Page 183 - Refining Biomass Residues for Sustainable Energy and Bioproducts

156 Refining Biomass Residues for Sustainable Energy and Bioproducts

100 μM on the growth of Coccomyxa subellipsoidea (Kovacik et al., 2017), and with
Cd (10 μM) on the viability of Scenedesmus quadricauda (Kovacik et al., 2016).
In the work of Torres et al. (2017), three of the contaminants tested [arsenic
(As), Zn, and chromium (Cr)] showed a detrimental effect on the growth of N. salina
only at the highest concentrations tested of these metals (3.12, 17.6, and 5.2 mg/L,
respectively). At the same levels, an increase in light intensity positively affected the
growth. This phenomenon was explained as a light-enhanced biomass production
occurring before the ions that were taken into the cells to cause damage. In the same
work, nickel (Ni) and Cu had a significant negative effect on the growth. This effect
was surprisingly lower at the highest concentration tested of these ions (10 and
5.2 mg/L, respectively). Also, the toxic effects of Ni and Cu were greater at higher
light intensities. This was attributed to the effect of these ions on the blocking of cell
division and the decreasing of protein production, which was emphasized at higher
light irradiations, impacting biomass productivity.
N. salina was effective in removing Cd, Pb, Co, Zn, and Cu, from the culture
medium. This ability was more effective at higher concentrations of these contaminants.
As for Se, As, Cr, and Ni, varying degrees of sorption were observed and they depended
on light and contamination levels (Torres et al., 2017). Similarly, Kovacik et al. (2016)
reported an increase in the intracellular accumulation of Cd and Ni in S. quadricauda,
promoted by higher concentrations of these metals in the culture medium.
Dissolved organic substances, such as humic acids (HAs), can influence the tol-
erance of algae cells to heavy metals by the formation of HA complexes or by
shielding cells from metal ions (Koukal et al., 2003). Kovacik et al. (2018) studied
the impact of HA on metal accumulation and toxicity of Cd, Ni, Pb, and Hg at
100 mM to S. quadricauda after 24 h exposure. In this study, algae were precul-
tured on solid or liquid media containing HA for 30 days and then exposed to
metals. The results showed that HA influenced the metal accumulation of Cd
(146%), Ni (181%), and Pb (115%), and there was no impact on the Hg accumu-
lation in the cells precultured on solid HA-enriched media.
Heavy metal removal efficiencies and toxicity limits mainly depend on the
microalgae species, and on the concentration and type of contaminants. Light inten-
sity and preculturing of algae cells in solid HA-enriched media have a significant
effect on both removal efficiency and toxicity of some contaminants to the cells, as
it has been discussed in this section.

7.3 Types of wastewaters and the adequacy of their
use to grow algae

Among the biggest problems facing modern society is the large production of resi-
dues from domestic, industrial, and agricultural sectors. These residues require ade-
quate treatment to minimize their impact on the natural environment caused by
heavy metals and pesticides, and the large loads of nitrogen and phosphorus, caus-
ing eutrophication of water bodies.