Different responses of fungi and bacteria to environmental stresses
The ratio of the whole pool of plants to bacterial OTUs in the community (Fig 3a) paralleled the trend in total fungal OTUs to plant species (Fig 3b). According to the results of PICRUSt2, the functional guild assemblage of bacterial OTUs that were associated with plant species were dominated by carbon fixation and oxidative phosphorylation(Fig S7b), all of which are associated with energy metabolisms (Eida et al. 2018). Therefore, we suggest that soil bacteria communities may play a pivotal role in plant colonization in stressful environments by providing more available energy.
The findings of this study are of general interest as a novel perspective on how plant-microbial interactions may vary along environmental gradients, a broad and far reaching topic with many implications for ecosystem function. It would be very interesting to see if the same trends hold true along environmental or plant community gradients in other main ecosystems, such as forests, shrublands and agricultural landscapes. In addition, this study clearly showed the ecological differences between soil fungi and bacteria on the aspect of bipartite networks, although they had been known having the different pH niches (Rousk et al. 2010; Peay et al. 2016) and dependence on plants (Yang et al. 2019; Ni et al. 2021).
Conclusion
Along an broad gradient of environmental variables and plant species richness, number of fungal OTUs specifically linked to each plant species was positively related to plant species richness, while bacteria showed the opposite trend. We suggest that in a more extreme high-stress environment that decreases plant diversity, plants and fungi have fewer excess resources to invest in specific relationships, showing up as lower associated microbiome diversity, but that bacteria may partially replace this role of fungi in high stress/low productivity environments.
This appears to be the first instance in which an analysis of this type, finding plant-host links at the whole community level, has been carried out. It would be interesting to conduct similar studies in other ecosystems around the world.