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

Shiladitya Dassarma,Edward W Schwieterman

doi:10.1017/s1473550418000423

Abstract

AbstractWe 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 bacterio chlorophyll 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 centred 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.

Highlights

The major events sparking life on Earth on our 4.6-billion-year-old planet remain enigmatic, there is general agreement that first life likely arose about 3.7–4.1 billion years ago, during the early Archean or late Hadean eons (Abramov and Mojzsis, 2009; Deamer, 2011; Bell et al, 2015; Knoll, 2015)
What were the important evolutionary events predating the rise of photosynthesis during the early history of life on Earth? the events during this very early time are not clear, in this paper, we discuss a speculative hypothesis for early evolution, called the ‘Purple Earth,’ which posits the rise of retinal pigment-based phototrophic life forms on Earth’s surface prior to anoxygenic and oxygenic photosynthesis
In order to address the limited time of presence of the vegetation red edge (VRE), astrobiologists interested in remote biosignatures have begun to consider and catalog surface reflectance signatures from a diverse array of known pigmented organisms including oxygenic and anoxygenic photosynthesizers, rhodopsin-based phototrophs and non-photosynthetic microbes that use pigments as a UV screen or antioxidant, or for other purposes (DasSarma, 2006; Kiang et al, 2007b, 2007a; Cockell, 2014; Hegde et al, 2015; Poch et al, 2017; Schwieterman, 2018; Schwieterman et al, 2018; 2015)

Summary

Introduction

The major events sparking life on Earth on our 4.6-billion-year-old planet remain enigmatic, there is general agreement that first life likely arose about 3.7–4.1 billion years ago, during the early Archean or late Hadean eons (Abramov and Mojzsis, 2009; Deamer, 2011; Bell et al, 2015; Knoll, 2015). The events during this very early time are not clear, in this paper, we discuss a speculative hypothesis for early evolution, called the ‘Purple Earth,’ which posits the rise of retinal pigment-based phototrophic life forms on Earth’s surface prior to anoxygenic and oxygenic photosynthesis.

Results

Conclusion