Green light billions of years ago drove evolution of photosynthesis in cyanobacteria

Aquatic environments of the archaic era, dominated by green light, played a key role in the evolution of light absorption systems by cyanobacteria. Research by a team of scientists from Japan, published in Nature Ecology & Evolution, showed that the presence of oxidized iron (Fe(III)) compounds in the water led to the formation of a specific light spectrum, forcing photosynthetic organisms to adapt to the new conditions.

Three windows of light in Earth’s history

Earth’s photosynthesis went through three dominant stages: the blue light window (before the appearance of cyanobacteria), the green light window (during the archaic era) and the white light window(after the Great Oxidation Event – GOE). The oxidation of the aquatic environment and the formation of the ozone layer changed the spectrum of light reaching photosynthetic organisms. Cyanobacteria, originally specialized in using green light, had to adapt their antenna system to the new conditions.

Green light dominance in the aquatic environment

Numerical simulations conducted by the researchers showed that the oxidation of iron (Fe(II)) by cyanobacteria and other bacteria capable of using light for this process resulted in the formation of iron hydroxide (Fe(OH)₃) particles, which effectively absorbed UV and blue light, transmitting mainly wavelengths in the 500-600 nm range. This phenomenon led to the formation of a so-called green light window, particularly intense at the boundary between aerobic and anaerobic zones – typically at depths of 20-50 m, according to models at Fe(OH)₃ concentrations of 10 µM.

Evolutionary response of cyanobacteria to new conditions

In response to the new light conditions, cyanobacteria developed phycobilisomes – complex antenna structures made up of three main phycobiliproteins: allophycocyanin (APC), phycocyanin (PC) and phycoerythrin (PE). The phycobiliprotein phycoerythrobilin (PEB), which absorbs green light and transfers energy to chlorophyll a (Chl a) via PC and APC, played a particularly important role. Phylogenetic analyses confirmed that the common ancestor of modern cyanobacteria possessed all the key elements of this system.

Experimental confirmation of the role of PEB

The growth of two cyanobacterial species was compared under laboratory conditions: Gloeobacter violaceus PCC 7421 (having PE, PC and APC) and Synechococcus elongatus PCC 7942 (lacking PE). Gloeobacter violaceus grew equally well under white and green light, while S. elongatus showed a marked slowdown in growth when exposed to green light. Moreover, genetic engineering made it possible to create S. elongatus transformants (PebAB-MX and PebAB-OX) capable of synthesizing PEB. These cells, despite lacking PE, efficiently absorbed green light and showed a growth advantage under conditions mimicking the archaic environment.

Green light as a selection factor

The green light spectrum, arising from the presence of Fe(OH)₃, provided a selection pressure favoring cyanobacteria capable of absorbing these wavelengths. Energy transfer from PEB to Chl a, implemented by PC and APC, was highly efficient. Calculations of Förster distances and electron couplings showed that ficobilin pigments, such as PEB, had a significantly higher energy transfer efficiency than other pigments, such as β-carotene.

The importance of the evolution of phycobilisomes

The evolution of phycobilisomes was a response to environmental light conditions. Through the development of pigments such as PEB, cyanobacteria gained the ability to use green light efficiently, which gave them an adaptive advantage and enabled them to colonize diverse aquatic environments. The adaptation of pigments to the changing light spectrum has not only influenced the evolution of organisms, but also global geochemical changes.

Applications

The study suggests that green light environments were an important factor shaping the evolution of early cyanobacteria. Gaining the ability to use it efficiently through the development of phycobilisomes gave them an evolutionary advantage, enabling them to survive and expand in diverse aquatic environments. The study’s findings are an important contribution to understanding the early evolution of life and photosynthesis on Earth.