The Comparison of Growth, Bioaccumulation of Heavy Metals, and Cia7 Gene Expression Levels between Two Strains of Chlamydomonas reinhardtii

Abstract

Chlamydomonas reinhardtii has been a valuable model organism in the study of heavy metal homeostasis and tolerance in photosynthetic organisms. This alga has a novel gene Cia7 isolated from one of the high CO2 requiring mutants designated as cia7. CIA7 has the presence of four conserved cysteines and thus is a putative metal-binding protein. The purpose of this study was to compare the growth and the bioaccumulation of metals of the wild type, D66 and the mutant, cia7 in the absence and presence of heavy metals. It also aimed to compare Cia7's expression levels in both strains in the absence and presence of heavy metals. The heavy metals used were Cd and Pb. Algal cultures were grown on different heavy metal concentrations of Cd or Pb for sixteen days. Chlorophyll absorbance and cell count were monitored every four day to assess algae growth. To test bioaccumulation, cells were exposed to 50 µM Cd or Pb and grown for one week prior to analysis using ICP-OES. Gene expression of Cia7 under heavy metal stress was determined by RT-PCR after exposing algal cell to Cd or Pb treatments. Results showed that heavy metal concentrations do not influence wild type growth as much as in the mutant. Exposure to Cd and Pb significantly affects (p<0.05) chlorophyll content and cell count in the mutant strain. Results showed a significant difference (p<0.05) in the bioaccumulation for D66 exposed to 50 µM Cd. Results also showed a significant difference (p<0.05) in the bioaccumulation for D66 exposed to 50 µM Pb. On the other hand, cia7 only showed bioaccumulation of Pb, not Cd. Gene expression results showed that is possible for Cia7 to be involved in specific heavy metal resistance to Cd.