Chapter 1. Origins of Photosynthesis and the Cyanobacteria

 

Photosynthesis originated early in the history of life on Earth, probably around 3.5 billion years ago. The Cyanobacteria were among the first to leave a fossil record and dominated the planet for at least 2 billion years before more complex forms of life evolved.

1. The first organisms to conduct full photosynthesis as we know it today were prokaryotic organisms known as Cyanobacteria

2.  Cyanobacteria today are still abundant and as diverse as ever

3. Photosynthesis consists of  two light-capturing cycles (photophosphorylation), and carbon fixation (Calvin cycle) 

4. Pieces of the complex process of photosynthetic evolved in separate groups of bacteria, and were combined through horizontal gene transfer 

5. With the establishment of photosynthesis in the ancient world, Nitrogen became the limiting element for the growth of Cyanobacteria and life in general. Biological nitrogen-fixation evolved among early bacteria and was incorporated into the metabolic machinery of Cyanobacteria through horizontal gene transfer.

 

Figure 1.1. These modern day stromatolites in shallow saline waters off the coast of 

Western Australia are built up, layer by  layer, by a complex community of 

microorganisms, including primarily Cyanobacteria, which create the sticky mucilaginous 

matrix that holds the community together. The matrix also traps sediments which at times

 buries the living matrix, upon which  the microbes migrate to the surface and begin a new layer. 

Nearly identical structures formed as early as 3.5 billion years ago, with the ancestors of 

Cyanobacteria the primary architects. Photo by Paul Harrison, posted on Wikimedia Commons, 

licensed by Creative Commons. 

 

Figure 1.2. Cyanobacteria have existed for over three billion years, and are still extensive 

and diverse  in form. A. Gloethece; B. Merismopedia; C. Oscilattoria;  D. Nostoc; E. Rivularia. 

Figure 1.3. The apparent remains of a cyanobacterium from 3.5 billion year-old rocks in 

Australia. Photograph from Awramik 1992.

Figure. 1.4.  The carbon fixation process, or the Calvin cycle, preceded the evolution of 

the light-capturing portions of photosynthesis, through processes called chemosynthesis.  

Carbon-fixation  was driven by the high-energy molecules ATP and 

NADPH that were created through the oxidation of sulfur-based molecules emanating from

 ancient underwater volcanic vents.  The difficult task of breaking highly stable carbon dioxide 

molecules apart, and  incorporating the carbon atoms into carbohydrate is achieved with 

incremental steps of energy absorption and creation of high-energy bonds within intermediary 

carbohydrate molecules, to the point where the the special enzyme RuDp could pull carbon atoms 

away  from the oxygen molecules of carbon dioxide.and transfer them to the carbohydrate 

intermediary.The intermediary molecules are then recycled to begin the process again.  

Drawings after Coulter, 1910.

Figure 1.5. The simplest form of photosynthesis, callled cyclic photophosphorylation, 

still practiced by some bacteria operates in a gear-like fashion to produce ATP that 

can be used in the Calvin Cycle to make glucose. Light first acts on chlorophyll in an 

Antenna complex to excite electrons, removing them from their base molecules. The 

excited electrons pass their energy on to hydrogen ions in the Cytochrome complex, and

then return to their original positions in the Antenna complex where they can enter a new 

cycle. The pool of hydrogen ions, concentrated in a thylakoid chamber, then drives 

synthesis of ATP in the ATP synthase complex.

Figure 1.6. In the more advanced form of photosynthesis, called non-cyclic , practiced by 

cyanobacteria, algae and higher plants, an extra Antenna system is added, which diverts 

some of the depleted electrons, giving them an extra burst of energy from sunlight,

and sending them to  an NADPH reductase complex, where NADPH is produced

and sent to the Calvin Cycle. That leaves a hole in the first Antenna complex, which

is filled by electrons pulled from water molecules in the Photolysis system. This leaves 

oxygen to be released into the water or atmosphere. Only this non-cyclic process then 

produces the all important photosynthetic byproduct of oxygen gas, that all higher life

is dependent on. 

Figure 1.7. Nitrogen is essential for the formation of proteins and other important molecules of life, but 

exists primarily in the form of highly inert nitrogen gas. The nitrogen molecules can be

 broken by lightning and then form into nitrogen oxides that can enter into biological 

processes as nitrates. More importantly, certain bacteria, including cyanobacteria, have the 

biochemical machinery to convert nitrogen molecules into ammonia (nitrogen fixation), which 

can be used by the bacteria or the plants to make protein.  For over three billion years, 

cyanobacteria were the primary nitrogen fixers on the planet, making the evolution of higher

forms of life possible. Pictured is a modern nitrogen cycle involving bacteria

living in root nodules of members of the legume family.