dc.description.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. | |