4.1. Variation of Volumes per Year and per Site
The leachate production from the Sargassum consortium collected in 2020 and 2021 shows variations between one collection and another. Noticeable differences were obtained in the quantities of leachate volumes generated; moreover, late leachate was noticeable in the production dynamics for all cases of Sargassum collected in 2021 with respect to the 2020 Sargassum samples. Furthermore, the cumulative volumes generated with the samples collected in 2020 were lower compared to the samples collected in 2021, specifically with respect to the collection sites.
Particularly, the
Sargassum consortium of AB and DB generated smaller volumes compared to sites AL and PL, for both collections. In this sense, this behavior suggests a variation in the content of the
Sargassum biomass with respect to the sites due to the structural and composition modifications of the
Sargassum during its journey through the route of origin, North Equatorial Recirculation Region [
34].
In addition to the quantities that arrive every certain period, it has been observed that there are changes in the composition of
Sargassum according to the temporality of the collection, as well as the incidence of one or another species (or subspecies) [
2,
35,
36,
37].
One of the main parameters that influence the leaching of an organic waste is the moisture content, which encourages the microorganisms present during the degradation of the biomass to act in the different stages of digestion [
35].
In some species of
Sargassum, it has been seen that a drying and rehydration process occurs during their stay in the tide and that influences their carbon fixation and the content of the carbon dissolved in the medium; this would justify the influence of the humidity present in the samples of
Sargassum collected in the different sites, as well as the influence of its degradability [
38].
The set of aerobic conditions accelerates the process of the degradation of organic matter, solubilizing it and generating leachate [
29]. On the other hand, [
31] mentions that, initially, organic substances can be oxidized to CO
2 and H
2O in the presence of oxygen; later, with the reduction of oxygen, they pass through a hydrolysis phase, in which the contained humidity of the waste increases and gives way to a process of the dissolution of organic matter [
39]. Moreover, it has been pointed out that in the processes of hydrolysis of seaweed, during the first weeks of the process, there is a large release of soluble organic matter [
40], so this would explain the gradual increase in the volume of leachate generated. Similarly, other studies [
41,
42] found that the solubilization of organic waste components occurs in the early stages of degradation and, as the process stabilizes, leachate generation decreases. In the present experiment, the behavior in the generation of leachate was similar to the results of the aforementioned studies, establishing that at the beginning of the process of degradation of
Sargassum, there was a rapid degradation of organic matter and, over time, the concentration of biodegradable compounds decreased, as well as the moisture present in the
Sargassum, which in turn reduced the volume of leachate generated.
As was mentioned before, the production of leachate from a waste is an indication of a biochemical decomposition of organic matter, where parameters such as humidity, pH, and temperature are crucial for the digestion stages to be efficient [
35,
43], which indicates that the behavior in the dynamic production of leachate is closely related to the microorganisms associated with the decomposition of organic matter [
35]. The process of the biochemical degradation of waste occurs in four main phases, hydrolysis, acidogenesis, acetogenesis, and methanogenesis [
44], which are carried out by specialized microorganisms, under specific conditions for each stage, and under different biomass origins. There is a difference between the decomposition of a marine and a terrestrial biomass, since they are from different environments; a marine biomass has resistance to salinity, and consequently different microbial ecosystems are involved in the stages of their degradation [
45,
46]. In this digestion, there is an assimilation of the main sources of proteins, carbohydrates, and fatty acids, where, in addition to generating a liquid fraction with soluble organic matter, a biogas is generated in the last phases, which is a mixture composed of CH
4 and CO
2, and to a lesser extent H
2S, O
2, and H
2 [
47]. The composition of the organic matter dissolved in a leachate generated by decomposition, as well as the proportions of the gases generated, may vary depending on the composition of the decomposing organic matter, rate, and microbial activity, as well as the conditions of the environment; in the case of
Sargassum sp. during its decomposition, it is possible that H
2S may prevail since its main polysaccharides include sulfated carbohydrates [
48].
Thus, we hypothesize that the variations in leachate production performances are due to differences in tissue composition found at each collection site, so more detailed analyses were performed on the samples collected in 2021.
