Chapter 5: Discussion
5.2 Saline Water Irrigation Changes Community Compositional
The health of the soil and crop yields are damaged by high salinity. It is found that the responses of the microbial community within such soil and their functional role towards plant stress tolerance are valuable. Still, there is a lack of clear understanding about these communities of microorganisms (Li et al., 2021). The main highlight of this study was that irrigation water affected the bacterial communities by changing the abundance of several bacterial taxa. The study exhibited that non-saline samples were highly heterogeneous, whereas the saline samples were closely placed in ordination space. Soil EC and irrigation water pH were the most significant variables driving the differences between the communities.
Our analyses showed that members of the bacterial phyla Proteobacteria, Actinobacteria, and Acidobacteroidota dominated both the types of samples and did not show much difference in the abundance. These are the most common groups found
in studies related to soils , including
saline soils (B. Ma & Gong, 2013; Valenzuela-Encinas et al., 2009). Mosqueira et al.
(2019) showed identical results to the current study. Regardless of the saline and non- saline soil conditions within the date palms grown in different geographic locations, they invariably contained Gammaproteobacteria and Alphaproteobacteria as the
dominant bacterial classes. Another study on sugarcane found that the most abundant classes of root and soil bacteria were Proteobacteria, similar to the current study of the date palm (Supriadi et al., 2020).
Furthermore, a recent study on the microbiomes from disturbed sandy soils of the Nadym region analyzed the 16S rRNA metagenomic libraries. Although it was indicated from that study that plant cover was the driving force of microbiome composition, the dominant bacterial phyla were the Proteobateria and Acidobacteria as found by the current study (Gladkov et al., 2019). In another study on Sorghum bicolor, similar bacterial phyla were identified Proteobacteria (Alpha and Beta), Actinobacteria, and Firmicutes (Guazzaroni et al., 2018). Moreover, it was found in both transgenic and non-transgenic sugarcane grown in different soils, the specific bacterial groups such as Actinobacteria, Alphaproteobacteria and Betaproteobacteria did not differ over time in different soils as it was indicated in the current study.
However, phylum Cholorflexi was highly abundant in the non-saline samples.
Previously, it was reported that oil palms grown in non-saline soils of Bernam were dominated by the bacterial phyla of Cholorflexi and Actinobacteria (Goh et al., 2020);
however other studies showed that the increase in salinity stress dramatically decreased the abundance of Cholorflexi (Yang et al., 2021). The dominance of the phyla Chloroflexi in the non-saline soil indicates some important implications of this phyla. It is found to play an important role in providing the filamentous scaffolding around which microbial aggregates are formed within the rhizosphere, to feed on the degraded fragments of the bacterial cells, to break down complex carbohydrates and polymeric organic compounds to low molecular weight substrates to support the growth of other bacterial communities within the rhizosphere (Speirs et al., 2019).
Contradictorily, phylum Firmicutes showed relatively higher abundance in the saline samples than non-saline. All of these phyla contained taxa commonly found in soils were indicated in previous studies, where it was indicated that these phyla could have a variety of impacts on soil health, including beneficial and pathogenic interactions (An & Berg, 2018; Berendsen et al., 2012; S.-H. Lee et al., 2008; Philippot et al., 2013). In a recent study on maize plants, the Firmicutes were more abundant in saline soils than the non-saline, similar to the results found in the present study (Hou et al., 2021). The Firmicutes are predominant in saline environments due to their gram- positive cell walls and spore-forming abilities (Mukhtar, Mehnaz, et al., 2018; Schimel et al., 2007). Studies have shown that Bacillus-like organisms play a key role in the biogeochemical cycles in saline soils and marine waters (Mehnaz et al., 2017;
Thanapun Taprig, 2013). Also, these halophilic Bacillus strains produce industrially important enzymes (proteases, amylases, cellulases, and lipases), promotes plant growth, and are involved in the bioremediation of different toxic chemicals and pollutants from saline environments (López-López et al., 2010; Mehnaz et al., 2017;
Mukhtar, Mehnaz, et al., 2018; Mukhtar, Mirza, et al., 2018).
In the present study, one of the most abundant genera found at the non-saline was the Rhizobium. The effect of salinity on the growth and survival of Rhizobium spp.
in culture media and soil was examined in a study that indicated that as the level of salinity increased through EC, and the Rhizobium became less abundant presented in the current study (Singleton et al., 1982). Another study revealed that the population abundance of rhizobia in cowpea was high, as indicated in the current study, which was associated with Ethiopian soils' season and cropping history (Kebede et al., 2021).
