4.2 Broader environmental adaptations of abundant bacteria
The current study found that abundant taxa were ubiquitous in more than half of sediment samples, while most rare taxa occurred only in a few samples (Fig. S1a). These results are in line with previous studies where abundant taxa with high local abundance have a widespread or ubiquitous distribution in terrestrial (Ji et al., 2020; Jiao and Lu, 2020; Wan et al., 2021a) and aquatic (Liu et al., 2015; Mo et al., 2018; Wan et al., 2021b) ecosystems. It is possible that abundant taxa may grow on a wider array of substrates and occupy a broader niche compared with rare taxa, hence supporting their persistence at higher abundance in a wider range of sites (Hambright et al., 2015). Indeed, our environmental threshold analysis further corroborated this assumption that abundant taxa exhibited potential broader response thresholds to the environmental variables compared to rare ones (Fig. 3a). For example, abundant taxa exhibited broader nutrient utilization potential than rare taxa, especially for TC, TN, NH4-N, and NO3-N.
Our results further demonstrated that abundant taxa displayed stronger phylogenetic signals for environmental preferences compared with rare taxa (Fig. 3b). This indicates closely related taxa in the abundant subcommunity exhibit more similar ecological preferences across environmental gradients than taxa in the rare subcommunity. Several recent reports have shown that the stronger phylogenetic signals for a given microbial community of environmental preferences, the greater the phylogenetic niche conservatism in the evolutionary history of environmental adaptation (Jiao and Lu, 2020; Wan et al., 2021a). Thus, the stronger phylogenetic signals of abundant taxa for environmental preferences may explain why they have broader environmental breadths than their rare counterparts. In addition, our results revealed that the correlation between community composition and phylogeny in the rare subcommunity was stronger than that in the corresponding abundant subcommunity. This finding implies that abundant taxa are better in maintaining ecological niches than rare taxa under altered environmental conditions (Ji et al., 2020). Together, the above findings indicated that abundant bacterial taxa possessed stronger adaptations to various environmental conditions than rare bacteria in sediments of the Yarlung Tsangpo River.