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.

Effects of pH on the growth rate exhibited of the wild-type and Cd-resistant Dictyosphaerium chlorelloides strainsEnvironmental factors such as pH affect the growth rate, productivity, and the toxicity of metals in microalgae. This research reports the exponential growth of both wild-type (Dc1M wt ) and cadmium-resistant (Dc RCd100 ) strains of Dictyosphaerium chlorelloides under different pH ranges at 1 to 15 days of exposure. The results reveal that Dc RCd100 strain had a maximum growth rate at pH ranges close to neutrality as time increases (pH 8.0, m = 0.93 on 10 th day), besides with limited growth capacity to moderately acidic pH (pH 4.0, m = 0.67 at 1 st day; pH 5.0 m = 0.16, 0.13 at 10 and 15 th days respectively). Whereas, Dc1M wt strain grows with a maximum growth rate at pH 9.0, with an increase at longer exposure time (m = 2.17 on the 15 th day). However, this strain was unable to maintain growth rates at the lowest pH tested at all exposure times. These results suggested that Dc RCd100 shows changes in cell division as well as in the photosynthetic activity to cope with slightly acidic pH ranges, although it was unable to grow in alkaline environments.

The successful selection of a particular type of bioelement and its association to the appropriate transducer determines the specificity of a biosensor. Therefore, from a strain of chloroficea Dictyosphaerium chlorelloides, modified in laboratory to tolerate high Cr(VI) concentrations, the possible interferences of other heavy metals on photosynthetic activity were studied. After exposing wild type and Cr(VI)-resistant cells to increasing Ag(+1), Co(+2), Hg(+2), Cr(+3), Cu(+2), Zn(+2), Fe(+3) and Cd(+2) concentrations, both photosynthetic quantum yields was compared. Photosynthetic electron transport rates were measured with a TOXY-PAM chlorophyll fluorometer, non-linear regression analysis of each of the toxicity tests was done, and means of both groups were compared using unpaired t test. The results show no significant differences between both cell types when they were exposed to Ag(+1), Co(+2), Hg(+2), Cr(+3), Cu(+2), Fe(+3) and Cd(+2) metal ions, and extremely significant differences (p < 0.0001) to Zn(+2) exposures. These results demonstrate the suitability of this Cr(VI)-resistant type D. chlorelloides strain as a suitable bioelement to be coupled to a biosensor based on dual-head microalgae strategy to detect and quantify Cr(VI) in water courses and waste water treatment plants. However, some disturbance may be expected, especially when certain analyte species such as zinc are present in water samples tested. The analysis of binary mixtures between Zn(+2) and other heavy metals showed a slight antagonistic phenomenon in all cases, which should not alter the potential Zn(+2) interference in the Cr(+6) detection process.