The toxic effects of hydrogen peroxide on four species of phytoplankton

Maritza Franco
California State University Long Beach
maritzafranco45@yahoo.com

Introduction

Natural oceanic rainwater can contain a lot of the reactive oxygen species (ROS) hydrogen peroxide (Willey et al. 2004). During photosynthesis, phytoplankton generate additional hydrogen peroxide in their chloroplasts and peroxisomes. The hydrogen peroxide can be released into the surrounding water or held in the membranes of the phytoplankton. Hydrogen peroxide can oxidize proteins and alter cell structures (Barros et al. 2005). We investigated the effects of hydrogen peroxide on the survival of four important phytoplankton species: Emiliania huxleyi, Heterosigma akashiwo, Heterocapsa rotundata, and Rhodomonas sp.

Methods

The experiment consisted of subjecting all four phytoplankton species: Emiliania huxleyi, Heterosigma akashiwo, Heterocapsa rotundata, and Rodomonas sp, to a variety of concentrations of hydrogen peroxide. They were then placed in an environmental chamber with high light at 15° celcius for 90 minutes.The treatments were then fixed with lugols in order to do a cell count.

The figure represents replicates of the control along with seven treatments with hydrogen peroxide concentration increasing as it goes to the right. All the treatments were placed in an incubator for 90min.

Results

Mortality, as a percent of the control, of H. akashiwo exposed to different hydrogen peroxide concentrations. At 2mM and higher, H. akashiwo showed significantly higher mortality (ANOVA p < 0.01).

Mortality, as a percent of the control, of E. huxleyi exposed to different hydrogen peroxide concentrations. At 20mM and higher, E. huxleyi showed significantly higher mortality (ANOVA p < 0.01).

Mortality, as a percent of the control, of H. rotundata exposed to different hydrogen peroxide concentrations. At 2mM and higher H. rotundata showed significantly higher mortality (ANOVA p < 0.01).

Mortality, as a percent of the control, of Rhodomonas sp. exposed to different hydrogen peroxide concentrations. At 200mM, Rhodomonas sp. had higher mortality (ANOVA p < 0.01). Growth in all treatments was lower than in the no-peroxide controls.

Survival, as a percentage of the control, for two algal species exposed to two hydrogen peroxide concentrations. For both concentrations, H. akashiwo had higher mortality than E. huxleyi.

Fluorescence, as a percentage of the control, for two algal species exposed to two hydrogen peroxide concentrations. For both concentrations, E. huxleyi had lower chlorophyll.

Discussion

• As we predicted, H. rotundata and Rhodomonas sp. were very sensitive to hydrogen peroxide.
• Contrary to our expectations, E. huxleyi was more resistant to high hydrogen peroxide levels. However, H. akashiwo’s chloroplasts appeared to be more resistant to degradation.
• Responses to oxidative stress differ among species. E. huxleyi stores hydrogen peroxide on the outside membranes while H. akashiwo stores it next to its chloroplast within the cell. This difference in natural hydrogen peroxide distribution in these cells probably accounts for their differences in resistance to elevated levels.