivotal evolutionary event with far-reaching consequences. As oxygen accumulated in Earth's atmosphere, obligate anaerobes (for which oxygen is toxic) became restricted to strictly anaerobic environments, yielding ground to new classes of bacteria equipped with mechanisms to detoxify oxygen and utilize it for The oxidation of complex organic compounds to harvest essential energy.