Re: green oceans

On 3/28/2025 11:04 AM, erik simpson wrote:

Archaean green-light environments drove the evolution of cyanobacteria’s light-harvesting system
 Abstract
Cyanobacteria induced the great oxidation event around 2.4 billion years ago, probably triggering the rise in aerobic biodiversity. While chlorophylls are universal pigments used by all phototrophic organisms, cyanobacteria use additional pigments called phycobilins for their light-harvesting antennas—phycobilisomes—to absorb light energy at complementary wavelengths to chlorophylls. Nonetheless, an enigma persists: why did cyanobacteria need phycobilisomes? Here, we demonstrate through numerical simulations that the underwater light spectrum during the Archaean era was probably predominantly green owing to oxidized Fe(III) precipitation. The green-light environments, probably shaped by photosynthetic organisms, may have directed their own photosynthetic evolution. Genetic engineering of extant cyanobacteria, simulating past natural selection, suggests that cyanobacteria that acquired a green-specialized phycobilin called phycoerythrobilin could have flourished under green-light environments. Phylogenetic analyses indicate that the common ancestor of modern cyanobacteria embraced all key components of phycobilisomes to establish an intricate energy transfer mechanism towards chlorophylls using green light and thus gained strong selective advantage under green-light conditions. Our findings highlight the co-evolutionary relationship between oxygenic phototrophs and light environments that defined the aquatic landscape of the Archaean Earth and envision the green colour as a sign of the distinct evolutionary stage of inhabited planets.
 https://www.nature.com/articles/s41559-025-02637-3 (Open access)
 

This abstract could have been better written.  Chloroplasts use light between 400 and 700 nm wave length (blue to red), but chlorophyll does not absorb green light (reflects green light).  So it uses blue and red light for photosynthesis.  Apparently the oceans were green because of oxidized Fe(III), but this doesn't make sense because oxidized Fe(III) absorbs green light and reflects red light.  It is the reduced form (Fe(II)) that reflects green light, and so would make the green light more common.  They may be relying on the Fe(III) to be removed from solution by precipitation, so that it would not absorb green light, but that wouldn't make the oceans green, just absorb less green light. Phycobilin absorbs blue and green and reflects yellow to red.  It literally fills in the hole that chlorophyll has because chlorophyll reflects green light.  Having both light absorbing molecules allows cyanobacteria to exploit the full visible spectrum of light between 400 and 700 nm.
Ron Okimoto