Table 2 presents the proximal characterization of the
Sargassum consortium collected in 2021 in the four sites, where in the first instance, the percentages of humidity are gradually reduced, taking in the function of the collection distances, from the high seas (86.15%, AL) to the coast (80.55%, PL), so that the organic contents (volatile solids) are also variable with respect to the collection sites, which coincides with Zhao et al. [
38], who mention that the changes caused by dehydration and rehydration during their residence at sea are related to the carbon content. Carbohydrate content is variable in the
Sargassum consortium according to the collection site; the AL samples contain the highest percentage of total carbohydrates with 24.64% and since in the early stages of a biochemical degradation, hydrolysis of the main carbon sources occurs [
44], which in the case of
Sargassum are the carbohydrate biopolymers in these samples, a greater volume of leachate was obtained (58.67 mL), while with the AB samples containing the lowest percentage of carbohydrates (18.49%), the lowest volume of leachate (17.3 mL) occurred on day 24 of experimentation.
On the other hand, regarding the presence of polyphenolic compounds (lignin-like) in
Sargassum tissue, it represents a main system of the stress response and recalcitrance to degradation [
28], so the high “lignin-like” content in DB samples (41.24%) gives them greater recalcitrance compared to the other samples, thus causing less degradation and low leachate production. Additionally, according to the spatial distribution of the collection sites, variations were presented in the content of these polyphenolic compounds. In the tissue collected in AL, 37.96% was obtained; AB, 36.79%; DB, 41.24%; and the samples collected from the beach, 40.59%. Since these compounds play an important role in the biosorption process of metallic contaminants [
14], it is possible to relate the increase with the content of the inorganic material (ash content) with respect to the
Sargassum path, so that in samples where the “lignin-like” content is higher (AB with 41.24%), there is the lowest percentage of ash content (19.54%) in relation to the other sites. Finally, a percentage of content not yet identified (determined as “other”) also shows an increase in spatial distribution from the high seas (14.77%) to the coast (18.97%).
Regarding CHNS elemental analysis results, a percentage of carbon content of 34.15% was obtained in the samples of the
Sargassum consortium collected offshore (AL), which is similar to those reported by other authors [
27,
49]. As for the AB, DB, and PL samples, they presented a carbon content of 27–28% similar to that reported by [
26]; however, as for the C:N ratio, in AL and PL, a ratio of 24:1 was obtained, which falls in the ideal range for the optimal digestion or fermentation of organic matter [
27], which does not happen with samples collected in AB and DB as they have a C:N ratio of less than 20:1.
These variations were presented in the
Sargassum consortia; however, in the case of the species
S. fluitans and
S. natans (which have been reported as the most abundant [
50,
51]), it is observed in
Table 3 that the percentages of
S. natans are higher in the samples collected in DB and AB with 85.1% and 63.1%, respectively, while the samples collected in AL present a higher percentage proportion of
S. fluitans (78.4%). As for their part in the PL consortium, 48.2% was registered for
S. fluitans and 51.8% for
S. natans, which shows according to the sites where the consortium resides and that the populations of the species are variable. In an evaluation of the same type of collection in 2021, Alzate-Gaviria et al. [
28] report that the “lignin-like” content is higher in
S. natans compared to
S. fluitans, and since these compounds give it the recalcitrant characteristic, it is possible to assume that by containing a greater presence of
S. natans, the samples collected in AB and DB caused a greater resistance to biodegradation and therefore a lower volume of leachate, contrary to what happened in the sample collected in AL.
The degradation of species present in the consortium affects the behavior of the generation of leachate, which is evident in the case of
S. fluitans, since in the consortium where the percentage of this species predominates, the behavior between the leachate generation processes of the species is very similar to the consortium process,
Figure 5a, which does not occur when the predominate percentage is that of
S. natans.
From the results observed in
Figure 5b,c,
S. natans alone presented a variable leachate production while in
S. natans in AB, 30.8 mL was produced on day 31, and in
S. natans in DB, only a maximum volume of 10.8 mL was reached in 38 days (
Table 1). In the case of
S. fluitans collected in AB and DB, the highest peak in leachate production was generated on day 17 (
Figure 5b,c). This suggests that the generation of leachate is a function of the percentage proportions of the species present in the consortium, as well as the synergy that exists between the microorganisms involved in the decomposition of each species (
Figure 5d).
