Partitioning beta diversity is an effective way to unravel the response mechanism of organisms to climate change, especially in climate-sensitive arid regions . Turnover and nestedness, showing different implications for biodiversity conservation , are often invoked to disentangle the spatial patterns of compositional beta diversity in biogeography and microbial ecology . In the semi-arid Hulun Lake, we investigated the driving mechanisms of beta diversity and community uniqueness for bacteria, archaea and fungi towards deeper water based on the Baselga's  and Legendre's  frameworks, respectively. Our results suggest that (1) the relative abundance of most dominant phyla had significant water-depth patterns across the three taxa. (2) For the LCBD and the total beta diversity and its turnover component, water-depth patterns were significantly observed for bacteria and archaea, but not for fungi. (3) There was a high correlation between bacteria and archaea regarding the LCBD, the total beta diversity and its turnover components. (4) The relative importance of water and sediment environmental factors on the LCBD, total beta diversity and their additive components was demonstrated to vary for the studied microbial taxonomic groups.
4.1 Water-depth patterns of the LCBD, total beta diversity and their partitioned components
For the total LCBD, we found that bacteria and archaea both showed significant decreasing patterns along the water depth, while fungi did not (Fig. 2a). Such variation may be relevant to sites with special ecological conditions like species combinations and environmental changes. Previous studies have shown that sites with high LCBD values not only have unique species combinations , but also indicate human interference or higher proportions of allochthonous species . As the water level fluctuates, indeed, shallower sites are most likely the consequence of land flooding or mean annual precipitation and water evaporation, which largely changes the habitat environment of autochthonous species. Intriguingly, such variations are also observed for bacterial and archaeal communities in semi-arid grassland soils, which reveals that higher precipitation can effectively regulate microbial assemblies and strengthen their interactions in water-limited areas . For LCBDRepl, such decreasing water-depth patterns are also observed among bacteria and archaea, albeit not significant for bacteria (Fig. 2b). To our knowledge, this is the first study that such parallel patterns in LCBD and its components are observed across microbial taxonomical groups, implying that bacterial and archaeal community uniqueness may be affected by similar ecological progress [58, 59] such as environmental selection. Notably, when bacteria and archaea responded to the variation in water depth, fungi unexpectedly showed non-significant patterns towards deep water regarding LCBD, LCBDRepl and LCBDNes. During the historical events induced by drought, eukaryotic communities including protist and fungi generally present strong resilience in freshwater ecosystems ; that is, fungal assemblages in the dry sediment can effectively restore community structure after water refill. Compared to bacteria and archaea, there is thus lower proportions of allochthonous species (i.e., species gain or loss) for fungal communities in this semi-arid lake.
Previous studies have reported that species turnover is crucial for disentangling the underlying mechanism of beta diversity in aquatic ecosystem . Likewise, for bacteria and archaea, our results indicated that the total beta diversity and its turnover component had consistent increasing patterns along water-depth difference (Fig. 2d, e), which largely consolidates the fact that species turnover contributes to the total beta diversity for aquatic microbial taxa. Moreover, such predominance is also observed in species of other lake environments. For instance, for bacterioplankton, turnover shows a higher contribution to total beta diversity than nestedness in the 25 shallow lakes of southern Brazil . It should be noted that when considering some specific spatial factors such as geographical distance between lakes, the predominance of bacterioplankton turnover may be replaced by nestedness . In the Grand Galibier Massif of the French South-Western Alps, however, high turnover is exhibited in Crenarcheal, bacterial and fungal community distribution, which is greatly associated with plant species composition but not geographical distance . Additionally, regarding nestedness, our studies showed no significant patterns for bacteria, archaea and fungi (Fig. 2f). In particular, for fungi, there was no significant depth-related pattern in total beta diversity and its two components. This may be because fungi has large difference with bacteria and archaea regarding biological characteristics, such as resilience and trophic position. Most importantly, nestedness is well applied to indicate dispersal limitation, while turnover can be as an indicator of species sorting based on environmental filters of microbial communities . Accordingly, for the total beta diversity and turnover component, such parallel pattern between bacteria and archaea may be governed by similar ecological progress [58, 59] like environmental selection.
