Morphological and physiological changes exhibited by a Cd-resistant Dictyosphaerium chlorelloides strain and its cadmium removal capacity.

Changes induced on freshwater microalga Dictyosphaerium chlorelloides (Dc(wt)) acclimated in the laboratory until their survival in culture media enriched with cadmium 100 µM have been studied. Cadmium removal by living cells of this Cd-resistant (Dc(CdR100)) strain was tested in cultures exposed to 100 µM Cd during 30 days. Cell dimensions were measured under light microscopy, and cell growth was studied. Photosynthetic yield (ΦPSII) was analyzed and the photosynthetic oxygen development and respiration response was obtained. Results show that Dc(CdR100) strain exhibited significant cell morphology changes in comparison to Dc(wt) cells, which affected both surface area and cell biovolume. Malthusian fitness analysis showed that Dc(CdR100) strain living in Cd-enriched culture had developed a lower capacity of nearly 50% growth, and its photosynthetic oxygen development and respiration response were significantly reduced in both light and dark photosynthetic phases. Dc(CdR100) strain showed a very high capacity to remove cadmium from the aquatic environment (over 90%), although most of the removed heavy metal (≈70%) is adhered to the cell wall. These specific characteristics of Dc(CdR100) cells suggest the possibility of using this strain in conjunction with Dc(wt) strain as bioelements into a dual-head biosensor, and in bioremediation processes on freshwater polluted with Cd.

GENETIC ADAPTATION AND ACCLIMATION OF PHYTOPLANKTON ALONG A STRESS GRADIENT IN THE EXTREME WATERS OF THE AGRIO RIVER-CAVIAHUE LAKE (ARGENTINA)(1).

We tested if different adaptation strategies were linked to a stress gradient in phytoplankton cells. For this purpose, we studied the adaptation and acclimation of Dictyosphaerium chlorelloides (Naumann) Komárek et Perman (Chlorophyta) and Microcystis aeruginosa (Kütz.) Kütz. (Cyanobacteria) to different water samples (from extremely acid, metal-rich water to moderate stressful conditions) of the Agrio River-Caviahue Lake system (Neuquén, Argentina). Both experimental strains were isolated from pristine, slightly alkaline waters. To distinguish between physiological acclimation and genetic adaptation (an adaptive evolution event), a modified Luria-Delbrück fluctuation analysis was carried out with both species by using as selective agent sample waters from different points along the stress gradient. M. aeruginosa did not acclimate to any of the waters tested from different points along the stress gradient nor did D. chlorelloides to the two most acidic and metal-rich waters. However, D. chlorelloides proliferated by rapid genetic adaptation, as the consequence of a single mutation (5.4 × 10(-7) resistant mutants per cell per division) at one locus, in less extreme water and also by acclimation in the least extreme water. It is hypothesized that the stress gradient resulted in different strategies of adaptation in phytoplankton cells from nonextreme waters. Thus, very extreme conditions were lethal for both organisms, but as stressful conditions decreased, adaptation of D. chlorelloides cells was possible by the selection of resistant mutants, and in less extreme conditions, by acclimation.