Uptake of two oxyanions (pertechnetate and selenite) by a unicellular phytoplankton species: Chlamydomonas reinhardtii.
C. Morlon*, C. Adam, J. Garnier-Laplace
SETAC Europe 13th annual meeting - Hamburg, Germany - 27/04-01/05/2003
* étudiant en contrat de thèse à l'IRSN
Few ecotoxicological studies deal with anions. Yet, they are often very mobile, bioavailable and potentially toxic. Moreover, general membrane uptake mechanisms differ from those known for cations. Selenium and technetium are predominantly found as anionic species in oxic environments. They may be accumulated by microorganisms such as phytoplankton, and transferred to the whole food-chain via trophic pathways.
Our study deals with the interaction of these two oxyanions with Chiamydomonas reinhardtii, a unicellular green alga representative of the freshwater phytoplankton community. Cells were exposed to Tc99 or Se 75 in well known simple inorganic media. After a short exposure period, quantities of adsorbed and absorbed radionuclides were measured using b liquid scintillation.
The aims of the performed experiments were three-fold:
(i) to describe the kinetics of accumulation
(ii) to characterise transport capacities using a Michaelis-Menten approach
(iii) to identify key nutrients playing a role in the cell membrane transport.
In the case of technetium, the absorbed fraction represented up to 70% of total technetium fixed by algal cells after one hour of exposure; the estimated absorbed flux was low, about 0.003 nmol.m-2.h-1.nM-1. Sulphate inhibited pertechnetate uptake, suggesting that sulphate transporters may be involved. As for selenite, the adsorbed fraction was negligible compared to the absorbed one. The absorption was linear with time, with an estimated absorbed flux of about 0.3 nmol.m-2.h-1.nM-1. Michaelis-Menten uptake patterns suggested that selenite may be accumulated via high affinity transporters when present at low concentration, and via low affinity transporters when present at higher concentrations.
Further experiments are underway to characterise and identify the involved transport mechanisms.