In addition, we found that there is a strong correlation between bacteria and archaea regarding the total beta diversity and its turnover component (Fig. 4a, b), which further underpins the above synchrony among bacteria and archaea toward deep water. As previously reported, such cross-taxon congruence can occur if different organisms covary in space regarding alpha or beta diversity . Notably, such synchrony is also observed in macroorganisms. For example, in the Espinhaç Range and Mantiqueira Range of southeastern Brazil, total beta diversity of both galling insects and host plant show similar patterns along the elevational gradients, mainly driven by turnover rather than nestedness . Intriguingly, for bacteria and archaea, such congruence is also found for the total LCBD (Fig. 3a). This may reflect the fact that beta diversity (including total beta diversity and LCBD) in semi-arid lake ecosystems is taxonomically dependent among bacteria and archaea. Our findings supported the conclusion that the total beta diversity of two high-dependent taxa is contributed by a similar component (i.e., turnover). It should be noted that, consistent with Yeh et al , no correlation was significantly detected between fungi and other microbial taxa, regardless of total beta diversity or LCBD. Conversely, in a arid inland river basin of northwest China, soil bacteria shows a high correlation with fungi for the total beta diversity and species turnover, and meanwhile, both of them respond sensitively to variations in such environmental variables and geographic distances . Accordingly, compared to soil fungi, the relationship between sediment fungi and other taxa is more susceptible to climate change and environmental heterogeneity.
4.2 Environmental determinants of the LCBD, total beta diversity and their components
Previous studies have shown that eutrophication and climate change are two processes promoting the proliferation or expansion of algal blooms . Similarly, for bacteria and archaea, we found that variations in total LCBD and LCBDRepl were greatly contributed by the water factors such as NO3−-N or NO2−-N (Fig. 5a), which may be also related to the eutrophication and the local climates. Hulun Lake, as a large hypereutrophic steppe lake, has clearly shown that microbial taxa such as bacteria and archaea frequently participate in the cycling of carbon, nitrogen and phosphorus in aquatic ecosystem . Over the past few years, however, the warming and drying climate has gradually accelerated the shrinking of water area and the decrease of water level in Hulun Lake . These climate changes have constantly caused the variations in water or sediment factors, and further driven the site uniqueness of microbial communities. The resulting water depth variation has a nonnegligible effect on determining environmental factors in lacustrine ecosystems . For example, in Lake Azul of São Miguel Island, environmental variables such as light intensity, nutrient availability or disturbance regimes vary largely with water depth, and thereby resulted in the distinct distribution of biological assemblages . Considering the vertical variations of light, nutrients and other physical context, water depth is well accepted to be a better determinant driving the site uniqueness of bacterial and diatom communities in Lugu Lake . Consistent with this, for bacteria and archaea, our results revealed a strong negative correlation between water depth and the total LCBD or LCBDRepl (Fig. S3a), implying that the drivers of bacterial and archaeal community uniqueness could be substantially constrained by water depth. In addition, climate change has a profound impact on the terrestrial input of inorganic nutrients and organic matter in aquatic ecosystems, thereby increasing the rate of eutrophication for water bodies . In particular, drought, as an extreme hydrologic event, will accelerate the occurrence of eutrophication when nutrients are overenriched [71, 72]. As noted above, the uniqueness of bacterial and archaeal communities may be predominantly caused by eutrophication and climatic change.
Contrary to the LCBD, our findings revealed that bacterial and archaeal total beta diversity and turnover component were mainly influenced by sediment factors like TN rather than water factors (Fig. 5b). As such, the observed total beta diversity and species turnover may be associated with the trophic status or nutrient supply. Although numerous nutrients and organic matters are stored in sediments of lakes and wetlands , much less is known about the effect of nutrients on the total beta diversity and its components across microbial taxa. In the shallow lakes of the southern Brazil, high species turnover is observed in bacterioplankton, strongly conditioned by environmental factors such as TN and total dissolved nitrogen . Additionally, nutrients, including water NH4+-N, NO3−-N and NO2−-N, show great direct effects on bacterial species turnover in aquatic microcosms, and such turnover rates do not decrease with lower temperatures . Admittedly, nutrient status is closely related to the sustainability of lake ecosystems, and meanwhile, the growths of aquatic organisms are largely governed by nitrogen concentrations such as TN, NO3−-N, NO2−-N and NH4+-N . Collectively, the total beta diversity and species turnover of bacteria and archaea were well explained by sediment TN, even after accounting for the water variables.