Han et al. (2020) analyzed the compositions of soybean rhizobium bacteria microbiota
in three different types of soil. It was found that the rhizosphere community and diversity within soybean plantations varied significantly in different soils, with a decline in saline soil as in the current study (Han et al., 2020). Members of Rhizobium most commonly form microsymbionts with nodulating legumes (Lima Guimarães et al., 2015). Many studies indicate that non-legumes also react to the presence of rhizobia in the rhizosphere. It has been observed that these symbiotic bacteria cause root hair to curl on maize, rice, and oat plants (Alikhani et al., 2006). Rhizobium belongs to the Alphaproteobacteria, and it is well known that environmental stress decreases the level of Alphaproteobacteria in soils (Chodak et al., 2013; Thiem et al., 2018). Therefore, this can be the possible reason for the greater abundance of these taxa in non-saline samples in the present study.
Furthermore, in the current study, it was found that the rhizosphere of the date palms grown in a saline environment consisted of an abundance of Micromonospora, Pseudomonas, and Mycobacterium. This result is correlated with the study conducted by Ferjani et al. (2015), as it was indicated that the date palms in the desert ecosystem of Tunisia had a high dominance of Pseudomonas, Pantoea, and Mycobacterium genera. Furthermore, in another study, a large abundance of Pseudomonas was found in the cotton plants (Gossypium hirsutum) grown in saline soils. That study indicated that Pseudomonas played a role in the salinity tolerance of the cotton, which may relate to the current study results. Similarly, Pseudomonas may play a role in date palms salinity tolerance (Egamberdieva et al., 2015). Furthermore, it was indicated that the halophytic plant Salicornia bigelovii soil was occupied by large communities of Micromonospora, as found in the current study (El-Tarabily et al., 2019).
Further the study showed that the growth under the saline environment by the Micromonospora, which could play a similar role for the date palms grown in the saline soils. In another study on the role of Mycobacterium under the saline conditions of the wheat. It was found that Mycobacterium had a beneficial impact on wheat's growth during the salinity stress due to its ability to produce different biologically active compounds such as enzymes and phytohormone auxin (Egamberdieva, 2012).
As found in the present studies, their presence in the saline-grown date palm rhizosphere indicates similar Mycobacterium functions for the date palms' survival in the saline conditions. The identified microbial communities Micromonospora, Pseudomonas, and Mycobacterium in the saline soils of date palms in the current study indicated the potential effect of PGPR of these microbial communities that have been demonstrated in various studies (Ahemad & Kibret, 2014; El-Tarabily et al., 2019).
Furthermore, in the current study, it was found that the rhizosphere of the date palms grown in a saline environment consisted of an abundance of Micromonospora, Pseudomonas, and Mycobacterium. This result is correlated with the study conducted by Ferjani et al. (2015), as it was indicated that the date palms in the desert ecosystem of Tunisia had a high dominance of Pseudomonas, Pantoea, and Mycobacterium genera. Various studies indicated that Pseudomonas played a role in the salinity tolerance of the cotton (Egamberdieva et al., 2015). In a halophytic plant, Salicornia bigelovii soil, large communities of Micromonospora (El-Tarabily et al., 2019).
Another study on wheat highlighted the beneficial impact of Mycobacterium under the saline conditions due to its ability to produce different biologically active compounds such as enzymes and phytohormone auxin (Egamberdieva, 2012). A similar role of Mycobacterium can be expected in our study as well. The identified microbial
communities Micromonospora, Pseudomonas, and Mycobacterium in the saline soils of date palms in the current study indicated the potential effect of PGPR of these microbial communities that have been demonstrated in various studies (Ahemad &
Kibret, 2014; El-Tarabily et al., 2019). In addition, PGPR isolates of Mycobacterium were tolerated to high temperatures and salt concentrations, thus conferring a potential role of this microbial community identified in the date palm rhizosphere as a possible competitive advantage for the date palm survival in arid and saline soils (Egamberdieva et al., 2019; Shrivastava & Kumar, 2015). Therefore, the irrigation water source may be responsible for influencing the rhizosphere bacterial communities. Moreover, when under stress, the selection of microorganisms and promoting their survival under stressful conditions might be governed by the plant to minimize stress and provide nutrients and shelter to only beneficial bacterial communities. Understanding the specificity of such interactions and mechanisms of these processes can enable us to focus our efforts on species that can potentially be used as beneficial bio inoculants for crops.