Bacteria are the most abundant microorganisms in the world and their diversity has not yet been completely elucidated30–34. In the environmental field, bacteria have been useful in the analysis of water quality35,36, in the evaluation of soil contamination37,38, and as the indicators of food quality 39,40. Regarding biodiversity, studies on the diversity of bacterial communities in treatment plants have identified bacterial genera and/or species41,42. In addition, in a recent report, bacterial morphotypes were used as biodiversity indicators in moorland ecosystems in Colombia. However, in that study, the morphotypes were identified using conventional bacterial taxonomic characteristics and 16S rDNA sequences7. In the present study, bacterial morphotypes only macroscopically and microscopically identified were evaluated. Although bacteria were not identified at the genus and/or species level, our results demonstrate that bacterial morphotypes can be used as biodiversity indicators. The inference is based on the fact that the bacterial morphotypes identified in the lagoons and pure leachate are characterized based on different taxonomic characteristics, rendering them different from each other. Morphotypes 34, 38, and 41 were identified as dominant morphotypes because they were found in all samplings. However, this represents a minimal percentage because each lagoon and the pure leachate contain unique morphotypes, which are representatives of their own biodiversity. In addition, our results allowed the evaluation of two indexes related to species richness (Simpson and Shannon), one index related to the evenness of the bacterial morphotype distribution (Pielou’s), and the Chao1 index, an estimator of the number of species.
The Simpson index showed that there is high diversity among bacterial morphotypes in the six sampled lagoons and the pure leachate, showing that these microorganisms are present throughout the treatment plant contributing to its operation. Reportedly, a high bacterial diversity indicates that the bacterial community is in constant interaction defining inter- and intra-specific relationships for the functioning of the ecosystem 43,44. In the sampled lagoons and the pure leachate, we consider the role of bacteria to be crucial for the operation of the treatment plant.
The global analysis results of the Shannon species richness index suggested that lagoon 6 is the most diverse, i.e., the one presenting the largest number of morphotypes. This is important because this lagoon is supplied with the waste that the leachate treatment plant cannot process. In addition, evaporation occurs in this lagoon, and the lagoon is subjected to physicochemical treatments that bacteria could use as a nutrient source. Bacteria present in lagoons 3, 5, 7, and 8 (lagoons for the collection and conduction of leachate to the plant) may be resistant to physicochemical treatments, resulting in a higher diversity in lagoon 6. Further, these results suggested that the resident bacteria were able to degrade pollutants that were not degraded by the previous chemical treatment45,46. Most of the isolated morphotypes were gram-positive spore-forming bacilli. This is in contrast to previous studies performed in this type of ecosystems where gram-negative bacteria were the main representatives47–49. Spore-forming bacteria may be advantageous compared with non-spore-forming ones, considering that under adverse conditions, they can form spores to protect themselves until the appropriate conditions for growth and development are present50,51.
According to the results of the global analysis of the Pielou’s evenness index, all bacterial morphotypes were evenly distributed in the studied lagoons, except for lagoons 5 and 8. Although the Simpson index indicated a diverse bacterial population, the bacterial morphotypes were not evenly distributed in these lagoons. Consequently, these lagoons presented the lowest values in terms of abundance and evenness of morphotypes. These results are important because they demonstrated that bacteria were not evenly distributed in an environmental matrix34. Besides the low diversity of bacterial morphotypes in lagoons 5 and 8, these results indicated that some of the morphotypes were present in a higher proportion than the others, suggesting that these lagoons, which collect leachate and are not subjected to any treatment, present optimal conditions for the development of certain bacteria that has the ability to produce metabolic waste that limits the growth of other bacteria or that limit the nutrients from their environment when optimally developed, thereby hindering further bacterial colonization52.
The bacterial morphotype compilation described in the present study contributes to the knowledge of bacterial diversity in – scarcely explored ecosystems, such as the leachate treatment plant located in the Antiguo Vertedero de Navarro. Some studies have proposed the identification of bacterial phylotypes and determination of their abundance and ecological attributes 34,53,54. In the present study, we proposed the use of bacterial morphotypes as biodiversity indicators because a wide morphotype diversity was observed in the six studied lagoons and the pure leachate. Moreover, the correct operation of the plant is probably attributed to the resident bacteria, which could be spore-forming extremophilic bacteria. Furthermore, because these morphotypes may represent bacterial species having the potential of degrading different compounds, further studies for the identification of genus and/or species as well as degradation studies are warranted. Finally, the results of this study and use of bacterial morphotypes as biodiversity indicators may constitute a valuable input for biodiversity, taxonomic, and systematics studies.