Regarding the LCBD, total beta diversity and their components, fungi have almost nonsignificant water-depth patterns and drivers (Fig. 2; Fig. 5), which may be related to its resilience in response to climate disturbances (e.g., precipitation or drought). As noted above, fungal assemblages in the dry sediment can effectively restore community structure after water refill . Indeed, fungi is more drought-tolerant than bacteria since its hyphae can effectively obtain water from the water-filled micropores . Based on such nonsignificant strong contribution from water factors like TP and TN (Fig. 5), we speculated that fungal communities may be relevant to the trophic status or nutrient supply of lake water, possibly disturbed by precipitation or drought.
To better clarify these studies, there are three main perspectives for the interpretation. First, historical legacies may be the key factor underlying the water-depth patterns in beta diversity. As proposed by Cardoso , the observed patterns of biodiversity cannot be fully explained without considering the effects of historical factors, which can be reflected in the case of the invertebrate taxa  and diatoms  in the Orinoco river basin. In the Neotropical region, the distinct river forms and riparian ecosystems are shaped by past geological and climatic events, namely Andean uplifts and glacier retreats, which greatly affects the evolution of ecosystem and biological succession . Hulun Lake, having experienced the processes of swamping, drying, and sharp increase or decrease of water level, is constantly evolving between saline, brackish and freshwater lakes owing to the high evaporation . With special geographical location of high latitude, Hulun Lake shows a half-year frozen period and its salinity is 300-400 mg L−1 higher than that of unfrozen status . Under the interplay of natural factors and anthropogenic activities , Hulun Lake shows a long history documenting the frequent outbreaks of severe eutrophication , such as the cyanobacteria bloom , which largely influences the nutrient status of this lake. Due to the historical legacies, nutrients  and salinity  have crucial roles in shaping microbial structure in aquatic ecosystems. Therefore, for microbial taxa, such water-depth pattern in beta diversity may be affected by historical factors.
Second, climate is a pivotal driver of biodiversity patterns , which is widely observed in bacterial biogeographic patterns in arid and semi-arid grasslands [85, 86]. Hulun Lake, as a semi-arid lake, affected by the temperate continental climate , is exceptionally fragile and suffers frequently from extreme climatic phenomena such as dramatic drought and low temperatures. Notably, drought intensification and precipitation alterations can effectively drive the turnover of composition structures, which is well known in the case of soil microbiome  and stream invertebrates . Moreover, species turnover is also considered as the legacy of climatic or geological events . For example, for the microbenthic communities in the Paraíba do Norte or Mamanguape estuaries, beta diversity is individually affected by species turnover in dry season, while turnover and nestedness in wet season . Furthermore, in response to temperature changes, organisms evolving in temperate zones generally have stronger thermal adaptations and dispersal capabilities than that in tropical regions . Accordingly, compared to tropical zones, species turnover is often lower in temperate regions . Even so, our results reveal that bacterial and archaeal total beta diversity were mainly explained by species turnover (Fig. 2), implying that microbial communities may be relatively sensitive to climate change. Given that the role of climatic variation and dispersal ability on species turnover plays key importance , patterns in microbial beta diversity may be substantially relevant to climate changes.
Third, similar patterns are underpinned by equivalent ecological processes . Such processes, including drift, selection and dispersal, governs the turnover of biological community composition in space . Simultaneously, turnover can replace species via environmental selection and historical or spatial restriction , and its predominance can evaluate beta diversity across different taxa, type of dispersal or trophic position . In our studies, the total beta diversity (Fig. 2d) and turnover (Fig. 2e) of bacteria and archaea consistently increased with the water depth difference, implying that their parallel patterns are supported by similar processes like environmental selection. Note that the same patterns are not only observed in total beta diversity and turnover, but also in LCBD and LCBDRepl. Thus, for bacteria and archaea, consistent patterns in LCBD and LCBDRepl are also theoretically governed by same ecological processes such as environmental filtering. From macro-organisms to microbes, whether it is regional or local variation in community structures, such parallel patterns are dictated by similar processes . Thereby, parallel patterns can serve as a powerful tool for understanding the forces driving species turnover or replacement across taxonomic groups, even after accounting for the absence of theoretical derivations.