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1. Introduction
Natural radioactive elements, such as uranium and thorium, are widely distributed over the earth crust. However, anthropogenic activities have a marked effect on the natural cycle of these radionuclides, contributing for an outstanding increase in the dispersion of these elements in the environment [1].
The bioaccumulation of toxic substances by microbial cells considerably decreases their concentration in the environment; consequently, their bioavailability decreases. The use of microalgal cells as biomarkers is becoming an efficient tool for the prevention of contamination, considering that biochemical and physiological changes are not observable at organic levels. Bioremediation, on the other side, is being considered as an effective technology for the treatment and removal of contaminants from wastewaters, particularly with a great potential to clean aquatic systems [2]. In this context, Sar et al. [3] studied the uptake capacity of uranium and thorium by soil-isolated Pseudomonas cells, for the bioremediation of nuclear wastes. The removal of thorium was also studied by Picardo et al. [4], with the inactivated brown seaweed Sargassum filipendula as biosorbent material. In that case, the authors found that the biomass of seaweed was able to recover thorium from solution, however, at very low concentrations, around 1.0 mg/L. This confirms that the lack of biological activity markedly decreases the uptake capacity of the biomass, in comparison to metabolically mediated processes.
According to Barsanti and Gualtieri [5], the estimated number of algal species ranges from one to ten million, most of them microalgal cells. These data emphasize this unexplored field of research, as only a limited number of species has already been studied, from the physiological and biochemical levels [6]. Genera Monoraphidium and Scenedesmus belong to the order Chlorellales, frequently present as pure culture in plankton [7].
The most traditional way to quantify microalgal population is direct counting, from a known volume of culture, using a counting chamber or hemocitometer—a flat chamber with known area [8, 9]. The observation field is used to quantify the amount of cells present on that space of known volume, being used for all counting chambers. However, factors such as culture density, size, and shape of cells or colonies can markedly affect the choice of a suitable counting chamber. Table 1 shows some characteristics of commercial cell-counting chambers, regarding cell size and...