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Biorefinery of microalgae biomass cultivated in wastewaters 151

2 1
(OH ) per N assimilated, while assimilation of NH generates about 1.3 hydrogen
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1
ions (H ) per N assimilated (Raven and Giordano, 2016).
The toxicity level of NH 3 depends on the algae species, the temperature, and
the presence of other compounds in the culture medium, but particularly on the
1
pH. Ammonia, in its ionized form (NH ), is harmless to algae, because its trans-
4
port into the cell is controlled by specific mechanisms preventing its intracellu-
lar accumulation. NH 3 , however, does penetrate freely through biological
membranes (Abeliovich, 2004). The acquisition of NH 1 in algae mostly
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involves transporters of the Amt/Mep family (Raven and Giordano, 2016). The
Amt/Mep/Rh family are the family of genes present in almost all prokaryotic
1
and eukaryotic lineages, responsible to uptake NH /NH 3 to be used in the syn-
4
thesis of N-containing metabolites (McDonald and Ward, 2016). Raven and
Giordano (2016) and references therein attribute the toxicity of NH 3 to its move-
ments through the plasmalemma and tonoplast by an aquaporin-related channel
combined with NH 1 transport at these membranes by AMT. Another explana-
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1
tion given by these researchers is a futile cycling of NH through low-affinity
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1
NH transporters. Other studies have attributed the toxic effect of intracellular
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NH 3 to the uncoupling of the electron transport in photosystem II and to its
competence with H 2 O in the oxidation reactions leading to O 2 generation
(Morales-Amaral et al., 2015).
The dissociation process of ammonia is described in the following equation:
1
NH 3 1 H 2 O2NH 1 OH 2 (7.1)
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The acid dissociation constant (pK a )of Eq. (7.1) is 9.25. Thus, at pH , 9.25,
1
NH 3 will accept protons and become NH , but when the pH of the solution exceeds
4
1
9.25, NH will be deprotonated and become NH 3 . Since the pH in algae cultures
4
generally ranges between 6 and 10 (Abeliovich, 2004), a very significant part of the
total ammonia can be present in the media as unionized NH 3 .
Toxicity levels of NH 3 to algae have been reported in some works with highly
varying values (Kallqvist and Svenson, 2003; Lin et al., 2007; Paskuliakova et al.,
2018; Zhao et al., 2014). Lin et al. (2007) found out that three algae strains (two of
them isolated from a high ammonia leachate pond) were inhibited by leachate; this
phenomenon was linked to ammoniacal nitrogen (NH 3 -N) concentrations higher
than 670 mg/L. However, almost half of this value has been reported for Chlorella
sp., whose growth was inhibited at a value of 392.4 mg/L of NH 3 -N (Lu et al.,
2018). Abeliovich and Azov (1976) reported the inhibition of photosynthesis and
growth of Scenedesmus obliquus at NH 3 concentrations over 2.0 mM (28.5 mg/L of
NH 3 -N) and at pH values over 8.0. The inhibitory effect seemed to be limited to
the normal function of the chloroplasts, and dark respiration was not affected.
Moreover, Kallqvist and Svenson (2003) reported that the specific toxicity of NH 3 -
N was found to be 2.34 μM (0.0328 mg/L NH 3 -N) for the green algae Nephroselmis
1
pyriformis. These researchers also reported a toxicity of NH to be about 100 times
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less than that of NH 3 .