# Haematococcus lacustris Carotenogensis: A Historical Event of Primary to Secondary Adaptations to Earth’s Oxygenation

## TL;DR

This paper explores how green algae evolved carotenoid biosynthesis enzymes in response to Earth's oxygenation events.

## Contribution

It reveals the molecular evolutionary mechanisms linking lycopene cyclases to historical oxygenation events.

## Key findings

- Green lineage LCYEs emerged after the Great Oxygenation Event and diverged into LCYBs via gene duplication.
- Positive selection in LCYEs and LCYBs correlates with major oxygenation events, influencing enzyme activity and carotenoid production.
- Carotenoid profiling shows oxidative adaptation has been preserved through evolution.

## Abstract

(1) Background: Oxygen has exerted a great effect in shaping the environment and driving biological diversity in Earth’s history. Green lineage has evolved primary and secondary carotenoid biosynthetic systems to adapt to Earth’s oxygenation, e.g., Haematococcus lacustris, which accumulates the highest amount of secondary astaxanthin under stresses. The two systems are controlled by lycopene ε-cyclase (LCYE) and β-cyclase (LCYB), which leave an important trace in Earth’s oxygenation. (2) Objectives: This work intends to disclose the underlying molecular evolutionary mechanism of Earth’s oxygenation in shaping green algal carotenogensis with a special focus on lycopene cyclases. (3) Methods: The two kinds of cyclases were analyzed by site-directed mutagenesis, phylogeny, divergence time and functional divergence. (4) Results: Green lineage LCYEs appeared at ~1.5 Ga after the first significant appearance and accumulation of atmospheric oxygen, the so-called Great Oxygenation Event (GOE), from which LCYBs diverged by gene duplication. Bacterial β-bicyclases evolved from β-monocyclase. Enhanced catalytic activity accompanied evolutionary transformation from ε-/β-monocyclase to β-bicyclase. Strong positive selection occurred in green lineage LCYEs after the GOE and in algal LCYBs during the second oxidation, the Neoproterozoic Oxygenation Event (NOE). Positively selected sites in the catalytic cavities of the enzymes controlled the mono-/bicyclase activity, respectively. Carotenoid profiling revealed that oxidative adaptation has been wildly preserved in evolution. (5) Conclusions: the functionalization of the two enzymes is a result of primary to secondary adaptations to Earth’s oxygenation.