# Adaptive Plasticity of Phytochelatin Synthase Under Chromium Stress and Sulfur Availability in Scenedesmus acutus

## TL;DR

This study explores how a key enzyme in algal stress response adapts to chromium and sulfur conditions in Scenedesmus acutus.

## Contribution

The study reveals that PCS in Scenedesmus acutus is regulated by sulfur availability and contributes to chromium tolerance via redox and sulfur metabolism.

## Key findings

- SaPCS expression and protein levels are mainly regulated by sulfur availability, not chromium stress.
- Two SaPCS isoforms are more abundant in chromium-tolerant algal strains.
- Phytochelatin production is negligible, with chromium detoxification relying on glutathione and antioxidant cycles.

## Abstract

Phytochelatin synthases (PCSs) are pivotal enzymes in heavy metal detoxification, yet also implicated in sulfur homeostasis and redox regulation. In this study, we report the molecular and functional characterization of the PCS gene from the green alga Scenedesmus acutus (SaPCS), comparing wild-type and chromium-tolerant strains of this microalga. RT-qPCR, immunoblotting and mass spectrometry analyses revealed that SaPCS expression and protein abundance are primarily regulated by sulfur availability rather than by chromium stress. Two protein isoforms (~70 kDa full-length and ~34 kDa truncated) were detected, both more abundant in the chromium-tolerant strain than the wild-type and responsive to sulfur availability. Furthermore, three alternatively spliced transcript variants (SaPCSa, SaPCSb, SaPCSc) lacking the C-terminal domain coding region but retaining a functional or partially disrupted N-terminal catalytic domain were identified, contributing to the post-transcriptional diversification of PCSs. Mass spectrometry analyses showed negligible phytochelatin production in response to chromium treatment, indicating that detoxification of this metal in S. acutus relies mainly on glutathione (GSH) conjugation and the ascorbate–GSH antioxidant cycle. Overall, these results suggest that SaPCS may promote chromium tolerance by modulating sulfur and redox metabolism rather than by driving phytochelatin accumulation, highlighting the remarkable functional plasticity of PCSs in algal stress responses.