Unravelling the formation of a palmelloid-like phenotype in the green microalga Raphidocelis subcapitata when exposed to pollutants

Introduction: Unicellular microorganisms may present different defence and survival strategies, when exposed to pollutants or adverse environments. For instance, the microalgae belonging to the genera Chlamydomonas, Chlorella, and Dunaliella, in response to high salinity or toxics, can form a structure (comprising non-motile cells surrounded by a matrix of polysaccharides) called “palmelloid”, due to its similarity with the morphology of the alga Palmella.

Objectives: This work aimed to verify the formation of a palmelloid-like (multinucleated) phenotype in the freshwater microalgae Raphidocelis subcapitata caused by exposure to inorganic or organic pollutants. The kinetics of the formation of the palmelloid-like phenotype and its reversibility were also studied.

Methods: The microalga was incubated with heavy metals (Cd or Zn) and organic compounds (the antibiotic erythromycin (ERY) or the herbicide metolachlor (MET)) at various concentrations. After 24, 48, and 72 h, the palmelloid-like phenotype formation was evaluated.

Results: In the absence of stress or when exposed to ERY or Zn up to 200 µgL^-1 (where growth was reduced or halted), R. subcapitata exhibited, primarily, a single nucleus. At sub-lethal and environmentally relevant concentrations of MET (100-200 µgL^-1) or Cd (200 µgL^-1), the alga transitioned to a multinucleated state, similar to the “palmelloid” morphology observed in different green algae. The multinucleated state was reversed when the microalgae were re-inoculated in a fresh medium (without pollutants).

Conclusions: The development of a palmelloid-like phenotype in R. subcapitata when exposed to sub-lethal levels of inorganic or organic pollutants is not a universal defence and response strategy to stress, as it depends on the pollutant, its concentration, and the duration of the exposure. The multinucleated state is a reversible process. The insights provided in this study contribute to increasing the knowledge of the strategies used by R. subcapitata to manage severe stress induced by pollutants.