**3.2 Physicochemical parameters**

The values of the physicochemical parameters taken in the field and the nutrients of the samples analyzed in the laboratory are shown in **Table 1**. It is included the coefficient of variation, which showed that physicochemical parameters showed a low proportion of variation in most of them (2–18%), except in depth where it was obtained 37%. On the other hand, the nutrients also showed a low variation ranging from 14 to 20%.

The concentrations of NO2 were found at an interval between 0.621 mg/L obtained at 20 meters and 0.751 mg/L in the sample taken directly under the dump; for NO3 the concentrations interval was between 6.9 mg/L in the sampling point of 20 m, and 14 mg/L


*The nutriments are shown as total nitrogen (TN) and total phosphorus (TP). Variation coefficient (VC) is expressed as the ratio between the standard deviation and the arithmetic mean.*

#### **Table 1.**

*Values of environmental factors (physical and chemical) in the Cerro de la Estrella treatment plant dump and in the different sampling points (at different distances).*

**Figure 4.** *(a-d). Nutrient concentrations (NO2 , NO3 , NH4+ y PO4 <sup>3</sup>) at the different sampling points.*

registered in the sample taken directly under the dump; moreover, the lowest concentration of NH4+ was found in the sample taken directly under the dump with 0.0129 mg/L and the highest value was obtained in the sample of the spot 20 m with 0.0174 mg/L, observing that the water from the dump, was enriched by mixing with the water of the ecosystem, which is more rich in this compound in the further sampling points. The concentrations of ortho PO4 <sup>3</sup>were between 5.4 and 7.1 mg/L, having the lowest value in the sample from 40 m and the highest in the sample from 10 m (**Figure 4**).

Regarding the linear correlation analysis, it can be observed that NO2, NO3 and NH4+ have a higher correlation value compared to the parameters measured in field, this in Ref. to species richness, instead in terms of abundance it is observed that NO2 and NH4+ together with turbidity are those that have a higher correlation, nevertheless, the correlation was lower when comparing it to the one obtained with species richness (**Table 2**).

## **3.3 Isolation and growth of microalgae**

As a result of the isolation methods (capillary pipetting and seeding in agar plates), it was possible to achieve the growth of three species of microalgae, two belonging to the Division Chlorophyta (*Chlamydomonas* sp. and *Chlorella* sp.) and one of the Division Bacillariophyta (*Nitzschia* cf. *amphibia*).

#### *3.3.1 Growth curves*

For the growth of isolated microalgae, the three strains were seeded in enriched liquid culture medium, based in the Bold Basal formula and cell counting were made for 10 days, having as a result the following:

*Water Cleaning by Means of Microalgae in the Channels of Xochimilco, Mexico DOI: http://dx.doi.org/10.5772/intechopen.104711*


#### **Table 2.**

*Linear correlation analysis of the physicochemical parameters on the richness and abundance of the species.*

#### **Figure 5.**

*Growth curve of* Chlamydomonas *sp.*

#### *Chlamydomonas* sp.

In the following graph it is shown the growth behavior of microalga *Chlamydomonas* sp. in liquid culture medium presenting a maximum growth at ten days (**Figure 5**).

*Chlorella* sp.

Regarding *Chlorella* sp. it was observed that its growth was exponential even after 14 days (**Figure 6**).

*Nitzschia* cf. *amphibia.*

This microalga presented a heterogeneous growth in time, having its maximum growth point at thirteen days and then decrease (**Figure 7**).

## **3.4 Bioassays**

#### *3.4.1 Growth curves*

As for the growth of microalga *Chlamydomonas* sp., it can be observed that there was an exponential increase of organisms during the first five days with a slight

**Figure 6.** *Growth curve of* chlorella *sp.*

**Figure 7.** *Growth curve of* Nitzschia *cf.* amphibia*.*

decrease at day six, recovering on the seventh day and from there presented marked fluctuations in the number of individuals until the end of the experiment. In the case of *Chlorella* sp. it was observed the same behavior as *Chlamydomonas* sp. because the maximum growth was reached at day five, presenting a decrease of organisms from day six until day ten, nevertheless, at day eleven there was an upturn of organisms maintaining it during three more days, to have a downbeat in the last two days.

On the other hand, the growth of *Nitzschia* cf. *amphibia* had an exponential growth in the first four days and a decease during the next two days, at seventh and eighth day it presented an upturn and then continue with ups and downs until the end of experiment (**Figure 8**).

#### *3.4.2 Nutriments and heavy metals comparison between treatments*

The concentrations of NO2 in water of bioassay were higher after incubation time, as opposed to expected, however this could be since not being an axenic culture

## *Water Cleaning by Means of Microalgae in the Channels of Xochimilco, Mexico DOI: http://dx.doi.org/10.5772/intechopen.104711*

**Figure 8.** *Growth curves of the three microalgae in the bioassays.*

it could present nitrifying bacteria that can oxidate NH4+, increasing the NO2 at the end of incubation (**Figure 9**).

In the case of NO3-, it was observed that in treatments with *Chlamydomonas* sp. and *Chlorella* sp. there was a decrease in the quantities of this nutriment, contraire to the bioassays with *Nitzschia* cf. *amphibia* where its values increased after three days (**Figure 10**).

Moreover, in NH4+ there was a decrease of this nutriment with the three microalgae from day three to the end of the experiment (**Figure 11**).

Regarding phosphate it did not show a continuous decrease in any of the treatments with the three microalgae, so there were ups and downs over the course of the days in all treatments, nevertheless, *Chlamydomonas* sp. and *Nitzschia* cf. *amphibia* presented the lowest quantities of this nutriment at day 12 (**Figure 12**).

**Figure 10.** *Nitrates values of each treatment.*

**Figure 11.** *Ammonium values of each treatment.*

**Figure 12.** *Phosphates values of each treatment.*

**Figure 13.** *Nickel values of each treatment.*

**Figure 14.** *Lead values of each treatment.*

As regards heavy metals, in the case of copper a graph is not included because the values obtained were zero from the beginning of the experiment in the three treatments.

For nickel, it is noted that the quantities of this metal were not completely reduced, instead there were sharp fluctuations in values especially in *Chlorella* sp. and *Nitzschia* cf. *amphibia*, only in the bioassay with *Chlamydomonas* sp. it was observed a decrease of the metal from day nine (**Figure 13**).

In the case of lead treatments, it was observed that the three species of microalgae decreased the quantities of this metal, however, the bioassay with *Chlamydomonas* sp., was the one that obtained the lowest concentration of lead at the end of the 15 days of experimentation (**Figure 14**).


#### **Table 3.**

*Percentage of removal of each compound with each microalga.*

**Table 3** shows the removal percentage of each nutrient by the three microalgae in the different bioassays. Negative numbers indicate that there was no nutrient removal, but that its value increased.
