Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures

TL;DR

This paper explores the early evolution of retinal-based phototrophy on Earth, its impact on biosignatures, and its potential as an indicator of extraterrestrial life, emphasizing the 'Purple Earth' hypothesis.

Contribution

It introduces the hypothesis that retinal-based light-harvesting systems predominate before photosynthesis and discusses their implications as biosignatures for exoplanet exploration.

Findings

Retinal pigments absorb strongly at 568 nm, complementing chlorophyll.

Retinal-based phototrophy may have preceded oxygenic photosynthesis.

Retinal biosignatures could be detectable indicators of life on exoplanets.

Abstract

We propose that retinal-based phototrophy arose early in the evolution of life on Earth, profoundly impacting the development of photosynthesis and creating implications for the search for life beyond our planet. While the early evolutionary history of phototrophy is largely in the realm of the unknown, the onset of oxygenic photosynthesis in primitive cyanobacteria significantly altered the Earth's atmosphere by contributing to the rise of oxygen ~2.3 billion years ago. However, photosynthetic chlorophyll and bacteriochlorophyll pigments lack appreciable absorption at wavelengths about 500-600 nm, an energy-rich region of the solar spectrum. By contrast, simpler retinal-based light-harvesting systems such as the haloarchaeal purple membrane protein bacteriorhodopsin show a strong well-defined peak of absorbance centered at 568 nm, which is complementary to that of chlorophyll pigments. We propose a scenario where simple retinal-based light-harvesting systems like that of the purple chromoprotein bacteriorhodopsin, originally discovered in halophilic Archaea, may have dominated prior to the development of photosynthesis. We explore this hypothesis, termed the 'Purple Earth,' and discuss how retinal photopigments may serve as remote biosignatures for exoplanet research.