4.2. Leachate Characteristics: pH and Electric Conductivity (EC)
Overall, pH monitoring during leachate generation is indicative of a distinction in the performance of the microbiota associated with the degradation of
Sargassum as a consortium, and consequently in its degradation by species. The behavior in the results of the pH measurements in the leachate generated from
S. fluitans in AL indicates a lower pH (7.2) on day 26 of collection (
Figure 6a), coinciding with the highest point of leachate volume production at the same site (AL) but as a consortium, due to the largest amount of
S. fluitans. On the contrary, it is observed that the lowest pH, in the leachate generated with
S. fluitans, was obtained for the AB, DB, and PL sites (
Figure 6b–d), recorded on the days with the highest volume production, because there is an adaptation of the microorganisms involved in the digestion of the biomass to the initial pH conditions; once adapted by acidogenic microorganisms, the pH is gradually reduced, so that it is in a pH range of 6.5–7.5, as reported by [
52], and this agrees with when optimal biomass hydrolysis conditions are reached.
Although pH is an important parameter for the development of microorganisms of the different stages of degradation, the nature and chemical composition of a biomass influences which genera and microbial species may be found, so that in a residual biomass with a higher proportion of cellulose, there will be greater diversity of cellulase-producing species [
35]; in this sense, given the information on the difference in carbon sources between
S. fluitans and
S. natans [
28,
50], the microorganisms responsible for degrading the tissue of
S. fluitans may be different from those responsible for the degradation of
S. natans, so a marked decrease in pH was not observed and that is reflected in the slow production of leachate with
S. natans with the exception of site AB, where it is observed that after 26 days, there is a more noticeable reduction in pH and increase in the volume of leachate.
In this sense, the microorganisms responsible for degrading the tissue of
S. fluitans may be different from those responsible for the degradation of
S. natans, so that a marked decrease in pH was not observed and that is reflected in the slow production of leachate with
S. natans with the exception of
S. natans in AB, where it is observed that after 26 days (
Figure 6a), there is a drastic reduction in pH and increase in the volume of leachate. In
Figure 6d, it is observed that in the leachate of the PL samples starting at pH 6 for
S. natans, pH 7.2 for
S. fluitans, and an average pH value of 6.5 in the consortium and that during the degradation of the biomass both by species and in the consortium, the pH of the leachate increased gradually during the first days until about 20 days, and the pH was stably maintained the rest of the days of experimentation, suggesting the difference in carbon sources between
S. fluitans and
S. natans [
28,
50].
In
Figure 6d, it is observed that in the leachate of the PL samples starting at pH 6 for
S. natans, pH 7.2 for
S. fluitans, and pH 6.5 in the consortium and that during the degradation of the biomass both by species and in the consortium, the pH of the leachate increased gradually during the first days until about 20 days, and the pH remained stable the rest of the days of experimentation. Generally during the first phases of decomposition of the matter, the pH of the leachate is less than 7 due to the acids generated during the stages of hydrolysis and acidogenesis; however, with respect to time, the pH increases due to the consumption of these acids by acetogenic microorganisms, and once the pH range 6.8–7.4 is reached, it is possible to obtain the highest load of dissolved substances, which is optimal for methanogenic microorganisms involved in anaerobic digestion systems [
44,
53]; consequently, the volume of leachate was reduced by going from an acidic to alkaline pH in coincidence with that reported for
Sargassum leachate [
49] given its bioconversion of the material dissolved in the liquid fraction to biogas during the methanogenic stage.
The electrical conductivity denotes the salinity of the solubilized compounds, providing valuable information as to the maturity of the leachates, that is, if they are suitable for use as a substrate. In this study, the EC results are presented in
Figure 7 (data corresponding to the days indicated for leachate collection, i.e., it is not accumulated leachate). The behavior of the EC both for the leachate generated as a consortium and species of the sites AB, DB, and PL is observed as upward behavior coinciding with the times of the greatest production of the leachate volume, followed by a reduction in EC (
Figure 7b–d).
The literature indicates that it is common for the EC to present an increase with respect to time, which is related to the increase in the concentration of degradation products of complex organic compounds [
54]; in this case, the EC values are higher than in the literature for dump site leachate, in landfills of continental waste, so it is suggested that the EC is higher in this case due to the marine origin of the biomass. On the other hand, for AL, there is a decrease in EC (both consortium and species) during the last days of generation; this coincides with the volume and moreover this can be considered as an accelerated degradation of biomass [
41]. In the case of fresh
Sargassum in AL, this is reflected to a greater extent with leachate production.