Alteration of photosystem II activity by atrazine on Chlamydomonas reinhardtii synchronized and asynchronized cell cycle cultures

Abstract

Inhibition of photosystem II (PSII) activity by atrazine was investigated in the green alga Chlamydomonas reinhardtii during different states of the cell cycle. The algal cultures were maintained under continuous light or under light/dark cycle (16/8 h) to obtain homogenized cell cycle distribution. The cycle state of algal population was determined by the DNA content using flow cytometry and defined as newly divided cells before the initiation of DNA replication (G₀/G₁) and cells at the end of the replication cycle with fully duplicated DNA content (G₂/M). Under different synchronized states of the cell population, the photosynthetic activity was investigated after treatment at 10, 100, and 1000 µmol L⁻¹ atrazine exposed for 24 h by using fluorescence parameters related to PSII activity measured with a plant efficiency analyzer and pulse-amplitude modulated methods. In this study, we found that the atrazine effect was different depending on cell cycle phases and the period of illumination. Algal cells under light–dark cycle showed inhibition of the PSII electron transport leading to an increase of heat energy dissipation by the PSII reaction center. Algal cells grown under continuous light was shown to be more resistant to atrazine than the cells grown under light–dark cycle.

Keywords:

• green alga
• cell cycle
• chlorophyll fluorescence
• photosynthesis
• Chlamydomonas reinhardtii

Acknowledgments

This work was supported by research grants awarded to R. Popovic by the Natural Sciences and Engineering Research Council of Canada (NSERC). F. Perreault is supported by an NSERC PhD fellowship.