My thesis research activity concerns the study of the detoxification mechanism developed by phytoplanktonic algae as response to heavy metal exposure, consisting in synthesis of phytochelatins (PC

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Abstract

Heavy metals such as zinc, copper, cadmium, lead, mercury, nickel and cobalt are ubiquitarian elements in the environment, even if they are minor components of the biosphere. They are usually sequestered in the sediment, in the soil and mine deposits, but, due to human activities, their concentration can reach values 50-100 times higher in polluted and industrialized coastal areas. They can be accumulated, directly or through the food chain, in the tissues of living organisms. In seawater, the toxicity of heavy metals and their bioavailability depends on their chemical forms, since they can be complexed with organic and inorganic molecules.

In the course of evolution, aquatic microorganisms have developed many adaptative strategies to neutralize the toxic effects of the excessive presence of heavy metals in water. The main detoxification mechanisms are based on the strategy to limit the amount of toxic metal ions in the cell.

My thesis research activity concerns the study of the detoxification mechanism developed by phytoplanktonic algae as response to heavy metal exposure, consisting in synthesis of phytochelatins (PC

).

Phytochelatins are cysteine-rich peptides with a general structure (-Glu-Cys)

-Gly,

with n values generally ranging from 2 to 6; these intracellular peptides, polymers of

glutathione, are capable to sequester potential toxic metals species in harmless

complexes.

Abstract

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Short-term incubation experiments were carried out by exposing marine microalgae to known amount of Cd, Cu and Pb and quantifying the pool of cellular PC

by using high performance liquid chromatography (HPLC) with a reverse-phase column, after derivatization with the fluorescent tag monobromobimane (mBrB). The microalgal species used in the experiments were: Phaeodactylum tricornutum, Thalassiosira weissflogii e Skeletonema costatum (diatoms), Dunaliella tertiolecta, (green algae) e Emiliania huxleyi (coccolithophore). Our results showed that the cellular pool of phytochelatins increases as the concentration of the inorganic metal fraction increases in solution, indicating that PC

production is cellular response to the metal bioavailable fraction, constituted by the inorganic form of the metal itself.

Experiments were carried out to evaluate the possibility to use the intracellular pool of PC

as biomarkers of metal exposure, in algal bioassay applied to elutrites of sediments collected in highly impacted coastal areas.

Two kinds of bioassay have been developed, both on stationary cells and on exponential

cultures, by using three species of marine microalgae: P. tricornutum, T. weissflogii e

S. costatum. The results showed that the presence of PC

can be effectively used as a

biomarker of metal exposure to the bioavailable metal fraction present in the elutriates

of contaminated marine sediments. Exponential cultures of T. weissflogii have been

shown to be the most sensitive to the presence of toxic metal forms. Finally, our results

showed that the presence of PC

is an early signal of metal bioavailability, occurring

before that toxic effects, such as the inhibition of the cellular growth, can be observed.