Effect of Cadmium on the marine diatoms Seminavis robusta and Phaeodactylum tricornutum: A comparative study

Abstract

Background: Heavy metals cause a harmful effect on marine ecosystems because of their toxicity, persistence and bioaccumulation properties. Cadmium (Cd) is speculated to be a unique heavy metal due to its diverse toxic effects. It is released into the environment through point sources and also capable of long range transports. An extensive laboratory study was carried out in order to explore the toxic effects of cadmium on diatoms. Diatoms were chosen as the study organisms because of their vital role as primary producers in the marine ecosystem. Experiment: Two marine benthic diatom species Seminavis robusta and Phaeodactylum tricornutum were separately cultured in aquil medium and exposed to seven different concentrations of cadmium (Cd2+) (0.01, 0.05, 0.25, 1.25, 6.25, 31.25, 156.25 mg/l) for approximately 4 days (day and night cycles). The toxic effects were analyzed in three different biological levels. Growth rate (at population level), chlorophyll fluorescence and cell granularity (at cell physiology level) and gene expression changes (at molecular transcription level) were analyzed. Results and conclusion: In Seminavis robusta, cadmium at low level concentrations is suggested to induce the expression of CA1 gene that might have resulted in the increase of growth rate. In Phaeodactylum tricornutum, the overall growth rate did not seem to be suppressed by cadmium. Genes responsible for the cellular uptake of cadmium, VIT1 and ZIPT1 were induced in P. tricornutum, while S. robusta did not show an indication of uptake of cadmium through these genes. NTF2L gene was shown to be significantly (p=0.001) upregulated at very high levels of cadmium in S. robusta. A similar trend was observed in P. tricornutum (p=0.01). The role of NTF2L in cadmium efflux in diatoms is presented for the first time in this study. The genes used to monitor oxidative stress, CAT1, SOD1, GSR2 were not significantly activated in any of the two species on exposure to cadmium under the given experimental conditions. The results of this study do not support the hypothesis that S. robusta due to its expanded gene inventory, shows a better tolerance to heavy metal stress.